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Atari 7800 Software Guide

7800 SOFTWARE GUIDE


INTRODUCTION

The 7800 is a product which combines the ATARI 2600 hardware with
a new graphics chip called MARIA. The entire 2600 library of
cartridges will run on the 7800 as they do on the 2600, but new
cartridges designed to access the improved hardware will be able
to take advantage of a large number of improvements.

OVERVIEW OF 7800

Ignoring the 2600 environment, which is identical to the ATARI
2600, the 7800 environment is characterized by the following:

     (2)  6116's    -    4K bytes of RAM.
          6532      -    I/O.
          TIA       -    sounds, some input ports.
          Expanded cartridge slot.
          SALLY (6502) - microprocessor running at 1.79 MHz.
          MARIA     -    all video.

Additionally, there is a protection circuit which verifies that
each cartridge has the correct encrypted data before enabling
7800 mode. Encryption will be covered in another document, but
see Appendix 1, 7800 Memory Map, for information about reserving
space for encryption.

6116's

There are two (2) 6116 2Kx8 RAM chips on the 7800 PC board.
Together they occupy addresses x '1800' to x'27FF'. They are
also partly accessible (shadowed) at addresses x '0040' - x
'00ff' and x '0140' - x '00FF' to extend zero page (quick access)
RAM and first page (stack) RAM. Refer to the memory map appendix
for further information.

6532

This chip is used only for I/O in 7800 mode, whereas in 2600 mode
it also supplies all RAM and timers. Its functions are more
limited because its speed is not fast enough for normal
operation. Any access to this chip (joystick and switch I/O)
will cause the microprocessor to slow to 1.19 MHz. The ports and
switches connected through the 6532 are: joysticks (directional),
pause, game select, game reset, and difficulty switches. The
64532 can be used to generate output through the joystick ports
as well. For address information on 6532 ports and switches,
refer to Appendix 2, Standard 7800 Equates.

TIA

The TIA is only partly accessible in 7800 mode. While it occupies
addresses x'0000' - x'003F' in 2600 mode, only the section at
x'0000' - x'001F' is available in 7800 mode. The only
significant (useable) registers of these are the sound related
registers and the input ports (fire buttons, paddle controllers).
Any access to the TIA will cause the processor to slow from 1.79
MHz to 1.19 MHz.

CARTRIDGE SLOT

The cartridge slot is larger for 7800 mode cartridges. The
additional lines are: three (3) address lines (now all 16 address
lines appear on the cartridge connector); the READ/WRITE line, so
that RAM may be added to any cartridge very simply; the phase 2
clock line in order to add another microprocessor on the
cartridge and have it synchronized with the existing Sally chip;
an audio line so that one may mix in audio signals generated on
the cartridge; a composite video line, so that external video
signals may be included; and the HALT line, to enable the
cartridge to distinguish MARIA ROM accesses from SALLY ROM
accesses.

SALLY (6502)

This is the microprocessor, which is also used in the ATARI 5200.
The only thing special about the Sally chip is that is has a HALT
line, which allows the functionality described above.

MARIA

This is the custom chip which is the heart of the 7800. It
handles all graphics and video including the VSYNC and VBLANK
signals.

OVERVIEW OF MARIA

GRAPHICS

MARIA does not employ the concepts of players, missiles, and
playfield, as do the 2600 and 5200. Instead MARIA uses an
approach to graphics commonly used in coin-operated games. Each
raster of the display may be thought of as a bit map. This map
is contained in an area of the MARIA chip called the Line RAM.
Information is first stored into the Line RAM, then later read
from Line RAM and displayed on the screen.

Consider for a moment just one raster of display. One would
compose this raster's graphics by storing data into Line RAM.
This is done by specifying what data should be put at what
horizontal location. Graphics may be specified in small pieces,
and overlapped. The order in which pieces of a raster are
specified determines object priority with the last object
specified on top.

When graphics data is specified to be stored into Line RAM, it
will reference any one of eight (8) color palettes. Each pixel
of data will take on any one of three (3) colors from the
specified palette, or may be turned off (transparent). Again,
the Line RAM contains only one raster of graphics information.
There are actually two Line RAM buffers. While one is being read
(displayed), the other is being written for display the next
raster. This means that the construction of graphics for a
raster may take as long as, but no longer than, one raster, and
that graphics must be stored into Line RAM on a raster by raster
basis.

The only limit to the number, and size of objects on one scan
line is tha mount of time it takes to loan each into Line RAM, as
all loading must occur during one scan line.

DISPLAY

There are a total of 262 rasters per frame (1/60th second). The
"visible" screen (during which MARIA attempts display) starts on
raster 16 and ends on raster 258. The area found visible on all
television sets starts on raster 41 and ends on raster 232, 192
scan lines later. Any display outside this area may not appear
on all televisions. See Appendix 4, Frame Timing, for more
details.

Display is accomplished automatically by MARIA and consists of
two tasks: constructing the Line RAM, and displaying the
graphics. These happen simultaneously in MARIA. Construction of
Line RAM is automatically initiated every raster by MARIA, and is
directed by a predefined list of instructions called the Display
List. Line RAM construction occurs through a process called DMA
(Direct Memory Access). This means that the 6502 (SALLY)
processing is suspended while MARIA comes in and interrogates the
RAM and ROM for Display List and graphics information. DMA will
occur every "visible" scan line and lasts no longer than one scan
line. Because the Line RAM being constructed is displayed on the
following scan line, MARIA will read each Display List one line
before it is actually displayed. All Line RAM is cleared on a
line by line basis and BACKGRND color will be displayed if no
data is written.

