Inter-CPU communication is done through a byte-wide bi-directional register.

68k to Z80 (request)

Writes to the Z80 are made through REG_SOUND. Any byte can be sent, its meaning is only determined by the way the Z80 code handles it (except for 3 special cases, as seen below).

When a byte is sent, the corresponding value is latched in NEO-C1 (NEO-SUD in CD systems ?), and an NMI is triggered on the Z80 if enabled. The value can then be read on the Z80's port $00.

(What chip is used on first gen chipset ?)

Z80 to 68k (answer)

The Z80's port $0C is used to reply to the 68k. The value is also buffered in the same chips, but there are no interrupts generated. The value can be read by using the same register, REG_SOUND.

Many sound drivers acknowledge sound commands by echoing them back with bit 7 set to 1 when they are processed.

Special commands

Commands $01 and $03 are always expected to be implemented in the Z80 code, as they are used by the System ROM for initialization purposes. During the MVS power up self-tests, if the Z80 doesn't reply to command $01 in time, the "Z80 ERROR" message is displayed and the system locks up.

Command $01

It is sent by the system ROM when the slot is switched. As the M1 ROM will be swapped, all sounds need to be stopped, NMI need to be activated, $01 needs to be sent back to the 68k and the Z80 code has to sit in a loop in RAM. After receiving the reply, the system ROM can then switch slots without crashing the Z80.

Command $02

It is used by cartridge systems to play the eye catcher music. The pattern (melody) was certainly imposed by SNK, but developers often chose their own instruments parameters. Boot music. No reply is expected.

Command $03

It is used to ask for a soft reset of the Z80, which needs to be done under 100ms. No reply is expected.

Minimal command handlers

These are sufficient handlers for both init commands:

;"MAKOTO V3" style handler
Command01_Handler:
	di			; Disable interrupts
        xor  a
        out  ($0C),a
        out  ($00),a
        ; Init banks and stuff...
        ld   sp,#fffc
        ld   hl,stayinram
        push hl
        retn                    ; RETN to stayinram

stayinram:
        ld   hl,#fffd
        ld   (hl),#c3	        ; (FFFD)=C3, opcode for JP
        ld   (#fffe),hl	        ; (FFFE)=FFFD (JP FFFD)
        ld   a,#01
        out  (#0c),a            ; Tell 68k that we're ready
        jp   #fffd              ; Sit in RAM loop

Command03_Handler:
	di			; Disable interrupts
	ld   sp, $FFFF		; Clear call stack
	ld   hl,0
	push hl
	retn			; RETN to 0

68k/Z80 communication

Contents