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

A78 Header Fields

0000	04	41	54	41	52	49	37	38	30	30	00	00	00	00	00	00	.ATARI7800......
0010	00	43	6f	6d	6d	61	6e	64	6f	00	00	00	00	00	00	00	.Commando.......
0020	00	00	00	00	00	00	00	00	00	00	00	00	00	00	00	00	................
0030	00	00	02	00	00	00	03	01	01	00	00	00	00	00	00	00	................
0040	01	00	00	05	00	00	00	00	00	00	00	00	00	00	00	00	................
0050	00	00	00	00	00	00	00	00	00	00	00	00	00	00	00	00	................
0060	00	00	00	00	41	43	54	55	41	4c	20	43	41	52	54	20	....ACTUAL CART
0070	44	41	54	41	20	53	54	41	52	54	53	20	48	45	52	45	DATA STARTS HERE

color	purpose	size	offset
	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
	reserved
	slot passthrough device	1 byte	63 / 0x3F
	v4+ mapper	1 byte	64 / 0x40
	v4+ mapper options	1 byte	65 / 0x41
	v4+ audio	2 bytes	66 / 0x42
	v4+ interrupt	2 bytes	68 / 0x44
	header end magic-text	28 bytes	100 / 0x64

cart type 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 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
  12 = Mega7800

TV type details:

  bit 0 : 0=NTSC, 1=PAL
  bit 1 : 0=component, 1=composite
  bit 2 : 0=single region, 1=multi-region

save device details:

  bit 0 : HSC
  bit 1 : SaveKey/AtariVox

slot passthrough device details:

  bit 0 : XM

v4+ mapper details:

  0 = Linear
  1 = SuperGame
  2 = Activision
  3 = Absolute
  4 = Souper

v4+ mapper options details: (The meaning of this field varies depending on the mapper chosen.)

  Linear
  bit 7    : Bankset ROM
  bits 0-1 : Option at @4000 (0=none, 1=16K RAM, 2=8K EXRAM/A8, 3=32K EXRAM/M2)

  SuperGame
  bit 7    : Bankset ROM
  bits 0-2 : Option at @4000 (0=none, 1=16K RAM, 2=8K EXRAM/A8, 3=32K EXRAM/M2, 4=EXROM, 5=EXFIX, 6=32k EXRAM/X2)

v4+ audio:

  bit 5    : ADPCM Audio Stream @420
  bit 4    : COVOX @430
  bit 3    : YM2151 @460
  bits 0-2 : POKEY (0=none, 1=@440, 2=@450, 3=@450+@440, 4=@800, 5=@4000)

v4+ interrupt:

  bit 2 : YM2151
  bit 1 : POKEY 2 (@440)
  bit 0 : POKEY 1 (@450 | @800 | @4000)

A78 Header Version Notes

The current header version stands at 4, but not all flash carts and emulators are capable of reading v4 specific fields, nor do all games have v4 headers.

If game authors choose not to use/support any v4 features, then the header version should be kept at "3". If a v4 header is used, it's encouraged that v3 headers are also used where possible, to allow for maximal compatibility with emulators and flash carts.

If an emulator or flash cart are v4 compatible, and they receive a rom with a v4 header, the v4 fields should take precedence over anything encoded in the equivalent v3 fields.

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