Display List

DMA is mainly concerned with reading the Display List. This is a
list of instructions for where to find graphics data, where to
put it on the screen, and other details for constructing a scan
line. The Display List is made up of many "headers." Most
headers are four (4) bytes long (the exeption is discussed
later). If the second byte of a header is zero, it indicates the
end of the Display List, and DMA will stop allowing the 6502 to
continue processing. The format of the header is as follows:


          _______________________________________________
         |     |     |     |     |     |     |     |     |
         | A7  | A6  | A5  | A4  | A3  | A2  | A1  | A0  |
         |_____|_____|_____|_____|_____|_____|_____|_____|
         |     |     |     |     |     |     |     |     |
         | P2  | P1  | P0  | W4  | W3  | W2  | W1  | W0  |
         |_____|_____|_____|_____|_____|_____|_____|_____|
         |     |     |     |     |     |     |     |     |
         | A15 | A14 | A13 | A12 | A11 | A10 | A9  | A8  |
         |_____|_____|_____|_____|_____|_____|_____|_____|
         |     |     |     |     |     |     |     |     |
         | H7  | H6  | H5  | H4  | H3  | H2  | H1  | H0  |
         |_____|_____|_____|_____|_____|_____|_____|_____|

                                 or

          _______________________________________________
         |                                               |
         |            L O W    A D D R E S S             |
         |_____|_____|_____|_____|_____|_____|_____|_____|
         |                 |                             |
         |  P A L E T T E  |             W I D T H       |
         |_____|_____|_____|_____|_____|_____|_____|_____|
         |                                               |
         |          H I G H    A D D R E S S             |
         |_____|_____|_____|_____|_____|_____|_____|_____|
         |                                               |
         |     H O R I Z O N T A L    P O S I T I O N    |
         |_____|_____|_____|_____|_____|_____|_____|_____|


         where:

         ADDRESS  (A15-A0)    - Address of graphics information.
         PALETTE  (P2-P0)     - Refers to color palettes 0-7.
         WIDTH    (W4-W0)     - 2's complement of width.
                                Specifies number of bytes of
                                graphics data to fetch: values
                                1-31.

         HORIZONTAL
         POSITION (H7-H0)     - X location on the screen where
                                left edge of graphics is to be
                                placed.
                                  0-159 => Visible
                                160-255 => Not visible.
                                Wrap around occurs at 255/0
                                boundary.


Each header is concerned with one graphics item, which can be any
width. If ten objects should appear on a scan line, the Display
List for that scan line would be ten (10) headers long, followed
by two (2) bytes, the first of which is ignored, and the second
of which should be zero to end DMA.

A Display List may cross only one page boundary, so it can be no
more than 512 bytes long. Additionally, Display Lists must be in
RAM, due to the required access time.

Display List List

MARIA locates the Display Lists by reading a Display List List
(referred to as DLL from now on). This list is a series of 3
byte entries. Each entry points to a Display List. Included in
each entry is a value called OFFSET, which indicates how many
rasters should use the specified Display List. OFFSET is
decremented at the end of each raster until it becomes negative,
which indicates that the next DLL entry should now be read and
used. Each time graphics data is to be fetched OFFSET is added
to the specified High address byte, to determine the acrual
address where the data should be found. This allows one display
list to specify many rasters of graphics. Without OFFSET the
only approach to graphics is to have a Display List for each
raster, and a DLL for each Display List. Not only would this use
a lot of RAM, but it would also take quite a bit of processing
time to manipulate these Display Lists when objects move.
Because OFFSET is added to HIGH address byte, each raster of
graphics for an object must be seperated by x'100' bytes, or one
page.

The group of rasters specified by one DLL entry is called a
"zone." Again, the number of rasters in a zone equals OFFSET+1.
Larger zones mean less RAM is needed for DLLs, Display Lists and
Character Maps (see DMA MODES below). But upon consideration of
how to use zones, you will realize that to achieve smooth
vertical motion each stamp must be padded at top and bottom with
zeros. For example, if the top raster of an object is to appear
on the last line of a 16 high zone, it must have 15 lines of
zeros above it. If that object is 8 pixels (2 bytes) wide, and
its top line of data is located at x'CF04', then you
will need two bytes of zeros at x'D004', x'D104', x'D304',...,
and x'DE04' (remember that OFFSET decrements). As this can add
up to many pages of zeros, you can specify that MARIA should
interpret certain data as zeros, even if it isn't. This is
called "Holey DMA" because DMA will see "holes" in the data that
aren't really there. This can be enabled and disabled on a zone
by zone basis via a DLL entry. Holey DMA has been aimed at 8 or
16 raster zones, but will have the same effect for other zone
sizes. MARIA can be told to interpret odd 4K blocks as zero,
for 16 high zones, or odd 2K blocks as zeros for 8 high zones.
This will only work for addresses above x '8000'. This means
that these blocks can hold meaningful code, or tables, or
graphics data used in a zone where Holey DMA is not on.

