Atari 2600 Stella Programmer's Guide

TABLE OF CONTENTS                                          4

                          Steve Wright
     (Reconstructed by Charles Sinnett 6/11/93    Internet:
Table of Contents

TELEVISION PROTOCOL                                      1
     Diagram 1 - Atari TV Frame                          2
The TIA (as seen by the programmer)                      3
     1.0 General Description                             3
     2.0 The Registers                                   3
     3.0 Synchronization                                 4
          3.1 Horizontal Timing                          4
          3.2 Microprocessor Synchronization             4
          3.3 Vertical timing                            4
     4.0 Color and Luminosity                            5
     5.0 Playfield                                       5
     6.0 The Moveable Objects Graphics                   6
          6.1 Missile Graphics (M0, M1)                  6
          6.2 Ball Graphics (BL)                         6
          6.3 Player Graphics (P0, P1)                   7
     7.0 Horizontal Positioning                          8
     8.0 Horizontal Motion                               8
     9.0 Object Priorities                               9
     10.0 Collisions                                     10
     11.0 Sound                                          10
          11.1 Tone                                      10
          11.2 Frequency                                 10
          11.3 Volume                                    10
     12.0 Input Ports                                    11
          12.1 Dumped Input Ports (INPT0 thru INPT3)     11
          12.2  Latched Input Ports (INPT4, INPT5)       11

THE PIA (6532)                                           12
     1.0 General                                         12
     2.0 Interval timer                                  12
          2.1 Setting the timer                          12
          2.2 Reading the timer                          12
          2.3 When the timer reaches zero                12
     3.0 RAM                                             13
     4.0 The I/O ports                                   13
          4.1 Port B - Console Switches (read only)      13
     5.0 Port A - Hand Controllers                       13
          5.1 Setting for input or output                13
          5.2 Inputting and Outputting                   14
          5.3 Joystick Controllers                       14
          5.4 Paddle (pot) controllers                   14
          5.5 Keyboard controllers                       15
     6.0 Address summary table                           15

PAL/SECAM CONVERSIONS                                    16
     PAL                                                 16
     SECAM                                               16

     GENERAL DESCRIPTION                                 17
     DETAILED DESCRIPTION                                18
     1. Data and addressing                              18
     2. Synchronization                                  18
          A. Horizontal Timing                           18
          B. Vertical Timing                             18
          C. Composite Sync                              18
          D. Microprocessor Synchronization              19
     3. Playfield graphics Register                      19
          A. Description                                 19
          B. Normal Serial Output                        19
          C. Reflected Serial Output                     19
          D. Timing Constraints                          20
     4. Horizontal Position Counters                     20
          A. Description                                 20
          B. Ball position Counter                       20
          C. Player Position Counters                    20
          D. Missile Position Counters                   21
     5. Horizontal Motion Registers                      21
          A. General Description                         21
          B. Timing constraints                          21
     6. Moving Object Graphics Registers                 22
          A. General Description                         22
          B. Missile Graphics                            22
          C. Player Graphics                             22
          D. Vertical Delay                              23
          E. Ball Graphics                               24
     7. Collision Detection Latches                      24
          A. Definitions                                 24
          B. Reading  Collision                          24
          C. Reset                                       24
     8. Input ports                                      25
          A. General Description                         25
          B. Dumped  Input Ports (I0 through I3)         25
          C. Latched Input ports (I4, I5)                25
     8.5 Priority Encoder                                26
          A. Purpose                                     26
          B. Priority Assignment                         26
          C. Priority Control                            26
     9 Color Luminance Registers                         27
          A. Description                                 27
          B. Multiplexing                                27
     10. Color Phase Shifter                             27
     11. Audio Circuits                                  27
          A. Frequency Select                            27
          B. Noise-Tone Generator                        28
          C. Volume Select                               28
     Figure 1. Vertical Delay                            29
     Figure 2. Synchronization                           30
     Figure 3. Color-Luminance                           30
     Figure 4. Typical Horizontal Motion Circuit         31
     Figure 5. Playfield Graphics                        32
     Figure 6. Collision Detection                       33
     Figure 7. Audio Circuit                             34
     Figure 8. Input Ports                               35
     Figure 9. Game System                               36

Write Address Detailed Functions                         37
     WSYNC (wait for sync)                               37
     RSYNC (reset sync)                                  37
     VSYNC                                               37
     VBLANK                                              37
     PJ0 (PF1, PF2)                                      38
     CTRLPF                                              38
     NUSIZ0 (NUSIZ1)                                     39
     RESP0 (RESP1, RESM0, RESM1, RESBL)                  39
     RESMP0 (RESMP1)                                     39
     HMOVE                                               40
     HMCLR                                               40
     HMP0 (HMP1, HMM0, HMM1, HMBL)                       40
     ENAM0 (ENAM1, ENABL)                                41
     GRP0 (GRP1)                                         41
     REFP0 (REFP1)                                       41
     VDELP0 (VDELP1, VDELBL)                             41
     CXCLR                                               41
     COLUP0 (COLUP1, COLUPF, COLUBK)                     42
     AUDF0 (AUDF1)                                       42
     AUDC0 (AUDC1)                                       43
     AUDV0 (AUDV1)                                       43
     WRITE ADDRESS SUMMARY                               44
     READ ADDRESS SUMMARY                                45
     TIA O0..02 AND LUM TIMING                           46
     TIA WRITE TIMING CHARACTERISTICS                    47
     TIA READ TIMING CHARACTERISTICS                     48
     TIA COMP-SYN AND READY TIMING                       49
     RSYNC, RES0O, H01, H02, SHB, 02, 0O                 50

     (The TV picture according to Atari)
     For the purposes of Stella programming, a single television
     “frame” consists of 262 horizontal lines, and each line is
     divided by 228 clock counts (3.58MHz).  The actual TV
     picture is drawn line by line from the top down 60 times a
     second, and actually consists of only a portion of the
     entire “frame” (see diag. #1).  A typical frame will
     consists of 3 vertical sync (VSYNC) lines*, 37 vertical
     blank (VBLANK) lines, 192 TV picture lines, and 30 overscan
     lines.  Atari’s research has shown that this pattern will
     work on all types of TV sets.  Each scan lines starts with
     68 clock counts of horizontal blank (not seen on the TV
     screen) followed by 160 clock counts to fully scan one line
     of TV picture.  When the electron beam reaches the end of a
     scan line, it returns to the left side of the screen, waits
     for the 68 horizontal blank clock counts, and proceeds to
     draw the next line below.
     All horizontal timing is taken care of by hardware, but the
     microprocessor must “manually” control vertical timing to
     signal the start of the next frame.  When the last line of
     the previous frame is detected, the microprocessor must
     generate 3 lines of VSYNC, 37 lines of VBLANK, 192 lines of
     actual TV picture, and 30 lines of overscan.  Fortunately,
     both VSYNC and VBLANK can simply be turned on and off at
     the appropriate times, freeing the microprocessor for other
     activities during their execution.
     * (to signal the TV set to start a new frame)
     The actual TV picture is drawn one line at a time by having
     the microprocessor enter the data for that line into the
     Television Interface Adaptor (TIA) chip, which then
     converts the data into video signals.  The TIA can only
     have data in it that pertains to the line being currently
     drawn, so the microprocessor must be “one step ahead” of
     the electron beam on each line.  Since one microprocessor
     machine cycle occurs every 3 clock counts, the programmer
     has only 76 machine cycles per line (228/3 = 76) to
     construct the actual picture (actually less because the
     microprocessor must be ahead of the raster).  To allow more
     time for the software, it is customary (but not required)
     to update the TIA every two scan lines.  The portion of the
     program that constructs this TV picture is referred to as
     the “Kernel”, as it is the essence or kernel of the game.
     In general, the remaining 70 scan lines (3 for VSYNC, 37
     for VBLANK, and 30 for overscan) will provides 5,320
     machine cycles (70 lines x 76 machine cycles) for
     housekeeping and game logic.  Such activities as
     calculating the new position of a player, updating the
     score, and checking for new inputs are typically done
     during this time.
Diagram 1 - Atari TV Frame

