CDDA

CDDA stands for CD Digital Audio.

When loading a game, the "CDDA flag" byte at location $000107 in the 68k program is checked with bit 7 is masked off. Known values: 0, 2, $82.

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/tracl pairs and drops them to a specific location in its own RAM, which is then read back and executed by the BIOS when using the $C0044A call.

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 a CDDA track.

Putting the CDDA flag byte at $000107 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 what the return address will be (set with the 16bit value at $00013A).

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).

Direct method
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

Commands

 * $00: Read track and loop
 * $02: Pause ?
 * $05: Read track

CDDA functionalities are disabled if ($00013A)=$0000. Set to $FFFF if using the default $FEF8 value.