One of the bits of a DLL entry tells MARIA to generate a Display
List Interrupt (DLI) for that zone. The interrupt will actually
occur following DMA on the last line of the PREVIOUS zone. This
interrupt is non-maskable, and causes the processor to go to the
address specified by the NMI vector at x 'FFFA' and x 'FFFB'.
This interrupt in no way affects DMA, so processing will still be
suspended at the beginning of the next raster.

The format of a 3 byte DLL entry is as follows:

     _______________________________________________
    |     |     |     |     |                       |
    | DLI | H16 | H8  | 0   |      O F F S E T      |
    |_____|_____|_____|_____|_____|_____|_____|_____|
    |                                               |
    |        H I G H    D L    A D D R E S S        |
    |_____|_____|_____|_____|_____|_____|_____|_____|
    |                                               |
    |         L O W    D L    A D D R E S S         |
    |_____|_____|_____|_____|_____|_____|_____|_____|

      where:

      DLI           -    Display List Interrupt flag.
                         0 => No DLI.
                         1 => Interrupt after DMA on last line
                              of previous zone.

      H16           -    16 high zone Holey DMA enable.
                         0 => Not enabled.
                         1 => Enabled. DMA interprets odd 4K
                         blocks as zeros. (A12 high => data=0)

      H8            -    8 high zone Holey DMA enable.
                         0 => Not enabled.
                         1 => Enabled. DMA interprets odd 2K
                         blocks as zeros. (A11 high => data=0)

      OFFSET        -    OFFSET starting value.
                         4 bits only.

      DL ADDRESS    -    Address of Display List for this zone.

A Display List List may cross only one page boundary, so it can
be no more than 512 bytes long. Additionally, Display List Lists
must be in RAM, due to the required access time.

MODES

DMA Modes

There are two modes for specifying graphics data. The first,
called Direct mode, is what has just been explained, where a
Header (in the Display List) points directly to graphics data.
The other mode is called Indirect or Character mode, and is
somewhat different in that the Header points to a Character Map,
which in turn points to graphics data. Indirect mode is selected
by every header that requires it via an extended (5 byte long)
header. The format of this header is as follows:

          _______________________________________________
         |     |     |     |     |     |     |     |     |
         | A7  | A6  | A5  | A4  | A3  | A2  | A1  | A0  |
         |_____|_____|_____|_____|_____|_____|_____|_____|
         |     |     |     |     |     |     |     |     |
         | WM  |  1  | IND | 0   | 0   | 0   | 0   | 0   |
         |_____|_____|_____|_____|_____|_____|_____|_____|
         |     |     |     |     |     |     |     |     |
         | A15 | A14 | A13 | A12 | A11 | A10 | A9  | A8  |
         |_____|_____|_____|_____|_____|_____|_____|_____|
         |     |     |     |     |     |     |     |     |
         | P2  | P1  | P0  | W4  | W3  | W2  | W1  | W0  |
         |_____|_____|_____|_____|_____|_____|_____|_____|
         |     |     |     |     |     |     |     |     |
         | H7  | H6  | H5  | H4  | H3  | H2  | H1  | H0  |
         |_____|_____|_____|_____|_____|_____|_____|_____|

                                 or

          _______________________________________________
         |                                               |
         |            L O W    A D D R E S S             |
         |_____|_____|_____|_____|_____|_____|_____|_____|
         |                                               |
         |               M O D E    B Y T E              |
         |_____|_____|_____|_____|_____|_____|_____|_____|
         |                                               |
         |          H I G H    A D D R E S S             |
         |_____|_____|_____|_____|_____|_____|_____|_____|
         |                 |                             |
         |  P A L E T T E  |             W I D T H       |
         |_____|_____|_____|_____|_____|_____|_____|_____|
         |                                               |
         |     H O R I Z O N T A L    P O S I T I O N    |
         |_____|_____|_____|_____|_____|_____|_____|_____|

         where:

         ADDRESS  (A15-A0)    - Address of graphics information.
         MODE BYTE: WM        - Write mode bit.
                                0 => 160x2 or 320x1
                                1 => 160x4 or 320x2
         IND                  - 0 => Direct mode.
                              - 1 => Indirect mode.
         PALETTE  (P2-P0)     - Refers to color palettes 0-7.
         WIDTH    (W4-W0)     - 2's complement of width.
                                Specifies number of bytes of
                                graphics data to fetch: values
                                1-32.

         HORIZONTAL
         POSITION (H7-H0)     - X location on the screen where left
                                edge of graphics is to be placed.
                                  0-159 => Visible
                                160-255 => Not visible.
                                Wrap around occurs at 255/0
                                boundary.

There is an added bonus to five byte headers. Because the end of
DMA is indicated by the presence of a zero in the second byte of
a header, and in a five byte header the width byte is not the
second but the fourth, a width of zero is valid in an extended
header, and will be interpreted as a value of 32.

Indirect mode, when selected, only lasts as long as the
corresponding header is being processed. MARIA will return to
Direct mode before the next header is read.