The TIA (as seen by the programmer)
1.0 General Description
     The TIA is a custom IC designed to create the TV picture
     and sound from the instructions sent to it by the
     microprocessor.  It converts the 8 bit parallel data from
     the microprocessor into signals that are sent to video
     modulation circuits which combine and shape those signals
     to be compatible with ordinary TV reception.  A “playfield”
     and 5 moveable objects can be created and manipulated by
     A playfield consisting of walls, clouds, barriers, and
     other seldom moved objects can be created over a colored
     background.  The 5 moveable objects can be positioned
     anywhere, and consists of 2 players, 2 missiles, and a
     ball.  The playfield, players, missiles, and ball are
     created and manipulated by a series of registers in the TIA
     that the microprocessor can address and write into.  Each
     type of object has certain defined capabilities.  For
     example, a player can be moved with one instruction, but
     the playfield must be completely re-drawn in order to make
     it “move”.
     Color and luminosity (brightness) can be assigned to the
     background, playfield, and 5 moveable objects.  Sound can
     also be generated  and controlled for volume, pitch, and
     type of sound.  Collisions between the various  objects on
     the TV screen are detected by the TIA and can be read by
     the microprocessor .  Input ports  which can be read by the
     microprocessor give the status of some of the various hand
     held controllers.
2.0 The Registers
     All instructions to the TIA are achieved by addressing and
     writing to various registers in the chip.  A key point to
     remember is data written in a register is latched and
     retained until altered by another write operation into that
     register.  For example, if the color register for a player
     is set for red, that player will be red every time it is
     drawn until that color register is changed.  All of the
     registers are addressed by the microprocessor as part of
     the overall RAM/ROM memory space.
     All registers have fixed address locations and pre-assigned
     address names for handy reference.  Many registers do not
     use all 8 data bits, and some registers are used to
     “strobe” or trigger events.  A “strobe” register executes
     its function the instant it is written to (the data written
     is ignored).  The only registers the microprocessor can
     read are the collision registers and input port registers.
     These registers are conveniently arranged so that the data
     bits of interest always appear as data bits 6 or 7 for easy
3.0 Synchronization
  3.1 Horizontal Timing
     When the electron beam scans across the TV screen and
     reaches the right edge, it must be turned off and moved
     back to the left edge of the screen to begin the next scan
     line. The TIA takes care of this automatically, independent
     of the microprocessor.  A 3.58 MHz oscillator generates
     clock pulses called color clocks which go into a pulse
     counter in the TIA.  This counter allows 160 color clocks
     for the beam to reach the right edge, then generates a
     horizontal sync signal (HSYNC) to return the beam to the
     left edge.  It also generates the signal to turn the beam
     off (horizontal blanking) during its return time of 68
     color clocks.  Total round trip for the electron beam is
     160 + 68 = 228 color clocks.  Again, all the horizontal
     timing is taken care of by the TIA without assistance from
     the microprocessor.
  3.2 Microprocessor Synchronization
     The microprocessor's clock is the 3.58 MHz oscillator
     divided by 3, so one machine cycle is 3 color clocks.
     Therefore, one complete scan line of 228 color clocks
     allows only 76 machine cycles (228/3 = 76) per scan line.
     The microprocessor must be synchronized with the TIA on a
     line-by-line basis, but program loops and branches take
     unpredictable lengths of time.  To solve this software
     sync. problem, the programmer can use the WSYNC (Wait for
     SYNC) strobe register.  Simply writing to the WSYNC causes
     the microprocessor to halt until the electron beam reaches
     the right edge of the screen, then the microprocessor
     resumes operation at the beginning of the 68 color clocks
     for horizontal blanking.  Since the TIA latches all
     instructions until altered by another write operation, it
     could be updated every 2 or 3 lines.  The advantage is the
     programmer gains more time to execute software, but at a
     price paid with lower vertical resolution in the graphics.
     NOTE:  WSYNC and all the following addresses' bit
     structures are itemized in the TIA hardware manual.  The
     purpose of this document is to make them understandable.
  3.3 Vertical timing
     When the electron beam has scanned 262 lines, the TV set
     must be signaled to blank the beam and position it at the
     top of the screen to start a new frame.  This signal is
     called vertical sync, and the TIA must transmit this signal
     for at least 3 scan lines.  This is accomplished by writing
     a 1 in D1 of VSYNC to turn it on, count at least 2 scan
     lines, then write a 0 to D1 of VSYNC to turn it off.
     To physically turn the beam off during its repositioning
     time, the TV set needs 37 scan lines of vertical blanks
     signal from the TIA.  This is accomplished by writing a 1
     in D1 of VBLANK to turn it on, count 37 lines, then write a
     0 to D1 of VBLANK to turn it off.  The microprocessor is
     of course free to execute other software during the
     vertical timing commands, VSYNC and VBLANK.
4.0 Color and Luminosity
     Color and luminosity can be assigned to the background
     (BK), playfield (PF), ball (BL), player 0 (P0), player
     1(P1), missile 0 (M0), and missile 1 (M1).  There are only
     four color-lum registers for these 7 objects, so the
     objects are paired to share the same register according to
     the following list:

