68k/Z80 communication: Difference between revisions
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Communication between the {{Chipname|68k}} CPU and the {{Chipname|Z80}} CPU is done through 2 byte-wide registers. | |||
There is no shared memory zone like in the Sega Genesis/Megadrive. | |||
=68k to Z80 (request)= | =68k to Z80 (request)= | ||
Writes to the | Writes to the Z80 are done through {{Reg|REG_SOUND}}. | ||
Any byte can be sent, the meaning of it is only determined by the way the Z80 code handles it (except for '''3 special cases''', as seen below). | Any byte can be sent, the meaning of it is only determined by the way the [[M1 ROM|Z80 code]] handles it (except for '''3 special cases''', as seen below). | ||
When a byte is sent, the corresponding value is latched in {{Chipname|NEO-C1}} ({{Chipname|NEO-SUD}} in CD systems ?), and an NMI is triggered | When a byte is sent, the corresponding value is latched in {{Chipname|NEO-C1}} ({{Chipname|NEO-SUD}} in CD systems ?), and an [[Z80 interrupts|NMI]] is triggered in the Z80 if enabled. The value can then be read on the Z80's side with [[Z80 port map|port $00]]. | ||
(What chip is used on first gen chipset ?) | (What chip is used on first gen chipset ?) | ||
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=Z80 to 68k (reply)= | =Z80 to 68k (reply)= | ||
The Z80's port $0C is used to reply to the | The Z80's port $0C is used to reply to the 68k. The value is also buffered in the same chip, but no interrupt is generated. | ||
The value can be read by using the same register, {{Reg|REG_SOUND}}. | The value can be read on the 68k's side by using the same register, {{Reg|REG_SOUND}}. | ||
Many [[sound driver]]s acknowledge sound commands by echoing them back with bit 7 set to 1 when they are processed. | Many [[sound driver]]s acknowledge sound commands by echoing them back with bit 7 set to 1 when they are processed. | ||
=Special commands= | =Special commands= | ||
Commands $01 | Commands $01, $02 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 [[system_self-test|self-tests]], if the Z80 doesn't reply to command $01 in time, the "[[Z80 ERROR]]" message is displayed and the system locks up. | During the MVS power up [[system_self-test|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== | ==Command $01: Prepare switch== | ||
It is sent by the system ROM just before the [[slot]] is switched. As the {{Chipname|M1 ROM}} has to be swapped, all sounds need to be stopped, | It is sent by the system ROM just before the [[slot]] is switched. As the {{Chipname|M1 ROM}} has to be swapped, all sounds need to be stopped, interrupts need to be enabled, $01 needs to be sent back to the 68k and the Z80 has to wait in a loop '''in RAM'''. After receiving the reply, the system ROM can then switch slots without crashing the Z80. | ||
==Command $02== | ==Command $02: Play eyecatcher music== | ||
It is used | It is used in cartridge systems to play the [[eyecatcher]] music. See [[boot music]]. No reply is expected. | ||
==Command $03== | ==Command $03: Reset== | ||
It is used to ask for a soft reset of the Z80, which needs to be done under 100ms. No reply is expected. | It is used to ask for a soft reset of the Z80, which needs to be done under 100ms as per SNK's recommendation. No reply is expected. | ||
=Minimal command handlers= | =Minimal command handlers= | ||
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<pre> | <pre> | ||
Command01_Handler: | Command01_Handler: | ||
di ; Disable interrupts | |||
xor a | xor a | ||
out ($0C),a | out ($0C),a ; Clear both buffers | ||
out ($00),a | out ($00),a | ||
; | ; Silence YM2610 here | ||
ld sp, | ld sp,$FFFC ; Reset SP | ||
ld hl,stayinram | ld hl,stayinram | ||
push hl | push hl | ||
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stayinram: | stayinram: | ||
ld hl, | ld hl,$FFFD | ||
ld (hl), | ld (hl),$C3 ; (FFFD)=$C3, opcode for JP | ||
ld ( | ld ($FFFE),hl ; (FFFE)=$FFFD (makes "JP FFFD") | ||
ld a, | ei | ||
out ( | ld a,$01 | ||
jp | out ($0C),a ; Tell 68k that we're ready | ||
jp $FFFD ; Quickly jump to RAM loop | |||
</pre> | </pre> | ||
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Command03_Handler: | Command03_Handler: | ||
di ; Disable interrupts | di ; Disable interrupts | ||
ld sp, $FFFF ; Clear call stack | ld sp,$FFFF ; Clear call stack | ||
ld hl,0 | ld hl,0 | ||
push hl | push hl | ||
Latest revision as of 05:27, 3 November 2016
Communication between the 68k CPU and the Z80 CPU is done through 2 byte-wide registers.
There is no shared memory zone like in the Sega Genesis/Megadrive.
68k to Z80 (request)
Writes to the Z80 are done through REG_SOUND. Any byte can be sent, the meaning of it 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 in the Z80 if enabled. The value can then be read on the Z80's side with port $00.
(What chip is used on first gen chipset ?)
Z80 to 68k (reply)
The Z80's port $0C is used to reply to the 68k. The value is also buffered in the same chip, but no interrupt is generated. The value can be read on the 68k's side 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, $02 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: Prepare switch
It is sent by the system ROM just before the slot is switched. As the M1 ROM has to be swapped, all sounds need to be stopped, interrupts need to be enabled, $01 needs to be sent back to the 68k and the Z80 has to wait in a loop in RAM. After receiving the reply, the system ROM can then switch slots without crashing the Z80.
Command $02: Play eyecatcher music
It is used in cartridge systems to play the eyecatcher music. See boot music. No reply is expected.
Command $03: Reset
It is used to ask for a soft reset of the Z80, which needs to be done under 100ms as per SNK's recommendation. No reply is expected.
Minimal command handlers
These are sufficient handlers for both init commands:
Command01_Handler:
di ; Disable interrupts
xor a
out ($0C),a ; Clear both buffers
out ($00),a
; Silence YM2610 here
ld sp,$FFFC ; Reset SP
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 (makes "JP FFFD")
ei
ld a,$01
out ($0C),a ; Tell 68k that we're ready
jp $FFFD ; Quickly jump to RAM loop
Command03_Handler: di ; Disable interrupts ld sp,$FFFF ; Clear call stack ld hl,0 push hl retn ; RETN to 0