In indirect mode, the width indicates how many Character Map
references to make, where each Character Map entry points to one
byte of graphics data (the Character Map can point to two (2)
consecutive bytes of graphics; see CTRL under REGISTERS). The
idea behind Character (Indirect) mode is to specify a great
amount of graphics with only one Header. The graphics start at
the horizontal location specified by the Header and each
character (graphics referred to by one Character Map entry) is
inserted to the right of the previous one. One Character may be
changed without affecting the others by altering the Character
Map entry corresponding to that character. This is ideally
suited for backgrounds such as the maze and dots in Ms. Pacman.

The Character Map is composed by W entries, where W is the
specified width and each entry is one byte long. Each entry is a
Low address byte of a character, and the High address byte is
specified by the Character Base register (see CHARBASE under
REGISTERS). This means that each character on a scan line must
have the same high address byte (sit on the same 256 byte page).

Display Modes

The normal display mode is 160 mode, where the screen is divided
into 160 pixels horizontally. Typically graphics are done in
160x2 mode, where there are two color bits specified for each
pixel, and these two color bits refer to one of the eight
palettes. Alternately, one may specify graphics in 160x4 mode,
where there are four color bits per pixel. In this mode, each
byte of graphics data would specify only two (2) pixels of
graphics. If higher resolution is preferred, 320x1 mode is the
common choice, where the screen is divided into 320 pixels
horizontally and each pixel has one color bit. A more colorful
320x2 mode is also available with two color bits per pixel.

Selection of a particular mode is accomplished through two
separate operations: specification of WRITE MODE, and
specification of READ MODE. WRITE MODE is specified via the WM
bit of an extended (5 byte) header, as described above. READ
MODE is specified via the CTRL register. Both of these
specifications will remain in effect until respecified. WRITE
MODE is not initialized by MARIA on power-up, and must be
initialized by the cartridge before any display occurs. The
reason for specifying WRITE MODE via an extended header, is to
allow the programmer to change for 160x2 to 160x4 (or from 320x2
to 320x1, or vice-versa) during the DMA for a particular scan
line. For more information about modes see CTRL under REGISTERS.

REGISTERS

The location of the MARIA registers which control the display is
shown in Appendix 1, 7800 Memory Map.

Palettes

The palette registers are used to specify colors for the
graphics. There are eight palettes, and each contains three
colors. The colors themselves are specified in the form:

           _______________________________________________
          |     |     |     |     |     |     |     |     |
          | C3  | C2  | C1  | C0  | L3  | L2  | L1  | L0  |
          |_____|_____|_____|_____|_____|_____|_____|_____|

where C3-C0 is the color, and L3-L0 is the luminosity, for a
total of 256 different hues.

The palette registers are labeled P0C1, P0C2, P0C3, P1C1, P1C2,
P1C3, P2C1, P2C2, P2C3,... P7C1, P7C2, P7C3. A pixel whose two
color bits are "10" and which refers to palette three (3) would
be colored based on the value in P3C2. Color zero of any palette
is transparent. Additionally, there is a register called
BACKGRND used to specify background color. All the palettes and
BACKGRND are READ/WRITE, but they must be read using
"Absolute,index" addressing of the 6502.

OFFSET

The OFFSET register is a 4 bit value which gets added,
automatically , to the high address byte on any graphics data
fetch, whether Direct or Indirect. This register is internal to
MARIA, and is set by each Display List List entry.

In a previous incarnation, the OFFSET register occupied a memory
address. This address is now vacant, but you should STORE ZERO
THERE ON POWER-UP TO ALLOW FOR FUTURE EXPANSION.

CHARBASE

the CHARBASE register serves to specify the high address for any
graphics data fetch in Character (Indirect) mode. As you recall,
The Character Map (pointed to by the Header in the Display List)
specifies the low address bytes of graphics data. Each of these
low address bytes is concatenated with the sum of CHARBASE +
OFFSET, to give the full 16 bit addresses of where the graphics
data should be found, The CHARBASE register is WRITE ONLY.

DPPH

DPPH stands for Display List Pointer Pointer High, and this is
the register which contains the high address byte of the Display
List List. This register is WRITE ONLY. The Display List List
may cross one page boundary, in which case DPPH is internally
incremented, then reset at the end of the visible screen, so it
is valid for the next frame. This register (and DPPL) should be
written to before DMA is turned on. Once DMA is on, DPPH and
DPPL may be written at any time, as they are only read at the
beginning of the screen.

DPPL

This register is used to specify the low address byte of the
Display List List. It, too, is WRITE ONLY.

MSTAT

MSTAT is a READ ONLY register which communicates the status of
Vertical Blank via bit 7 (MSB). When this bit is 1 VBLANK is on.
When VBLANK turns off, DMA will begin according to your Display
List. This transition occurs at raster 16 of the frame.

CTRL

The CTRL register is a WRITE ONLY register used to control many
of the modes of MARIA. Through this register one can control
whether the background color extends off the edge of the TV
(horizontally), beyond the area where graphics may be positioned;
or whether the background color stops at the horizontal limits of
graphics and this border area appears black. This border area is
an area which appears undependably on various television sets.

CTRL also specifies whether characters (in Character mode) are
one or two bytes wide. That is, in Character (Indirect) mode,
whether one, or two bytes of graphics data should be fetched at
the address pointed to by the Character Map entry and CHARBASE.
The advantage of two byte characters is that the same number of
pixels can be specified with half as many Character Map entries.
The disadvantage is that when changing one character, twich as
much of the screen is affected.