     color-lum register       Objects colored
     COLUMP0             P0, M0 (player 0, missile 0)
     COLUMP1             P1, M1 (player 1, missile 1)
     COLUMPF             PF, BL (playfield, ball)
     COLUMBK             BK (background)
     For example, if the COLUMP0 register is set for light red,
     both P0 and M0 will be light red when drawn.
     A color-lum register is set for both color and luminosity
     by writing a single 7 bit instruction to that register.
     Four of the bits select one of the 16 available colors, and
     the other 3 bits select one of 8 levels of luminosity
     (brightness).  The specific codes required to create
     specific color and lum are listed in the Detailed Address
     List of the TIA hardware manual.  As with all registers
     (except the strobe registers), the data written to them
     is latched until altered by another write operation.
5.0 Playfield
     The PF register is used to create a playfield of walls,
     clouds, barriers, etc., that are seldom moved.  This low
     resolution register is written into to draw the left half
     of the TV screen only.  The right half of the screen is
     drawn by software selection of whether a duplication or a
     reflection of the right half.
     The PF register is 20 bits wide, so the 20 bits are written
     into 3 addresses: PF0, PF1, and PF2.  PF0 is only 4 bits
     wide and constructs the first 4 bits of the playfield,
     starting at the left edge of the TV screen.  PF1 constructs
     the next 8 bits, and PF2 the last 8 bits which end at
     the center of the screen.  The PF register is scanned from
     left  to right and where a 1 is found the PF color is
     drawn, and where a 0 is found, the BK color is drawn.  To
     clear the playfield, obviously zeros must be written into
     PF0, PF1, and PF2.
     To make the right half of the playfield into a duplication
     or copy of the left half, a 0 is written to D0 of the
     CTLPF (control playfield) register.  Writing a 1 will
     cause the reflection to be displayed.
6.0 The Moveable Objects Graphics
     All 5 moveable objects (P0, M0, P1, M1, BL) can be assigned
     a horizontal location on the screen and moved left or right
     relative to their location.  Vertical positions, however,
     are treated in an entirely different manner.  In principle,
     these objects appear at whatever scan lines their graphics
     registers are enabled.  For example, let us assume the ball
     is to be positioned vertically in the center of the screen.
     The screen has 192 scan lines and we want the ball to be 2
     scan lines thick.  The ball graphics would be disabled
     until scan line 96, enabled for 2 scan lines, then disabled
     for the rest of the frame.  Each type of object (players,
     missiles, and ball) has its' own characteristics and
  6.1 Missile Graphics (M0, M1)
     The two missile graphics registers will draw a missile on
     any scan line by writing a 1 to the one bit enable
     missile registers (ENAM0, ENAM1).  Writing a 0 to these
     registers will disable the graphics.  The missiles' left
     edge is positioned by a horizontal position register, but
     the right edge is a function of how wide the missile is
     made.  Width of a missile is controlled by writing into
     bits D4 and D5 of the number-size registers (NUSIZ0,
     NUSIZ1).  This has the effect of stretching the missile
     out over 1, 2, 4, or 8 color clock counts (a full scan line
     is 160 color clocks).
  6.2 Ball Graphics (BL)
     The ball graphics register works just like the missile
     registers.  Writing a 1 to the enable ball register
     (ENABL) enables the ball graphics until the register is
     disabled.  The ball can also be stretched to widths of 1,
     2, 4, or 8 color clock counts by writing to bits D4 and D5
     of the CTRLPF register.
     The ball can also be vertically delayed one can line.  For
     example, if the ball graphics were enabled on scan line 95,
     it could be delayed to not display on the screen until scan
     line 96 by writing a 1 to D0 of the vertical delay
     (VDELBL) register.  The reason for having a vertical delay
     capability is because most programs will update the TIA
     every 2 lines.  This confines all vertical movements  of
     objects to 2 scan line jumps.  The use of vertical delay
     allows the objects to move one scan line at a time.
  6.3 Player Graphics (P0, P1)
     The player graphics are the most sophisticated of all the
     moveable objects.  They have all the capabilities of the
     missile and ball graphics, plus three move capabilities.
     Players can take on a shape such as a man or an airplane,
     and the player can easily be flipped over horizontally to
     display the mirror image (reflection) instead of the
     original image, plus multiple copies of the players can be
     The player graphics are drawn line-by-line like all other
     graphics.  The difference here is each scan line of the
     player is 8 bits wide, whereas the missiles and ball are
     one bit wide.  Therefore, a player can be thought of as
     being drawn of graph paper 8 squares wide and as tall as
     desired.  To color in the squares of this imaginary graph
     paper, 8 data bits are written into the players graphics
     registers (GP0, GP1).  This 8 bit register is scanned from
     D7 to D0, and wherever a 1 is found that square gets
     the player's color (from the color-lum register) and where
     a 0 is found, that square gets the background color.  To
     position a player vertically, simply leave all 0s in the
     graphics registers (GP0, GP1) until the electron beam is on
     the scan line desired, write to the graphics register line-
     by-line describing the player, then write all 0s to turn
     off the player's graphics until the end of that frame.
     To display a mirror image (reflection) instead of the
     original figure, write a 1 to D3 of the one bit
     reflection register (REFP0, REFP1).  A 0 written to these
     registers restores the original figure.
     Multiple copies of players as well as their sizes are
     controlled by writing 3 bits (D0, D1, D2) into the number-
     size registers (NUSIZ0, NUSIZ1).  These three bits select
     from 1 to 3 copies of the player, spacing of those copies,
     as well as the size of the player (each square of the
     player can be 1, 2, or 4 clocks wide).  Whenever multiple
     copies are selected, the TIA automatically creates the same
     number of copies of the missile for that player.  Again,
     the specifics of all this are laid out in the TIA hardware
     Vertical delay for the players works exactly like the ball
     by writing a 1 to D0 in the players' vertical delay
     registers (VDELP0, VDELP1).  Writing a 0 to these
     locations disables the vertical delay.
7.0 Horizontal Positioning
     The horizontal positions of objects are set by writing to
     their associated reset registers (RESP0, RESP1, RESM0, RESM1,
     RESBL) which are all strobe registers (they trigger their
     function as soon as they are addressed).  That causes the
     object to be positioned wherever  the electron bean was in
     its sweep across the screen when the register was reset.
     For example, if the electron beam was 60 color clocks into
     a scan line when RESP0 was written to, player 0 would be
     positioned 60 color clocks into the next scan line.
     Whether or not P0 is actually drawn on the screen is a
     function of the data in the GP0 register, but if it were
     drawn, it would show up at 60.  Resets to these registers
     anywhere during horizontal blanking will position objects
     at the left edge of the screen (color clock 0).  Since
     there are 3 color clocks per machine cycle, and it can take
     up to 5 machine cycles to write the register, the
     programmer is confined to positioning the objects at 15
     color clock intervals across the screen.  This coarse
     positioning is fine tuned by the Horizontal Motion,
     explained in section 8.0.
     Missiles have an additional positioning command.  Writing a
     1 to D1 of the reset missile-to-player register (RESMP0,
     RESMP1) disables that missile's graphics (turns it off) and
     repositions it horizontally to the center of its
     associated player.  Until a 0 is written to the register,
     the missile's horizontal position is locked to the center
     of its player in preparation to be fired again.
8.0 Horizontal Motion
     Horizontal motion allows the programmer to move any of the
     5 graphics objects relative to their current horizontal
     position.  Each object has a 4 bit horizontal motion
     register (HMP0, HMP1, HMM0, HMM1, HMBL) that can be loaded
     with a value in the range of +7 to -8 (negative values are
     expressed in twos complement from).  This motion is not
     executed until the HMOVE register is written to, at which
     time all motion registers move their respective objects.
     Objects can be moved repeatedly by simply executing HMOVE.
     Any object that is not to move must have a 0 in its motion
     register.  With the horizontal positioning command confined
     to positioning objects at 15 color clock intervals, the
     motion registers fills in the gaps by moving objects +7 to
     -8 color clocks.  Objects can not be placed at any color
     clock position across the screen.  All 5 motion registers
     can be set to zero simultaneously by writing to the
     horizontal motion clear register (HMCLR).
     There are timing constraints for the HMOVE command.  The
     HMOVE command must immediately follow a WSYNC (Wait for
     SYNC) to insure the HMOVE operation occurs during
     horizontal blanking.  This is to allow sufficient time for
     the motion registers to do their thing before the electron
     beam starts drawing the next scan line.  Also, for
     mysterious internal hardware considerations, the motion
     registers should not be modified for at least 24 machine
     cycles after an HMOVE command.
9.0 Object Priorities
     Each object is assigned a priority so when any two objects
     overlap the one with the highest priority will appear to
     move in front of the other.  To simplify hardware logic,
     the missiles have the same priority as their associated
     player, and the ball has the same priority as the
     playfield.  The background, of course, has the lowest
     priority.  The following table illustrates the normal
     (default) priority assignments.
               Priority            Objects
               1              P0, M0
               2              P1, M1
               3              BL, PF
               4              BK
     This priority assignment means that players and missiles
     will move in front of the playfield.  To make the players
     and missiles move behind the playfield, a "1" must be
     written to D2 of the CTRLPF register.  The following table
     illustrates how the priorities are affected:
               Priority            Objects
               1              PF, BL
               2              P0, M0
               3              P1, M1
               4              BK
     One more priority control is available to be used for
     displaying the score.  When a "1" is written to D1 of the
     CTRLPF register, the left half of the playfield takes on
     the color of player 0, and the right half the color of
     player 1.  The game score can now be displayed using the PF
     graphics register, and the score will be in the same color
     as its associated player.
10.0 Collisions
     The TIA detects collisions between any of the 6 objects it
     generates (the playfield and 5 moveable objects).  There
     are 15 possible two-object collisions which are stored in
     15 one bit latches.  Each collision register contains two
     of these latches which are read by the microprocessor on D6
     and D7 of the data bus for easy access.  A "1" on the data
     line indicates the collision it records has occurred.  The
     collision registers could be read at any time but is
     usually done during vertical blank after all possible
     collisions have occurred.  The collision registers are all
     reset simultaneously by writing to the collision reset
     register (CXCLR).
11.0 Sound
     There are two audio circuits for generating sound.  They
     are identical but completely independent and can be
     operated simultaneously to produce sound effects through
     the TV speaker.  Each audio circuit has three registers
     that control a noise-tone generator (what kind of sound), a
     frequency selection (high or low pitch of the sound), and a
     volume control.
  11.1 Tone
     The noise-tone generator is controlled by writing to the 4
     bit audio control registers (AUDC0, AUDC1).  The values
     written cause different kinds of sounds to be generated.
     Some are pure tones like a flute, others have various
     "noise" content like a rocket motor or explosion.  Even
     though the TIA hardware manual lists the sounds created by
     each value, some experimentation will be necessary to find
     "your sound".
  11.2 Frequency
      Frequency selection is controlled by writing to a 5 bit
     audio frequency register (AUDF0, AUDF1).  The value written
     is used to divide a 30KHz reference frequency creating
     higher or lower pitch of whatever type of sound is created
     by the noise-tone generator.  By combining the pure tones
     available from the noise-tone generator with frequency
     selection a wide range of tones can be generated.
  11.3 Volume
     Volume is controlled by writing to a 4 bit audio volume
     register (AUDV0, AUDV1).  Writing 0 to these registers
     turns sound off completely, and writing any value up to 15
     increases the volume accordingly.
12.0 Input Ports
     There are six input ports whose logic states can be read on
     D7 by reading the input port addresses (INPT0, thru INPT5).
     These six ports are divided into two types, "dumped" and
  12.1 Dumped Input Ports (INPT0 thru INPT3)
     These four ports are used to read up to four paddle
     controllers.  Each paddle controller contains an adjustable
     pot controlled by the knob on the controller.  The output
     of the pot is used to charge a capacitor in the console,
     and when the capacitor is charged the input port goes HI.
     The microprocessor discharges this capacitor by writing a
     "1" to D7 of VBLANK then measures the time it takes to
     detect a logic one at that port.  This information can be
     used to position objects on the screen based on the
     position of the knob on the paddle controller.
  12.2  Latched Input Ports (INPT4, INPT5)
     These two ports have latches that are both enabled by
     writing a "1" or disabled by writing a "0" to D6 of VBLANK.
     When disabled the microprocessor reads the logic level of
     the port directly.  When enabled, the latch is set for
     logic one and remains that way until its' port goes LO.
     When the port goes LO the latch goes LO and remains that
     way regardless of what the port does.  The trigger buttons
     of the joystick controllers connect to these ports.

