This article needs checking and research.
CDDA stands for "CD Digital Audio". When loading a game, the "CDDA flag" byte at location $107 in the 68k program header is checked with bit 7 masked off.
- $00 or $01 is for games which use the Z80 RAM default location $FEF8 to play CDDA tracks.
- $02 and up seems to be for games which either use a custom Z80 RAM location, or the BIOS call.
Z80 RAM check method
This method was probably implemented to simplify the porting of small games to the NeoGeo CD. Games that didn't need in-game file loading only had to have their vector table and sound driver code replaced. The sound driver itself translates the regular music commands to CDDA command/track pairs and drops them to a specific location in its RAM, which is then read back and executed by the BIOS when using the SYSTEM_IO ($C0044A) call (see difference between the MVS and CD versions of Puzzled: no calls for music, M1 replaced).
In the sound drivers of CD games, a table is used to figure out if a received command concerns the YM2610 (mostly sound effects) or is a CDDA track.
Putting the CDDA flag byte at $107 to 0 or 1 enables this method and sets the Z80 return address to $FEF8. Setting the CDDA flag to any other values allows to choose the address (word at $13A). If the address is 0, this method isn't used.
For example, Puzzled uses this method. The CDDA flag is set to 0 in the program header. The 68k sends the exact same "play music" Z80 commands as the MVS version of the game, the sound driver then converts them to CDDA ones and drops them at $FEF8. Command $0743 gets translated to $0532 (intro music, track 32).
BIOS call $C0056A. Command/track pair in D0. (track number in BCD format).
Metal Slug 2 waits for a location in the BIOS RAM to be non-zero before calling $C0056A (?):
#: move.b d0,$300001 ;Watchdog kick, interrupts are disabled ? tst.b $10F6D9 beq.s # jsr $C0056A
Viewpoint sends the command 4 ($04xx, xx being the track number): The CDDA flag ($107) is $02, and $13A is $0000.
- $00: Read track and loop
- $02: Pause ?
- $04: Read track and loop (confirmed)
- $05: Read track, no loop
- $06: Pause playback (mutnat, 0x0600)
- $07: Resume playback (mutnat, 0x0700)
Commands over $07 cause a Z80 I/O ERROR.
Set $13A to $0000 if not using the Z80 RAM. Set to $FFFF if using the default $FEF8 value.
BIOSF_SYSTEM_IO must be called every VBlank for direct CDDA commands to work.
CDDA data can be picked up through memory mapped registers CDDA_LEFTSTREAM and CDDA_RIGHTSTREAM.