This register also controls whether the color burst signal is
generated or not. If color burst is turned off, the graphics
are, of course, displayed in black and white, but with a greater
clarity than if the gray scale colors (x'00' - x'0F') were used.

Another bit of CTRL enables "Kangaroo" mode which eliminates
transparency, so that any pixel of color "0" will be background
color, rather than transparent. For the derivation of this name
see the ATARI coin-op game Kangaroo.

DMA may be turned on or off via the CTRL register. At power-up
DMA is off, and mist be turned on by the cartridge. This should
not be done until after DPPL and DPPH have been stored (so that
DMA doesn't try to read a DLL from an undefined location). DMA
should be turned on DURING VBLANK, and never during the screen
(rasters 16-258). If DMA is off the screen will continue to
display the background color.

Finally, CTRL is where the READ MODE portion of the graphics mode
is selected (remember the WRITE MODE portion is selected via an
extended header). WRITE mode controls the way data is written
into Line RAM, and READ mode controls the way Line RAM is
interpreted and translated to the screen. Because READ MODE
affects the scan line being displayed, changes to READ MODE
should happen at the beginning of the scan line to be affected.

The WRITE MODE selects between a.) 160x2 or 320x1 and b.) 160x4
or 320x2. The Read mode selects between a.) 320A or 320C, b.)
320B or 320D, and c.) 160A(x2) or 160B(x4). The following table
should be more informative:

                MODE     WM     RM1     RM0

                160A     0       0       0
                160B     1       0       0
                320A     0       1       1
                320B     1       1       0
                320C     1       1       1
                320D     0       1       0

320A mode is a true 320x1 mode. Pixels that are "on" refer to
color two (2) of the specified palette. Pixels that are off are
transparent (or background color if Kangaroo" mode is on). In
320B mode, which is a 320x2 display mode, only the most
significant palette bit is read. This means that either palette
zero (0) of palatte four (4) is used. If "Kangaroo" mode is off,
transparency will work differently for modes. Consider a pair of
320-size pixels which make up one 160-size pixel. If either
pixel of the pair is off, it will not be transparent, but will
take on background color instead. If both pixels are off, they
will be transparent. With "Kangaroo" mode on, things work as one
would expect them to work in this mode. Another factor
concerning 320 modes is that the horizontal positioning still
happens like 160 mode. This means that in 320 modes, objects can
only be positioned in 2 pixel increments.

320C and 320D are display modes somewhat similar to 320B and
320A, respectively. They are what you would get if you changed
WRITE mode without changing READ mode (such as changing modes
during a scan line). If you were in 320A mode, and wanted to
include a character with more colors on the line, changing modes
would give you 320C mode. Likewise, changing from 320B on the
fly would give you 320D mode. The way data is interpreted for
320C and 320D will be explained later on.

In 160x4 mode, again only the most significant palette bit is
read (note that 160x4 and 320B share the same WRITE mode sense).
Because there are more color bits than each palette can handle,
the palettes are combined in 160x4 mode so you may choose between
the combinations of 0-3 and 4-7. The net result of 160x4 mode is
twelve (12) colors, where color one (1) is P0C1 or P4C1, two (2)
is P0C2 or P4C2, five (5) is P1C1 or P5C1, six (6) is P1C2 or
P5C2, etc. and colors 0, 4, 8, and 12 are transparent.

The CTRL register is arranged as follows:

         _______________________________________________
        |     |     |     |     |     |     |     |     |
        | CK  | DM1 | DM0 | CW  | BC  | KM  | RM1 | RM0 |
        |_____|_____|_____|_____|_____|_____|_____|_____|

      Where:

      CK                 -    Color Kill.
                              0 => Normal color.
                              1 => No color burst.
      DM1, DM2           -    DMA control.
                              0 => Test A (DO NOT USE)
                              1 => Test B (DO NOT USE)
                              2 => Normal DMA.
                              3 => No DMA.
      CW                 -    Character Width.
                              0 => Two (2) byte characters.
                              1 => Single byte characters.
      BC                 -    Border Control.
                              0 => Background color border.
                              1 => Black border.
      KM                 -    "Kangaroo" Mode Switch
                              0 => Transparency
                              1 => "Kangaroo" mode: no transparency.
      RM1, RM0           -    Read Mode.
                              0 => 160x2, or 160x4
                              1 => Not used.
                              2 => 320B or 320D.
                              3 => 320A or 320C.

      (WARNING: TEST A (DM = 0) and TEST B (DM = 1) should NOT be
      used! These are for testing the chip at manufacturing time,
      and may cause irreconvertable problems, as well as possible
      DAMAGE TO THE BASE UNIT!)