THE PIA (6532)
1.0 General
     The PIA chip is an off-the-shelf 6532 Peripheral Interface
     Adaptor which has three functions:  a programmable timer,
     128 bytes of RAM, and two 8 bit parallel I/O ports.
2.0 Interval timer
     The PIA uses the same clock as the microprocessor so that
     one PIA cycle occurs for each machine cycle.  The PIA can
     be set for one of four different "intervals", where each
     interval is some multiple of the clock (and therefore
     machine cycles).  A value from 1 to 255 is loaded into the
     PIA which will be decremented by one at each interval.  The
     timer can now be read by the microprocessor to determine
     elapsed time for timing various software operations and
     keep them synchronized with the hardware (TIA chip).
  2.1 Setting the timer
     The timer is set by writing a value or count (from 1 to
     255) to the address of the desired interval setting
     according to the following table :
          Hex Address    Interval  Mnemonic
          294       1 clock        TIM1T
          295       8 clocks  TIM8T
          296       64 clocks TIM64T
          297       1024 clocks    T1024T
     For example, if the value of 100 were written to TIM64T
     (HEX address 296) the timer would decrement to 0 in 6400
     clocks (64 clocks per interval x 100 intervals) which would
     also be 6400 microprocessor machine cycles.
  2.2 Reading the timer
     The timer may be read any number of times after it is
     loaded of course, but the programmer is usually interested
     in whether or not the timer has reached 0.  The timer is
     read by reading INTIM at hex address 284.
  2.3 When the timer reaches zero
     The PIA decrements the value or count loaded into it once
     each interval until it reaches 0.  It holds that 0 counts
     for one interval, then the counter flips to FF(HEX) and
     decrements once each clock cycle, rather than once per
     interval.  The purpose of this feature is to allow the
     programmer to determine how long ago the timer zeroed out
     in the event the timer was read after it passed zero.
3.0 RAM
     The PIA has 128 bytes of RAM located in the Stella memory
     map from HEX address 80 to FF.  The microprocessor stack is
     normally located from FF on down, and variables are
     normally located from 80 on up (hoping the two never meet).
4.0 The I/O ports
     The two ports (Port A and Port B) are 8 bits wide and can
     be set for either input or output.  Port A is used to
     interface to various hand-held controllers but Port B is
     dedicated to reading the status of the Stella console
  4.1 Port B - Console Switches (read only)
     Port B is hardwired to be an input port only that is read
     by addressing SWCHB (HEX 282) to determine the status of
     all the console switches according to the following table:
          Data Bit  Switch         Bit Meaning
          D7        P1 difficulty  0 = amateur (B), 1 = pro (A)
          D6        P0 difficulty       0 = amateur (B), 1 = pro
          D5/D4          (not used)
          D3        color - B/W    0 = B/W, 1 = color
          D2        (not used)
          D1        game select    0 = switch pressed
          D0        game reset     0 = switch pressed
5.0 Port A - Hand Controllers
     Port A is under full software control to be configured as
     an input or an output port.  It can then be used to read or
     control various hand-head controllers with the data bits
     defined differently depending on the type of controller
  5.1 Setting for input or output
     Port A has an 8 bit wide Data Direction Register (DDR) that
     is written to at SWACNT (HEX 281) to set each individual
     pin of Port A to either input or output.  The Port A pins
     are labeled PA0 thru PA7, and writing a "0" to a pins' DDR
     bit sets that pin as input, and a "1" sets it as an output.
     For example, writing all 0's to SWACNT (the DDR for Port A)
     sets PA0 thru PA7 (all 8 pins of Port A) as inputs. If F0
     (11110000) were written to SWACNT then PA7, PA6, PA5 & PA4
     would be outputs, and PA3, PA2, PA1 & PA0 would be inputs.
  5.2 Inputting and Outputting
     Once the DDR has set the pins of Port A for input or output
     they may be read or written to by addressing SWCHA (HEX
  5.3 Joystick Controllers
     Two joysticks can be read by configuring the entire port as
     input and reading the data at SWCHA according to the
     following table:
               Data Bit  Direction Player
               D7        right          P0
               D6        left      P0
               D5        down      P0
               D4        up        P0
               D3        right          P1
               D2        left      P1
               D1        down      P1
               D0        up        P1
               (P0 = left player, P1 = right player)
     A "0" in a data bit indicates the joystick has been moved
     to close that switch.  All "1's" in a player's nibble
     indicates that joystick is not moving.
  5.4 Paddle (pot) controllers
     Only the paddle triggers are read from the PIA.  The
     paddles themselves are read at INP0 thru INPT3 of the TIA.
     The paddle triggers can be read at SWCHA according to the
     following table :
               Data Bit  Paddle #
               D7        P0
               D6        P1
               D5/D4          (not used)
               D3        P2
               D2        P3
               D1/D0          (not used)
  5.5 Keyboard controllers
     The keyboard controller has 12 buttons arranged into 4 rows
     and 3 columns.  A signal is sent to a row, then the columns
     are checked to see if a button is pushed, then the next row
     is signaled and all columns sensed, etc. until the entire
     keyboard is scanned and sensed.  The PIA sends the signals
     to the rows, and the columns are sensed by reading INPT0,
     INPT1, and INPT4 of the TIA.  With Port A configured as an
     output port, the data bits will send a signal to the
     keyboard controller rows according to the following table :
               Data Bit  Keyboard Row   Player
               D7        bottom         P0
               D6        third          P0
               D5        second         P0
               D4        top       P0
               D3        bottom         P1
               D2        third          P1
               D1        second         P1
               D0        top       P1
               (P0 = left player, P1 = right player)
     NOTE : a delay of 400 microseconds is necessary between
     writing to this port and reading the TIA input ports.
6.0 Address summary table
     Hex Address    Mnemonic  Purpose
     280       SWCHA     Port A; input or output  (read or
     281       SWACNT    Port A DDR, 0= input, 1=output
     282       SWCHB     Port B; console switches (read only)
     283       SWBCNT    Port B DDR (hardwired as input)
     284       INTIM          Timer output (read only)
     294       TIM1T          set 1 clock interval (838
     295       TIM8T          set 8 clock interval (6.7
     296       TIM64T         set 64 clock interval (53.6
     297       T1024T         set 1024 clock interval (858.2
     NOTE: one clock is also one microprocessor machine cycle

     1. The number of scan lines, and therefore the frame time
     increases from NTSC to PAL according to the following
                              NTSC                         PAL
                         scan micro          scan micro
                         lines     seconds   lines     seconds
               VBLANK    40   2548      48   3085
               KERNAL    192  12228          228  14656
               OVERSCAN  30   1910      36   2314
               FRAME          262  16686          312  20055
     2. Sounds will drop a little in pitch (frequency) because
     of a slower crystal clock.  Some sounds may need the
     AUDF0/AUDF1 touched up.
     3. PAL operates at 50 Hz compared to NTSC 60Hz, a 17%
     reduction.  If game play speed is based on frames per
     second, it will slow down by 17%.  This can be disastrous
     for most skill/action carts.  If the NTSC version is
     designed with 2 byte fractional addition techniques (or
     anything not based on frames per second) to move objects,
     then PAL conversion can be as simple as changing the
     fraction tables, avoiding major surgery on the program.
     1. SECAM is a little weird.  It takes the PAL software, but
     the console color/black & white switch is hardwired as
     black & white.  Therefore, it reads the PAL black & white
     tables in software and assigns a fixed color to each lum of
     black & white according to the following table:
                    Lum  Color
                    0    Black
                    2    Blue
                    4    Red
                    6    Magenta
                    8    green
                    A    cyan
                    C    yellow
                    E    white
     There is a trap here:  the manual is the same for NTSC,
     PAL, & SECAM.  This means that the descriptions for black &
     white must jive between NTSC & PAL.  If you make major
     changes to PAL black & white to achieve good SECAM color,
     NTSC black & white must be made similar.
     2. PAL sounds work fine on SECAM with one exception: when a
     sound is to be turned off, it must be one by setting
     AUDV0/AUDV1 to 0, not by setting AUDC0/AUDC1 to 0.
     Otherwise, you get an obnoxious background sound.

