Difference between revisions of "A78 Header Specification"

Revision as of 21:05, 20 January 2024

History and Purpose

The A78 header was originally created by Dan Boris, in tandem with his A7800 emulator. The header, which is added to the beginning of ROM images, serves to tell the emulator which hardware should be present for the game to run correctly.

The format has since been expanded to incorporate new hardware, and is now being used by multiple emulators and the Concerto 7800 flash cart.

A78 Header Fields

cart type word details::

  bit 0     = pokey at $4000
  bit 1     = supergame bank switched
  bit 2     = supergame ram at $4000
  bit 3     = rom at $4000
  bit 4     = bank 6 at $4000
  bit 5     = banked ram
  bit 6     = pokey at $450
  bit 7     = mirror ram at $4000
  bit 8     = activision banking
  bit 9     = absolute banking
  bit 10    = pokey at $440
  bit 11    = ym2151 at $460/$461
  bit 12    = souper
  bit 13    = banksets
  bit 14    = halt banked ram
  bit 15    = pokey@800

controller type byte details:

  0 = none
  1 = 7800 joystick
  2 = lightgun
  3 = paddle
  4 = trakball
  5 = 2600 joystick
  6 = 2600 driving
  7 = 2600 keypad
  8 = ST mouse
  9 = Amiga mouse
  10 = AtariVox/SaveKey
  11 = SNES2Atari

TV type details:

  bit 0 = 0:NTSC, 1:PAL
  bit 1 = 0:component, 1:composite

save device details:

  bit 0    = HSC
  bit 1    = SaveKey/AtariVox

expansion module:

  bit 0    = XM

v3 A78 Header Version Notes

The current header version stands at 3. There were fields in previous versions that were ambiguously defined, but are now spelled out. To be v3 compliant, a header must:

• Have a zero-terminated Cart Title field. Any post-title padding should be zeros, rather than spaces.
• All reserved bytes must contain zero.

Previous versions of the header were ambiguous as to the TV Type byte being an index or bitfield. V3 headers define TV type as a bitfield, and have included a bit to indicate whether composite display artifacts are appropriate for the game.

Adding Or Modifying an A78 Header on an Existing ROM

You can add or update an A78 header with the interactive command-line utility 7800header, which is distributed with 7800basic and the 7800AsmDevKit.

The utility displays the current header (or default header if the file is headerless) and you can set/unset hardware features with the displayed commands.

7800 A78 Header Info

    embedded game name : Commando
    rom size           : 131072
    cart format        : Pokey@4000 SuperGame
    controllers        : 7800joy1 7800joy2
    save peripheral    : none
    xm/xboard          : not enabled
    tv format          : NTSC

Commands: "save"  [name]   Save the A78 file and exit.
          "strip" [name]   Save a headerless .BIN and exit.
          "set [option]"   Set one of the options.
          "unset [option]" Unset one of the options.
          "name game name" Set the game name in the header.
          "exit"           Exit the utility. Unsaved changes will be lost.

Options:  rom@4000 bank6@4000 pokey@440 pokey@450 pokey@800 pokey@4000
   ram@4000 mram@4000 hram@4000 ym2151@460 supergame bankram
   absolute activision souper bankset tvpal tvntsc composite
   7800joy1 7800joy2 lightgun1 lightgun2 paddle1 paddle2
   tball1 tball2 2600joy1 2600joy2 driving1 driving2 keypad1
   keypad2 stmouse1 stmouse2 amouse1 amouse2 hsc savekey xm
> exit

Adding an A78 Header to Source Code

To add an A78 header to your DASM source code, you can add or include the header at the start of your source, and adjust the fields to encode the hardware your game requires.

For the current a78 assembly-language header, see the a78 header github