The coding of graphics data is straightforward for most of these
modes. In 160x2 mode, each pair of bits is arranged so that the
leftmost pixel's color is specified by the most significant pair
of bits, and the rightmost pixel by the least significant pair of
bits.

     bits             76      54      32      10
                      ___     ___     ___     ___
     pixels          |   |   |   |   |   |   |   |
                     |___|   |___|   |___|   |___|
                       |       |       |       |
                       |       |       |       |
                       |       |       |       |          160x2
                       |       |       |       |
                       |       |       |       |
                      / \     / \     / \     / \
                    _______________________________
     graphics      |   |   |   |   |   |   |   |   |
     byte          |___|___|___|___|___|___|___|___|
                         

In 160x4 mode, the data is read as follows: the left pixel's
color is specified by bits 3,2,7,6 (where 3 is MSB, 6 is LSB).
The right pixel is specified by bits 1,0,5,4 (where 1 is MSB, 4
is LSB).

     bits             3276                    1054
                      ___                     ___
     pixels          |   |                   |   |
                     |___|                   |___|
                       |                       |
                       +---------------+       |
                       |               |       |          160x4
                       |       +---------------+
                       |       |       |       |
                      / \     / \     / \     / \
                    _______________________________
     graphic       |   |   |   |   |   |   |   |   |
     byte          |___|___|___|___|___|___|___|___|
                         

320A mode is a direct mapping like 160x2, except that each bit
specifies the color of one pixel.

     bits            7   6   5   4   3   2   1   0
                    _______________________________
     pixels        |   |   |   |   |   |   |   |   |
                   |___|___|___|___|___|___|___|___|
                     |   |   |   |   |   |   |   |
                     |   |   |   |   |   |   |   |
                     |   |   |   |   |   |   |   |     320A(x1)
                     |   |   |   |   |   |   |   |
                    _______________________________
     graphics      |   |   |   |   |   |   |   |   |
     byte          |___|___|___|___|___|___|___|___|


320B mode works as follows:


     bits           73        62      51        40
                    ___       ___     ___       ___
     pixels        |   |     |   |   |   |     |   |
                   |___|     |___|   |___|     |___|
                     |\       / \     / \       /|
                     | \     /   \   /   \     / |
                     |  \   /     \ /     \   /  |
                     |   \ /       \       \ /   |
                     |    /       / \       \    |
                     |   | \     /   \     / |   |
                     |   |  \   /     \   /  |   |
                     |   |   \ /       \ /   |   |     320B(x2)
                     |   |    /         \    |   |
                     |   |   | \       / |   |   |
                     |   |   |  \     /  |   |   |
                     |   |   |   \   /   |   |   |
                     |   |   |    \ /    |   |   |
                     |   |   |     /     |   |   |
                     |   |   |    / \    |   |   |
                    _______________________________
     graphics      |   |   |   |   |   |   |   |   |
     byte          |___|___|___|___|___|___|___|___|


320C mode allows more colors than 320A, but cannot really be
called 320x2. In this mode, some of the graphics data goes to
specifying palettes, which is somewhat non-standard. If a pixel
is on, it is color two (2), and if it is off, it is transparent.
or background color (same as 320A and 320B). The palette is
determined by combining the most significant palette. The
palette for the leftmost pixel is specified by P2,D3, and D2
(where P means a palette bit, and D means graphics data bit), and
the graphics are specified by D7. The next pixel right uses the
same palette, and uses D6 for data. The next pixel right uses a
palette specified by P2, D1, and D0, and uses D5 for data. The
rightmost pixel uses the same palette, but D4 for data. The
mapping for 320C mode is as follows:

     palette
     bits                         <P2><D3><D2>  <P2><D1><D0>
                                  \__________/  \__________/ 
     color                             |             /
     bits           D7  D6  D5  D4     |            /
                                       |           /
     pixels          _   _   _   _     |          /
                    | | | | | | | |    |         /
                    |_| |_| |_| |_|    |        /
                     |   |   |   |     |       |
                     |   |   |   |     |       |      320C
                     |   |   |   |    / \     / \
     graphics       _______________________________
     byte          |   |   |   |   |   |   |   |   |
                   |___|___|___|___|___|___|___|___|


320D mode is a little confusing, too. Every pixel refers to the
same palette but palette bits affect the color of the pixels.
The only palette bit used for palette definition is the most
significant bit (same as 320B), so only palettes zero (0) and
four (4) will be referenced. For color selection there is really
more than one bit per pixel. The graphics data bits are used as
follows: each is the most significant bit for a two bit pair.
But the least significant bit of this pair is either P0 or P1
(where P again means palette bit). If the specified palette is 0
or 4 (where P1 and P0 are zero), this is a mormal 320x1 mode,
like 320A. But if the specified palette is 5, palette 4 will be
used, and certain pixels will be either color 1 or 3, and others
will be 0 or 2. A picture is worth a thousand words, so:

     palette
     bits           P2  P2  P2  P2  P2  P2  P2  P2

     color          DP  DP  DP  DP  DP  DP  DP  DP
     bits           71  60  51  40  31  20  11  00
                    _______________________________
     pixels        |   |   |   |   |   |   |   |   |
                   |___|___|___|___|___|___|___|___|
                     |   |   |   |   |   |   |   |
                     |   |   |   |   |   |   |   |
                     |   |   |   |   |   |   |   |    320D
                     |   |   |   |   |   |   |   |
                    _______________________________
     graphics      |   |   |   |   |   |   |   |   |
     byte          |___|___|___|___|___|___|___|___|


APPENDIX 1: 7800 MEMORY MAP

The memory map of the 7800, graphically illustrated on the next
page, is in many ways similar to that of the 2600, with the
addition not only of MARIA, but also of 4K of RAM. This RAM is
shadowed (responds to other addresses) in zero, first, second,
and third pages, the first two of these being significant. You
will notice the absence of the 128 bytes of 6532 RAM that make up
zero page RAM in the 2600. This is because of speed discrepancy
with the 6532. It's RAM has moved to an area in page four (4)
and may not exist in future versions of the MARIA chip, so it
should not be used.

