Z80 Program Code - Part 2

Let's take a look at the structured format of sound information. Here's a relatively straightforward example demonstrating the setup of the Checkpoint PCM sample.

; Voice 1, Checkpoint

ROM:6F91 data_voice1:    dw data_voice1_c   ; Offset to channel setup below

The sample is played through two channels simultaneously, presumably to boost its volume. 

; Voice 1, Channel Setup

ROM:6F99 data_voice1_c:  db 2               ; Number of channels

ROM:6F9A                 dw data_voice1_c1  ; Address of Channel 1 Setup

ROM:6F9C                 dw data_voice1_c2  ; Address of Channel 2 Setup

This block represents the default setup for the 32 byte block mentioned in the previous post. It's not actually 32 bytes, but the remainder of the space is padding to zero by the program code. I've cut the second entry short in the interests of space as it's very similar.

; Voice 1, Checkpoint (PCM Samples: Channel 1) - Default 0x20 area setup

ROM:6F9E data_voice1_c1: db 80h             ; Flags: Enable

ROM:6F9F                 db 1000110b        ; Flags: Mute & Channel Index

ROM:6FA0                 db 2               ; End Marker

ROM:6FA1                 dw 0

ROM:6FA3                 dw 1

ROM:6FA5                 dw data_voice1_c1p; Address of commands

ROM:6FA7                 db 0

ROM:6FA8                 db 20h            ; Offset for positioning information

ROM:6FA9                 db 0

ROM:6FAA                 db 0

ROM:6FAB                 db 0

; Voice 1, Checkpoint (PCM Samples: Channel 2)

ROM:6FAC data_voice1_c2: db 80h

; Snip: Similar to the above block

ROM:6FB9                 db 0

Here's where things gets a little interesting; what follows is a series of commands that correspond to a particular z80 routine, along with their arguments.

; Voice 1, Checkpoint (PCM Properties)

ROM:6FBA data_voice1_c1p:db 93             ; 93 = Command: PCM Set Pitch

ROM:6FBB                 db 48h            ;    value = pitch

ROM:6FBC                 db 82h            ; 82 = Command: PCM Sample Volumes

ROM:6FBD                 db 17h            ;    value = left channel vol

ROM:6FBE                 db 2Eh            ;    value = right channel vol

ROM:6FBF                 db 0DCh           ; DC = Command: Sample Index

ROM:6FC0                 db 28h            ;    value = checkpoint

ROM:6FC1                 db 99h            ; 99 = Command: PCM Finalize

ROM:6FC2 data_voice1_c2p:db 93h            ; 93 = Command: PCM Set Pitch

ROM:6FC3                 db 48h            ;    value = pitch

ROM:6FC4                 db 82h            ; 82 = Command: PCM Sample Volumes

ROM:6FC5                 db 2Eh            ;    value = left channel vol

ROM:6FC6                 db 17h            ;    value = right channel vol

ROM:6FC7                 db 0DCh           ; DC = Command: Sample Index

ROM:6FC8                 db 28h            ;    value = checkpoint

ROM:6FC9                 db 99h            ; 99 = Command: PCM Finalize

These commands index a table of routines which is as follows. Not all of these routines are used, as I imagine this area of the code is used across other Sega titles. I've highlighted in red the entries used by the above sample.

ROM:0B93 BigRoutineTable:
ROM:0B93    dw YM_Dec_Pos           ; YM: Decrement Position In Sequence (80)
ROM:0B95    dw YM_SetEndMarker      ; YM: Set End Marker. 
ROM:0B97    dw PCM_SetVol           ; PCM: Set Volume (Left & Right Channels) (82)
ROM:0B99    dw YM_Dec_Pos           ; YM: Decrement Position In Sequence (80)
ROM:0B9B    dw YM_Finalize          ; YM: End (84)
ROM:0B9D    dw YM_SetNoise          ; YM: Enable Noise Channel (85)
ROM:0B9F    dw loc_409              ; Unused?
ROM:0BA1    dw YM_SetModTab         ; YM: Enable/Disable Modulation table
ROM:0BA3    dw WriteSeqAddr
ROM:0BA5    dw SetSeqAddr           ; Set Next Sequence Address
ROM:0BA7    dw YM_GetLoopAdr        ; de = new YM loop address
ROM:0BA9    dw YM_SetNoteOffset     ; YM: Set Note/Octave Offset (8B)
ROM:0BAB    dw YM_DoLoop            ; YM: Loop Sequence Of Commands (8C)
ROM:0BAD    dw loc_46B              ; Unused?
ROM:0BAF    dw loc_471              ; Unused?
ROM:0BB1    dw YM_Enable_Correspnd  ; YM: (Unused) Enable corresponding channel (8F)
ROM:0BB3    dw YM_Disable_Correspnd ; YM: (Unused) Disable corresponding channel (90)
ROM:0BB5    dw YM_SetBlock          ; YM: Set Block - Called First When Setting Up (91)
ROM:0BB7    dw YM_DisableNoise      ; YM: Disable Noise Channel (92)
ROM:0BB9    dw PCM_SetPitch         ; PCM: Set Pitch (93)
ROM:0BBB    dw YM_MarkerData        ; FM: End Marker - Do not calculate, use value from data (94)
ROM:0BBD    dw YM_MarkerHigh        ; FM: End Marker - Set High Byte From Data (95)
ROM:0BBF    dw YM_ConnectRight      ; FM: Connect Channel to Right Speaker (96)
ROM:0BC1    dw YM_ConnectLeft       ; FM: Connect Channel to Left Speaker (97)
ROM:0BC3    dw YM_ConnectCentre     ; FM: Connect Channel to Both Speaker (98)
ROM:0BC5    dw PCM_Finalize         ; Write Commands to PCM Channel (99)

In case you were wondering the 0xDC command which is not shown in the table above triggers a separate piece of code, which also triggers drum samples and so forth in the music tracks. Anyway,  let's take a look at a simple routine - setting the pitch of a PCM sample.

ROM:03C4 PCM_SetPitch:
ROM:03C4   bit     6, (ix+1)       ; If channel is muted, don't set pitch
ROM:03C8   jp      nz, set_pitch
ROM:03CB   ld      a, a
ROM:03CC   ret
ROM:03CD set_pitch:
ROM:03CD   ld      a, (de)         ; a = New pitch (read from setup table in rom)
ROM:03CE   ld      (ix+16h), a     ; Set relevant area in 32 byte block that controls pitch
ROM:03D1   ret

The music tracks work in a similar way, but with a much longer and more complex series of commands.

Originally, I thought the Z80 might be used in a 'dumb' manner and simply stream preformatted audio data to the various chips. But its usage is much more sophisticated as demonstrated above.