     The TIA is an MOS integrated circuit designed to interface
     between an eight (8) bit microprocessor and a television
     video modulator and to convert eight (8) bit parallel data
     into serial outputs for the color, luminosity, and
     composite sync required by a video modulator.
     This circuit operates on a line by line basis, always
     outputting the same information every television line
     unless new data is written into it by the microprocessor.
     A hardware sync counter produces horizontal sync timing
     independent of the microprocessor. Vertical sync timing is
     supplied to this circuit by the microprocessor and combined
     into composite sync.
     Horizontal position counters are used to trigger the serial
     output of five (5) horizontally movable objects; two
     players, two missiles and a ball. The microprocessor can
     add or subtract from these position counters to move these
     objects right or left.
     The microprocessor determines all vertical position and
     motion by writing zeros or ones into object registers
     before each appropriate horizontal line.
     Walls, clouds and other seldom moved objects are produced
     by a low resolution data register called the playfield
     A fifteen (15) bit collision register detects all fifteen
     possible two object collisions between these six (6)
     objects (five moveable and one playfield). This collision
     register can be read and reset by the microprocessor. Six
     input ports are also provided on this chip that can be read
     by the microprocessor. These input ports and the collision
     register are the only chip addresses that can be read by
     the microprocessor. All other addresses are write only.
     Color luminosity registers are included that can be
     programmed by the microprocessor with eight (8) luminosity
     and fifteen (15) color values. A digital phase shifter is
     included on this chip to provide a single color output with
     fifteen (15) phase angles.
     Two (2) independent audio generating circuits are included,
     each with programmable frequency, noise content, and volume
     control registers.
1. Data and addressing
     Registers on this chip are addressed by the microprocessor
     as part of its overall RAM-ROM memory space.  The attached
     table of read-write addresses summarizes the addressable
     functions.  There are no registers that are both read and
     write.  Some addresses however are both read and write,
     with write data going into one register and read data
     returning from a different register.
     If the read-write line is low, the data bits indicated in
     this table will be written into the addressed write
     location when the 02 clock goes from high to low.  Some
     registers are eight bits wide, some only one bit, and some
     (strobes) have no bits, performing only control functions
     (such as resets) when their address is written.
     If the read-write line is high, the addressed location can
     be read by the microprocessor on data lines 6 and 7 while
     the 02 clock is high.
     The addresses given in the table refer only to the six (6)
     real address lines.  If any of the four (4) chip select
     lines are used for addressing, the addresses must be
     modified accordingly.
2. Synchronization
  A. Horizontal Timing
     A hardware counter on this chip produces all horizontal
     timing (such as sync, blank, burst) independent of the
     microprocessor,  This counter is driven from an external
     3.58 Mhz oscillator and has a total count of 228.  Blank is
     decoded as 68 counts and sync and color burst as 16 counts.
  B. Vertical Timing
     There are one bit, addressable registers on this chip for
     vertical sync and vertical blank.  The timing for these
     functions is established by the microprocessor by writing
     zero or one into these bits. (VSYNC, VBLANK )
  C. Composite Sync
     Horizontal sync and the output of the vertical sync bit are
     combined together to produce composite sync.  This
     composite sync signal drives a chip output pad to an
     external composite video resistor network.
  D. Microprocessor Synchronization
     The 3.58 MHz oscillator also clocks a divide by three
     counter on this chip whose output (1.19 Mhz) is buffered to
     drive an output pad called 00.  This pad provides the input
     phase zero clock to the microprocessor which then produces
     the system 02 clock (1.19 Mhz).
     Software program loops require different lengths of time to
     run depending on branch decisions made within the program.
     Additional synchronization between the software and
     hardware. This is done with a one bit latch called WSYNC
     (wait for sync).  When the microprocessor finishes a
     routine such as loading registers for a horizontal line, or
     computing new vertical locations during vertical blank, it
     can address WSYNC, setting this latch high.  When this
     latch is high, it drives an output pad to zero connected to
     the microprocessor ready line (RDY). A zero on this line
     causes the microprocessor to halt and wait.  As shown in
     figure 2, WSYNC latch is automatically reset to zero by the
     leading edge of the next horizontal blank timing signal,
     releasing the RDY line, allowing the microprocessor to
     begin its computation and register writing for this
     horizontal television line or line pair.
3. Playfield graphics Register
  A. Description
     Objects such as walls, clouds, and score) which are not
     required to move, are written into a 20 bit register called
     the playfield register. This register (Figure 5) is loaded
     from the data bus by three separate write addresses (PF0,
     PFl, PF2).  Playfield may be loaded at any time. To clear
     the playfield, zeros must be written into all three
  B. Normal Serial Output
     The playfield register is automatically scanned (and
     converted to serial output) by a bi-directional shift
     register clocked at a rate which spreads the twenty (20)
     bits out over the left half of a horizontal line. This
     scanning is initiated by the end of horizontal blank (left
     edge of television screen).  Normally the same scan is then
     repeated, duplicating the same twenty (20) bit sequence
     over the right half of the horizontal line.
  C. Reflected Serial Output
     A reflected playfield may be requested by writing a one
     into bit zero of the playfield control register (CTRLPF).
     When this bit is true the scanning shift register will scan
     the opposite direction during the right half of the
     horizontal line, reversing the twenty (20) bit sequence.
  D. Timing Constraints
     Even though the playfield bytes (PF0, PFl, PF2) may be
     written to any time, if one of them is changed while being
     serially scanned, part of the new value may both show up on
     the television horizontal line.
4. Horizontal Position Counters
  A. Description
     The playfield is a fixed graphics register, always starting
     its serial output when triggered by the beginning of each
     television line.  This chip also includes five "moveable"
     graphics registers, whose serial outputs are triggered by
     five separate horizontal position counters every time these
     counters pass through zero count. These position counters
     are clocked continuously during the unblanked portion of
     every horizontal line and their count length is exactly
     equal to the normal number of clocks supplied to them
     during this time. They will therefore pass through zero at
     the same time during each horizontal television line and
     the triggered outputs will have no horizontal motion. A
     typical horizontal counter is shown in figure 4.
     If extra clocks are supplied to these counters (or normal
     clocks suppressed) the zero crossing time will shift and
     the object will have moved left (extra clocks) or right
     (fewer clocks).  Some position counters have extra decoders
     (in addition to a zero decode) to trigger multiple copies
     of the same object across a horizontal line.
     All position counters can be reset to zero count by the
     microprocessor at any time, by a write instruction to the
     reset addresses (RESBL, RESM0, RESMl, RESP0, RESPl). If
     reset occurs during horizontal blank, the object will
     appear at the left side of the television screen.  Properly
     timed resets may position an object at any horizontal
     location consistent with the microprocessor cycle time.
  B. Ball position Counter
     The ball position counter has only the zero crossing decode
     and therefore cannot trigger multiple copies of the ball
  C. Player Position Counters
     Each player position counter has three decodes in addition
     to the zero crossing decode.  These decodes are controlled
     by bits 0,1,2 of the number size control registers (NUSIZ0,
     NUSIZ1), and trigger 1,2, or 3 copies of the player (at
     various spacings) across a horizontal line as shown on page
     ___. These same control bits are used for the decodes on
     the missile position counter, insuring an equal number of
     players and missiles.
  D. Missile Position Counters
     Missile position counters are identical to player position
     counters except that they have another type of reset in
     addition to the previously discussed horizontal position
     reset.  These extra reset addresses (RESMP0, RESMP1) write
     data bit 1 into a one bit register whose output is used to
     position the missile (horizontally) directly on top of its
     corresponding player, and to disable the missile serial
5. Horizontal Motion Registers
  A. General Description
     There are five write only registers on this chip that
     contain the horizontal motion values for each of the five
     moving objects. A typical horizontal motion register is
     shown in figure 4 . The data bus (bits 4 through 7) is
     written into these addresses (HMP0, HMPl, HMM0, HMMl, HMBL)
     to load these registers with motion values. These registers
     supply extra (or fewer) clocks to the horizontal position
     counters only when commanded to do so by an HMOVE address
     from the microprocessor. These registers may all be cleared
     to zero (no motion) simultaneously by an HMCLR command from
     the microprocessor, or individually by loading zeros into
     each register.  These registers are each four bits in
     length and may be loaded with positive (left motion),
     negative (right motion) or zero (no motion) values.
     Negative values are represented in twos complement format.
  B. Timing constraints
     These registers may be loaded or cleared at almost any
     time. The motion values they contain will be used only when
     an HMOVE command is addressed, and then all five motion
     values will be used simultaneously into all five horizontal
     position counters once. The only timing constraint on this
     operation involves the HMOVE command.  The HMOVE command
     must be located in the microprocessor program immediately
     after a wait for sync (WSYNC) command. This assures that
     the HMOVE operation begins at the leading edge of
     horizontal blank, and has the full blank time to supply
     extra or fewer clocks to the horizontal position counters.
     These registers should not be modified for at least 24
     Computer cycles after the HMOVE command.
6. Moving Object Graphics Registers
  A. General Description
     There are five graphics registers for moving objects on
     this chip. These graphics registers are loaded (written) in
     parallel by the microprocessor and like the playfield
     register are scanned and converted to serial output.
     Unlike the playfield register, which is always scanned
     beginning at the left side of each horizontal line, moving
     object graphics registers are scanned only when triggered
     by a start decode from their horizontal position counter.
     A typical graphics register is shown in figure 4 .
  B. Missile Graphics
     The graphics registers for both missiles are identical and
     very simple. They each consist of a one bit register called
     missile enable (ENAM0, ENAM1). This graphics bit is scanned
     (outputted) only when triggered by its corresponding
     position counter. There are control bits (bits 4, 5, of
     NUSIZ0, NUSIZ1) that can stretch this single graphics bit
     out over widths of 1, 2, 4, or 8 clocks of horizontal line
     time. (A full line is 160 clocks).
  C. Player Graphics
     The graphics registers for both players are identical and
     are rather complex. They each consist of eight bit parallel
     registers (GRP0, GRP1) and a bi-directional parallel to
     serial scan counter that converts the parallel data into
     serial output.  A one bit control register (REFP0, REFP1)