               FROM                    TO

1.  TIA        0000 00XX 0000 0000  -  0000 00XX 0001 1111

2.  MARIA      0000 00XX 0010 0000  -  0000 00XX 0011 1111

3.  6532       0000 0010 1000 0000  -  0000 0010 1111 1111
    PORTS

4.  6532       0000 010X 1000 0000  -  0000 010X 1111 1111
    RAM (DON'T USE)

5.  RAM        0001 1000 0000 0000  -  0010 0111 1111 1111

6.  RAM        00X0 000A 0100 0000  -  00X0 000A 1111 1111
    SHADOW

7.  RAM        001X X000 0000 0000  -  001X X111 1111 1111
    SHADOW

where: X means "Don't Care," and A means the bits may be 1 or 0,
but are not ignored. Entries 5 and 6 indicate that pieces of RAM
from x'1800' - x'27FF' appear in zero, and first pages. The last
entry indicates that the last 2K block (x'2000' - x'27FF') is
repeated at x'2800', x'3000', and x'3800' making this 6K area a
series of 2K shadows.

For encryption purposes, the 128 bytes from x'FF7A' - x'FFF9'
must be left free. Put FFs in this area until encrypted.

            _________________________________
     0000  |                                 |
           |          TIA Registers          |  
           |_________________________________|  001F
     0020  |                                 |
           |         MARIA Registers         |  
           |_________________________________|  003F
     0040  |                                 |
           |               RAM               |  
           |        (6116 Block Zero)        |
           |_________________________________|  00FF
     0100  |                                 |
           |         Shadow of Page 0        |
           |         (TIA and MARIA)         |
           |_________________________________|  013F
     0140  |                                 |
           |               RAM               |
           |         (6116 Block One)        |
           |_________________________________|  01FF
     0200  |                                 |
           |             Shadowed            |  
           |_________________________________|  027F
     0280  |                                 |
           |            6532 Ports           |  
           |_________________________________|  02FF
     0300  |                                 |
           |             Shadowed            |  
           |_________________________________|  03FF
     0400  |                                 |
           |            Available            |  
           |_________________________________|  047F
     0480  |                                 |
           |      6532 RAM -- Don't Use      |  
           |_________________________________|  04FF
     0500  |                                 |
           |            Available            |  
           |_________________________________|  17FF
     1800  |                                 |
           |               RAM               |  
           |_________________________________|  203F
     2040  |                                 |
           |        Block Zero Shadow        |  
           |_________________________________|  20FF
     2100  |                                 |
           |               RAM               |  
           |_________________________________|  213F
     2140  |                                 |
           |        Block One Shadow         |  
           |_________________________________|  21FF
     2200  |                                 |
           |               RAM               |  
           |_________________________________|  27FF
     2800  |                                 |
           |        Same as 2000-27FF        |  
           |_________________________________|  3FFF
     4000  |                                 |
           |            Available            |  
           |_________________________________|  FF79
     FF7A  |                                 |
           |     Reserved for Encryption     |  
           |_________________________________|  FFF9
     FFFA  |                                 |
           |       Available (vectors)       |  
           |_________________________________|  FFFF


APPENDIX 2: STANDARD 7800 EQUATES

INPTCTRL  EQU X '01' INPUT PORT CONTROL ("VBLANK" IN TIA)  WO
AUDC0     EQU X '15' AUDIO CONTROL CHANNEL 0               WO
AUDC1     EQU X '16' AUDIO CONTROL CHANNEL 1               WO
AUDF0     EQU X '17' AUDIO FREQUENCY CHANNEL 0             WO
AUDF1     EQU X '18' AUDIO FREQUENCY CHANNEL 1             WO
AUDV0     EQU X '19' AUDIO VOLUME CHANNEL 0                WO
AUDV1     EQU X '1A' AUDIO VOLUME CHANNEL 1                WO
INPT0     EQU X '08' PADDLE CONTROL INPUT 0                WO
INPT1     EQU X '09' PADDLE CONTROL INPUT 1                WO
INPT2     EQU X '0A' PADDLE CONTROL INPUT 2                WO
INPT3     EQU X '0B' PADDLE CONTROL INPUT 3                WO
INPT4     EQU X '0C' PLAYER 0 FIRE BUTTON INPUT            WO
INPT5     EQU X '0D' PLAYER 1 FIRE BUTTON INPUT            WO

