A78 Header Specification
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
|0070||44||41||54||41||20||53||54||41||52||54||53||20||48||45||52||45||DATA STARTS HERE|
|header version||1 byte||0 / 0x00|
|ATARI7800 magic-text||16 bytes||1 / 0x01|
|cart title||32 bytes||17 / 0x11|
|rom size without header||4 bytes||49 / 0x31|
|cart type||2 bytes||53 / 0x35|
|controller 1 type||1 byte||55 / 0x37|
|controller 2 type||1 byte||56 / 0x38|
|TV type||1 byte||57 / 0x39|
|save device||1 byte||58 / 0x3A|
|expansion module||1 byte||63 / 0x3F|
|header end magic-text||28 bytes||100 / 0x64|
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
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
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
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 controller 1 : Joystick controller 2 : None save peripheral : None xm/xboard : not enabled tv format : NTSC Commands: "save" Save the A78 file 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@450 pokey@4000 supergame supergameram supergamebankram absolute activision tvpal tvntsc savekey hsc xm joystick1 joystick2 lightgun1 lightgun2 > 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