     determines the direction (reflection) of the parallel to
     serial scan, outputing either D7 through D0, or D0 though
     D7.  This allows reflection (horizontal flipping) of player
     serial graphics data without having to flip the
     microprocessor data.
     The clock into the scan counter can be controlled (three
     bits of NUSIZ0 and NUSIZ1) to slow the scan rate and
     stretch the eight bits of serial graphics out over widths
     of 8, 16, or 32 clocks of horizontal line time. These same
     control bits are used in the player-missile motion counters
     to control multiple copies, so only three player widths (
     rates) are available.
  D. Vertical Delay
     Each of the player graphics registers actually consists of
     two 8 bit parallel registers.  The first (GRP0, GRP1) is
     loaded (written) from the microprocessor 8 bit data bus.
     The second is automatically loaded from the output of the
     first.  The reason for this is a complex subject called
     vertical delay.  A large amount of microprocessor time is
     required to generate player, missile and playfield graphics
     (table look up, masking, comparisons, etc.) and load these
     into this chip's registers.  For most game programs this
     time is just too large to fit into one horizontal line
     time. In fact for most games it will barely fit into two
     line times (127 microseconds). Therefore, individual
     graphics registers are loaded (written) every two lines,
     and used twice for serial output between loads.  This type
     of programing will obviously limit the vertical height
     resolution of objects to multiples of two lines. It also
     will limit the resolution of vertical motion to two lines
     jumps.  Nothing can be done about the vertical height
     resolution; however, vertical motion can be resolved to a
     single line by addition of a second graphics register that
     is automatically parallel loaded from the output of the
     first, one line time after the first was loaded from the
     data bus.  This second graphics register output is
     therefore always delayed vertically by one line. A control
     bit called vertical delay (VDEL0, VDEL1) selects which of
     these two registers is to be used for serial output. If
     this control bit is set by the microprocessor between
     picture frames, the object will be moved down (delayed) by
     one line during the next frame.  In most programming
     applications player 0 graphics and player 1 graphics are
     loaded (written) alternately, during the blank time just
     prior to each line as shown in (figure 1).  Since GRP0 and
     GRP1 addresses from the microprocessor alternate, they are
     delayed by one line from each other. The GRP0 address
     decode can therefore be used to load the delayed graphics
     register for player 1, and GRP1 likewise to load the
     delayed graphics register for player 0. The two vertical
     delay bits (VDEL0, VDELl) then select delayed or undelayed
     registers for player 0 and player 1 as serial outputs.
  E. Ball Graphics
     The ball graphics register is almost identical to the
     missile graphics register.  It also consists of a single
     enable bit (ENABL) whose output is triggered by the ball
     position counter.  It also has two control bits (bits 4, 5
     of CTRLPF) that can stretch this single graphics bit out
     over widths of 1, 2, 4, or 8 clocks of horizontal line
     time.  Unlike the missile graphics; however, the ball
     graphics register has capability for vertical delay similar
     to the player graphics.  A second graphics (enable) bit is
     alternately loaded from the output of the first, one line
     after the first was loaded from the data bus. A ball
     vertical delay bit (VDELBL) selects which of these two
     graphics bits is used for the ball serial output. The first
     graphics bit (ENABL) should be loaded during the same
     horizontal blank time as player 0 (GRP0), because GRP1 is
     used to load the second enable bit from the output of the
     first on alternate lines.
7. Collision Detection Latches
  A. Definitions
     The serial outputs from all the graphics registers
     represent real time horizontal location of objects on the
     television screen. If any of these outputs occur at the
     same time, they will overlap (collide) on the screen.
     There are six objects generated on this chip (five moving
     and playfield) allowing fifteen possible two object
     collisions. These overlaps (collisions) are detected by
     fifteen "and" gates whenever they occur, and are stored in
     fifteen individual latch register bits, as shown in figure
  B. Reading  Collision
     The microprocessor can read these fifteen collision bits on
     data lines 6 and 7 by addressing them two at a time. This
     could be done at any time but is usually done between
     frames (during vertical blank) after all possible
     collisions have serially occurred.
  C. Reset
     All collision bits are reset simultaneously by the
     microprocessor using the reset address CXCLR. This is
     usually done near the end of vertical blank, after
     collisions have been tested.
8. Input ports
  A. General Description
     There are 6 input ports on this chip whose logic state may
     be read on data line 7 with read addresses INPT0 through
     INPT5.  These 6 ports are divided into two types, "dumped"
     and "latched".  See Figure 8.
  B. Dumped  Input Ports (I0 through I3)
     These 4 input ports are normally used to read paddle
     position from an external potentiometer-capacitor circuit.
     In order to discharge these capacitors each of these input
     ports has a large transistor, which may be turned on
     (grounding the input ports) by writing into bit 7 of the
     register VBLANK. When this control bit is cleared the
     potentiometers begin to recharge the capacitors and the
     microprocessor measures the time required to detect a logic
     1 at each input port.
     As long as bit 7 of register VBLANK is zero, these four
     ports are general purpose high impedance input ports. When
     this bit is a 1 these ports are grounded.
  C. Latched Input ports (I4, I5)
     These two input ports have latches which can be enabled or
     disabled by writing into bit 6 of register VBLANK.
     When disabled, these latches are removed from the circuit
     completely and these ports become two general purpose input
     ports, whose present logic state can be read directly by
     the microprocessor.
     When enabled, these latches will store negative (zero logic
     level) signals appearing on these two input ports, and the
     input port addresses will read the latches instead of the
     input ports.
     When first enabled these latches will remain positive as
     long as the input ports remain positive (logic one). A zero
     input port signal will clear a latch value to zero, where
     it will remain (even after the port returns positive) until
     disabled. Both latches may be simultaneously disabled by
     writing a zero into bit 6 of register VBLANK.
8.5 Priority Encoder
  A. Purpose
     As discussed in the section on collisions, simultaneous
     serial outputs from the graphics registers represent
     overlap on the television screen.  In order to have color-
     luminosity values assigned to individual objects it is
     necessary to establish priorities between objects when
     overlapped. The priority encoder is shown in figure 3.
  B. Priority Assignment
     The lack of any objects results in a color-lum value called
     the background. The background (BK) has lowest priority and
     only appears when no objects  are outputing.  In order to
     simplify the logic, each missile is given the same color-
     lum value and priority as it's corresponding player (P0,
     M0) and the ball is given the same color-lum value and
     priority as the playfield (PF, BL).
     The following table illustrates the normal priority
                    Highest Priority    P0, M0
                    Second Highest P1, M1
                    Third Highest  PF, BL
                    Lowest Priority     BK
     Objects with higher priority will appear to move in front
     of objects with lower priority. Players will therefore move
     in front of playfield (clouds, walls, etc.).
  C. Priority Control
     There are two playfield control bits that affect priority,
     one called playfield priority (PFP) (bit 2 of CTRLPF) and
     one called score (bit 1 of CTRLPF).  When a one is written
     into the PFP bit the priority assignment is modified as
     shown below.
                    Highest Priority    PF, BL
                    Second Highest P0, M0
                    Third Highest  P1, M1
                    Lowest Priority     BK
     Players will then move behind playfield (clouds, wall,
     etc.).  When a one is written into the score control bit,
     the playfield is forced to take the color-lum of player 0
     in the left half of the screen and player 1 in the right
     half of the screen.  This is used when displaying score and
     identifies the score with the correct player.  The priority
     encoder produces 4 register select lines shown in figure 3)
     that are mutually exclusive.  These 4 lines select either
     background, player 0, player 1 or playfield, and only one
     of them can be true at a time.
9 Color Luminance Registers
  A. Description
     There are four registers (shown in figure 3) that contain
     color-lum codes. Four bits of color code and three its of
     luminance code may be written into each of these registers
     (COLUP0, COLUP1, COLUPF, COLUBK) by the microprocessor at
     any time. These codes (representing 16 color values and 8
     luminance values) are given in the Detailed Address List.
  B. Multiplexing
     The serial graphics output from all six objects is examined
     by the priority encoder which activates one of the four
     select lines into a 4 x 7 multiplexer. This multiplexer
     (shown in figure 3) then selects one of the four color-lum
     registers as a 7 line output.  Three of these lines are
     binary coded luminosity and go directly to chip output
     pads. The other four lines go to the color phase shifter.
10. Color Phase Shifter
     This portion of the chip (shown in figure 3) produces a
     reference color output (color burst) during horizontal
     blank and then during the unblanked portion of the line it
     produces a color output shifted in phase with respect to
     the color burst.  The amount of phase shift determines the
     color and is selected by the four color code lines from the
     Color-lum multiplexer.  Binary code 0 selects no color.
     Code 1 selects gold (same phase as color burst). Codes 2
     (0010) through 15 (1111) shift the phase from zero through
     almost 360 degrees allowing selection of 15 total colors
     around the television color wheel.
11. Audio Circuits
     Two audio circuits are incorporated on this chip.  They are
     identical and completely independent, although their
     outputs could be combined externally into one speaker.
     Each audio circuit consists of parts described below, and
     in figure 7.
  A. Frequency Select
     Clock pulses (at approximately 30 KHz) from the horizontal
     sync counter pass through a divide by N circuit which is
     controlled by the output code from a five bit frequency
     register (AUDF).  This register can be loaded (written) by
     the microprocessor at any time, and causes the 30 KHz
     clocks to be divided by 1 (code 00000) through 32 (code
     11111).  This produces pulses that are digitally adjustable
     from approximately 30 KHz to 1 KHz and are used to clock
     the noise-tone generator.
  B. Noise-Tone Generator
     This circuit contains a nine bit shift counter which may be
     controlled by the output code from a four bit audio control
     register(AUDC), and is clocked by the frequency select
     circuit.  The control register can be loaded by the
     microprocessor at any time, and selects different shift
     counter feedback taps and count lengths to produce a
     variety of noise and tone qualities.
  C. Volume Select
     The shift counter output is used to drive the audio output
     pad through four driver transistors that are graduated in
     size. Each transistor is twice as large as the previous one
     and is enable by one bit from the audio volume register
     (AUDV).  This audio volume register may be loaded by the
     microprocessor at any time.  As binary codes 0 through 15
     are loaded, the pad drive transistors are enabled in a
     binary sequence.  The shift counter output therefore can
     pull down on the audio output pad with 16 selectable
     impedance levels.
Figure 1. Vertical Delay
Figure 2. Synchronization
Figure 3. Color-Luminance