BACKGRND  EQU X '20' BACKGROUND COLOR                      R/W
P0C1      EQU X '21' PALETTE 0 - COLOR 1                   R/W
P0C2      EQU X '22'           - COLOR 2                   R/W
P0C3      EQU X '23'           - COLOR 3                   R/W
WSYNC     EQU X '24' WAIT FOR SYNC                       STROBE
P1C1      EQU X '25' PALETTE 1 - COLOR 1                   R/W
P1C2      EQU X '26'           - COLOR 2                   R/W
P1C3      EQU X '27'           - COLOR 3                   R/W
MSTAT     EQU X '28' MARIA STATUS                          RO
P2C1      EQU X '29' PALETTE 2 - COLOR 1                   R/W
P2C2      EQU X '2A'           - COLOR 2                   R/W
P2C3      EQU X '2B'           - COLOR 3                   R/W
DPPH      EQU X '2C' DISPLAY LIST LIST POINT HIGH          WO
P3C1      EQU X '2D' PALETTE 3 - COLOR 1                   R/W
P3C2      EQU X '2E'           - COLOR 2                   R/W
P3C3      EQU X '2F'           - COLOR 3                   R/W
DPPL      EQU X '30' DISPLAY LIST LIST POINT LOW           WO
P4C1      EQU X '31' PALETTE 4 - COLOR 1                   R/W
P4C2      EQU X '32'           - COLOR 2                   R/W
P4C3      EQU X '33'           - COLOR 3                   R/W
CHARBASE  EQU X '34' CHARACTER BASE ADDRESS                WO
P5C1      EQU X '35' PALETTE 5 - COLOR 1                   R/W
P5C2      EQU X '36'           - COLOR 2                   R/W
P5C3      EQU X '37'           - COLOR 3                   R/W
OFFSET    EQU X '38' FOR FUTURE EXPANSION -STORE ZERO HERE R/W
P6C1      EQU X '39' PALETTE 6 - COLOR 1                   R/W
P6C2      EQU X '3A'           - COLOR 2                   R/W
P6C3      EQU X '3B'           - COLOR 3                   R/W
CTRL      EQU X '3C' MARIA CONTROL REGISTER                WO
P7C1      EQU X '3D' PALETTE 7 - COLOR 1                   R/W
P7C2      EQU X '3E'           - COLOR 2                   R/W
P7C3      EQU X '3F'           - COLOR 3                   R/W

SWCHA     EQU X'280' P0,P1 JOYSTICK DIRECTIONAL INPUT      R/W
SWCHB     EQU X'282' CONSOLE SWITCHES                      RO
CTLSWA    EQU X'281' I/O CONTROL FOR SWCHA                 R/W
CTLSWB    EQU X'283' I/O CONTROL FOR SWCHB                 R/W


APPENDIX 3: DMA TIMING

There is some uncertainty as to the number of cycles DMA will
require, because the internal MARIA chip timing resolution is
7.16 MHz, while the 6502 runs at either 1.79 MHz or 1.19MHz. As
a result, it is not known how many extra cycles will be needed in
DMA startup/shutdown to make the 6502 happy. It is even possible
for the 6502 to be in the middle of a long (TIA or 6532) acces
when it is to be halted, so the uncertainty goes up to about 5
cycles.

All times listed below refer to 7.16 MHz cyles.

          DMA startup            5-9      cycles

          Header (4 byte)        8        cycles
          Header (5 byte)        12       cycles

          Graphics Reads:
          Direct                 3        cycles
          Indirect/1 byte        6        cycles
          Indirect/2 byte        9        cycles

          Character Map access   3        cycles

          Shutdown Times:
          Last line of zone      10-13    cycles
          Other lines of zone    4 - 7    cycles


End of VBLANK is made up of a DMA startup plus a Long shutdown.

DMA does not begin until 7 cpu (1.79 MHz) cycles into each scan
line. The significance of this is that there is enough time to
change a color, or change CTRL before DMA begins, and during
HBLANK (before display begins). This figure should, however, be
included in any DMA usage calculations.

Another timing characteristic is that there is one mpu (7.16 MHz)
cycle between DMA shutdown and generation of a DLI.


APPENDIX 4: FRAME TIMING


               34 cycles @1.79 MHz       80 cycles @1.79 MHz
              136 cycles @7.16 MHz      320 cycles @7.16 MHz
            <--68 pixels--> <-----------160 pixels------------->
     ______|_______________|____________________________________|
      ^    |               |                  ^                 |
      |    |               |                  |                 |
      15   |<---HBLANK---->|               VBLANK               |
      |    |               |                  |                 |
      v    |               |                  v                 |
___________|_______________|____________________________________|
 ^    ^    |               |                  ^                 |
 |    |    |               |                  |                 |
 |    25   |               |                  |                 |
 |    |    |               |                  |                 |
 |    v    |               |                  |                 |
 |    -----|---------------|------------------|-----------------|
 |    ^    |               |                  |                 |
 |    |    |               |                  |                 |
 |    |    |               |                  |                 |
 |    |    |               |                  |                 |
 |    |    |               |                  |                 |
243  192   |               |               VISIBLE              |
 |    |    |               |                  |                 |
 |    |    |               |                  |                 |
 |    |    |               |                  |                 |
 |    |    |               |                  |                 |
 |    v    |               |                  |                 |
 |    -----|---------------|------------------|-----------------|
 |    ^    |               |                  |                 |
 |    |    |               |                  |                 |
 |    26   |               |                  |                 |
 |    |    |               |                  |                 |
 v    v    |               |                  v                 |
___________|_______________|____________________________________|
      ^    |               |                  ^                 |
      |    |               |                  |                 |
      4    |               |               VBLANK               |
      |    |               |                  |                 |
      v    |               |                  v                 |
     ______|_______________|____________________________________|
           |<-------------------228 pixels--------------------->|
                                456 cycles @7.16 MHz
                                114 cycles @1.79 MHz



(Text version updated Nov. 24, 1999)
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