Figure 4. Typical Horizontal Motion Circuit
Figure 5. Playfield Graphics
Figure 6. Collision Detection
Figure 7. Audio Circuit
Figure 8. Input Ports

Figure 9. Game System

Write Address Detailed Functions
WSYNC (wait for sync)
     This address halts microporcessor by clearing RDY latch to
     zero.  RDY is set true again by the leading edge of
     horizontal blank.
     Data bits not used
RSYNC (reset sync)
     This address resets the horizontal sync counter to define
     the beginning of horizontal blank time, and is used in chip
     Data bits not used
     This address controls vertical sync time by writing D1 into
     the VSYNC latch
     D1 [1 = start vert sync, 0 = stop vertical sync]
     This address controls vertical blank and the latches and
     dumping transistors on the input ports by writing into bits
     D7, D6 and D1 of the VBLANK register.
     D1 [ 1 = start vert. blank, 0 = stop vert. blank]
     D6 [ 1 = Enable I4 I5 latches, 0 = disable I4 I5 latches]
     D7 [ 1 = dump I6I1I2I3 ports to ground, 0 = remove dump
     path to ground]
     Note : Disable latches (D6 = 0) also resets latches to
     logic true
PJ0 (PF1, PF2)
     These addresses are used to write into playfield registers
          1 horizontal line ( 160 clocks)
                                        Reflect Control
      PF0  PF1        PF2      PF0      PF1           PF2
      PF0   PF1            PF2          PF2           PF1
     each bit = 4 clocks
     This address is uded to write into the playfield control
     register (a logic 1 causes action as described below)
     D0 = REF (reflect playfield)
     D1 = SCORE (left half of playfield gets color of player 0,
     right half gets color of player 1)
     D2 = PFP (playfield gets priority over players so they can
     move behind the playfield)
     D4 & D5 = BALL SIZE
               D5   D4   Width
               0    0    1 clock
               0    1    2 clocks
               1    0    4 clocks
               1    1    8 clocks
     These addresses control the number and size of players and
     Missile Size   D5   D4   Width
               0    0    1 clock
               0    1    2 clocks
               1    0    4 clocks
               1    1    8 clocks
               Player-Missile number & player size
                    1/2 television line (80 clocks)
                            8 clocks per square

     These addresses are used to reset players, missiles and the
     ball.  The object will begin its serial graphics at the
     time of a horizontal line at which the reset address
     No data bits are used
     These addresses are used to reset the hoiz. location of a
     missile to the center of it’s corresponding player.  As
     long as this control bit is true (1) the missile will
     remain locked to the center of it’s player and the missile
     graphics will be siddabled.  When a zero is written into
     this location, the missile is enabled, and can be moved
     independently from the player.
     D1 = RESMP (missile-player reset)
     This address causes the horizontal motion register values
     to be acted upon during the horizontal blank time in which
     it occurs.  It must occur at the beginning of horiz. blank
     in order to allow time for generation of extra clock pulses
     into the horizontal position counters if motion is desired
     this command must immediately follow a WSYNC command in the
     No data bits are used
     This address clears all horizontal motion registers to zero
     (no motion)
     No data bits are used
     These addresses write data (horizontal motion values) into
     the horizontal motion registers.  These registers will
     cause horizontal motion only when commanded to do so by the
     horiz. move command HMOVE.
     The motion values are coded as shown below :

      D7 D6  D5 D4                
      0  1   1  1   +7   
      0  1   1  0   +6   
      0  1   0  1   +5   Move left
      0  1   0  0   +4   indicated number
      0  0   1  1   +3   of clocks
      0  0   1  0   +2   
      0  0   0  1   +1   
      0  0   0  0   0    No Motion
      1  1   1  1   -1   
      1  1   1  0   -2   
      1  1   0  1   -3   
      1  1   0  0   -4   move right
      1  0   1  1   -5   indicated number
      1  0   1  0   -6   of clocks
      1  0   0  1   -7   
      1  0   0  0   -8   
     WARNING : These motion registers should not be modified
     during the 24 computer cycles immediately following an
     HMOVE command.  Unpredictable motion values may result.
     These addresses write D1 into the 1 bit missile or ball
     graphics registers.
     D1 - [0 = dissables object, 1 = enables object]
     These addresses write data into the player graphics
     Note: serial output begins with D7, unless REFP0 (REFP1) =
     These addesses write D3 into the 1 bit player reflect
     D3 - [ 0 = no reflect, D7 of GRP on left, 1 = reflect, D0
     of GRP on left]
     These addresses write D0 into the 1 bit vertical delay
     registers, to delay players or ball by one vertical line.
     D0 - [0 = no delay, 1 = delay]
     This adderess clears all collision latches to zero (no
     No data bits are used
     These addresses write data into the player, playfield, adn
     background color-luminance registers
      COLOR       D7  D6 D5  D4 D3  D2 D1     LUM
      grey - gold 0   0  0   0  0   0  0      black
                  0   0  0   1  0   0  1      dark grey
      orange, brt-0   0  1   0  0   1  0      
                  0   0  1   1  0   1  1      grey
      pink -      0   1  0   0  1   0  0      
                  0   1  0   1  1   0  1      
      purp-blue,  0   1  1   0  1   1  0      light gret
                  0   1  1   1  1   1  1      white
      blue - lt.  1   0  0   0                
                  1   0  0   1                
      torq. -     1   0  1   0                
      grn. blue
                  1   0  1   1                
      grn. - yel. 1   1  0   0                
                  1   1  0   1                
      org. grn -  1   1  1   0                
      lt org.
                  1   1  1   1                
     These addresses write data into the audio frequency divider
                D4  D3 D2  D1 D0  30KHz divided
                0   0  0   0  0   no division
                0   0  0   0  1   divide by 2
                0   0  0   1  0   divide by 3
                ..  .. ..  .. ..  ...
                .   .  .   .  .
                1   1  1   1  0   divide by 31
                1   1  1   1  1   divide by 32
     These addresses write data into the audio control registers
     which control the noise content and additional division of
     the audio output.
                    D3 D2  D1 D0  Type of noise
                                  or division
                    0  0   0  0   set to 1
                    0  0   0  1   4 bit poly
                    0  0   1  0   div 15 -> 4
                                  bit poly
                    0  0   1  1   5 bit poly ->
                                  4 bit poly
                    0  1   0  0   div 2 : pure
                    0  1   0  1   div 2 : pure
                    0  1   1  0   div 31 : pure
                    0  1   1  1   5 bit poly ->
                                  div 2
                    1  0   0  0   9 bit poly
                                  (white noise)
                    1  0   0  1   5 bit poly
                    1  0   1  0   div 31 : pure
                    1  0   1  1   set last 4
                                  bits to 1
                    1  1   0  0   div 6 : pure
                    1  1   0  1   div 6 : pure
                    1  1   1  0   div 93 : pure
                    1  1   1  1   5 bit poly div
     These addresses write data into the audio volume registers
     which set the pull down impedance driving the audio output
                    D3 D2  D1 D0  Audio Output
                                  Pull down
                    0  0   0  0   No output
                    0  0   0  1   lowest
                    0  0   1  0   
                    .. ..  .. ..  
                    .  .   .  .
                    1  1   1  0   
                    1  1   1  1   highest
      6 bit   Address   7 6  5 4  3 2  1 0  Function
      addres  Name
      00      VSYNC                    1    vertical sync set-clear
      01      VBLANK    1 1            1    vertical blank set-
      02      WSYNC       s  t r  o b  e    wait for leading edge
                                            of horizontal blank
      03      RSYNC       s  t r  o b  e    reset horizontal sync
      04      NUSIZ0         1 1  1 1  1 1  number-size player-
                                            missile 0
      05      NUSIZ1         1 1  1 1  1 1  number-size player-
                                            missile 1
      06      COLUP0    1 1  1 1  1 1  1    color-lum player 0
      07      COLUP1    1 1  1 1  1 1  1    color-lum player 1
      08      COLUPF    1 1  1 1  1 1  1    color-lum playfield
      09      COLUBK    1 1  1 1  1 1  1    color-lum background
      0A      CTRLPF         1 1    1  1 1  control playfield ball
                                            size & collisions
      0B      REFP0               1         reflect player 0
      0C      REFP1               1         reflect player 1
      0D      PF0       1 1  1 1            playfield register byte
      0E      PF1       1 1  1 1  1 1  1 1  playfield register byte
      0F      PF2       1 1  1 1  1 1  1 1  playfield register byte
      10      RESP0       s  t r  o b  e    reset player 0
      11      RESP1       s  t r  o b  e    reset player 1
      12      RESM0       s  t r  o b  e    reset missile 0
      13      RESM1       s  t r  o b  e    reset missile 1
      14      RESBL       s  t r  o b  e    reset ball
      15      AUDC0               1 1  1 1  audio control 0
      16      AUDC1            1  1 1  1 1  audio control 1
      17      AUDF0            1  1 1  1 1  audio frequency 0
      18      AUDF1               1 1  1 1  audio frequency 1
      19      AUDV0               1 1  1 1  audio volume 0
      1A      AUDV1               1 1  1 1  audio volume 1
      1B      GRP0      1 1  1 1  1 1  1 1  graphics player 0
      1C      GRP1      1 1  1 1  1 1  1 1  graphics player 1
      1D      ENAM0                    1    graphics (enable)
                                            missile 0
      1E      ENAM1                    1    graphics (enable)
                                            missile 1
      1F      ENABL                    1    graphics (enable) ball
      20      HMP0      1 1  1 1            horizontal motion
                                            player 0
      21      HMP1      1 1  1 1            horizontal motion
                                            player 1
      22      HMM0      1 1  1 1            horizontal motion
                                            missile 0
      23      HMM1      1 1  1 1            horizontal motion
                                            missile 1
      24      HMBL      1 1  1 1            horizontal motion ball
      25      VDELP0                     1  vertical delay player 0
      26      VDEL01                     1  vertical delay player 1
      27      VDELBL                     1  vertical delay ball
      28      RESMP0                   1    reset missile 0 to
                                            player 0
      29      RESMP1                   1    reset missile 1 to
                                            player 1
      2A      HMOVE       s  t r  o b  e    apply horizontal motion
      2B      HMCLR       s  t r  o b  e    clear horizontal motion
      2C      CXCLR       s  t r  o b  e    clear collision latches
      6 bit   Address   7 6  5 4  3 2  1 0  Function
      addres  Name                          
      s                                     D7             D6
      0       CXM0P     1 1                 read       MO P1  M0 P0
      1       CXM1P     1 1                 read       M1 P0  M1 P1
      2       CXP0FB    1 1                 read       P0 PF  P0 BL
      3       CXP1FB    1 1                 read       P1 PF  P1 BL
      4       CXM0FB    1 1                 read       M0 PF  M0 BL
      5       CXM1FB    1 1                 read       M1 PF  M1 BL
      6       CXBLPF    1                   read       BL PF  unuse
                                            collision         d
      7       CXPPMM    1 1                 read       P0 P1  M0 M1
      8       INPT0     1                   read pot port
      9       INPT1     1                   read pot port
      A       INPT2     1                   read pot port
      B       INPT3     1                   read pot port
      C       INPT4     1                   read input
      D       INPT5     1                   read input
                                            Note : I0, I2, I2, I3
                                            can be grounded under
                                            software control.
                                            I4, I5 can be converted
                                            to latched inputs under
                                            software control


RSYNC, RES0O, H01, H02, SHB, 02, 0O
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