These are the results of my testing in MAME debugger:
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EntryPoint: ; First thing that runs after powering on move.b ($00a11100),d5 ; Get BUSREQ state andi.l #1,d5 ; D5 = 1, MAME memory disassembly says Z80_HAS_BUS flag = 1 (true) move.b ($00a11200),d4 ; Get RESET state, but this doesn't work, it always returns 0 ; move.w ($00a11200),d4 ; Same thing, it always returns 0 ; move.l ($00a11200),d4 ; Again, always 0 andi.l #1,d4 ; D4 = 0 no matter what, but memory disassembly says Z80_IS_RESET flag = 1 (true) ;-----This part doesn't work. No matter how long I wait, Bit #0 of BUSREQ never becomes 0, and Z80_HAS_BUS flag never becomes 0 (false)-----; ; move.w #$0100,($00a11100) ; Write 0100h to the BUSREQ port to REQUEST CONTROL of the Z80 bus ;.LocalLoop1: ; btst #0,($00a11100) ; Check Bit #0 of the byte at the BUSREQ port ; bne .LocalLoop1 ; Loop until Bit #0 of the byte at BUSREQ = 0, then we control the bus ;----------This part works fine.-----------; move.w #$0100,($00a11200) ; Write 0100h to the RESET port to STOP HOLDING the Z80 in a reset state ; Memory disassembly now says Z80_IS_RESET flag = 0 (false) move.w #$0100,($00a11100) ; Write 0100h to the BUSREQ port to REQUEST CONTROL of the Z80 bus ; Memory disassembly now says Z80_HAS_BUS flag = 0 (false) .LocalLoop: btst #0,($00a11100) ; Check Bit #0 of the byte at the BUSREQ port bne .LocalLoop ; Loop until Bit #0 of the byte at BUSREQ = 0, then we control the bus ; Access Z80 memory here... move.w #0,($00a11100) ; Write 0000h to the BUSREQ port to RELEASE CONTROL of the Z80 bus ; Memory disassembly says Z80_HAS_BUS flag = 1 (true)
- The Z80 manual says that the BUSREQ and RESET pins are both "active low." From what I've read, that seems to mean a low voltage signal (a "0" from a software standpoint) turns them ON, and a high signal (a "1" in software) turns them OFF. If that is correct, then it matches the way the RESET port works (0000h = RESET Z80 ON; 0100h = STOP RESETTING Z80), but it's the reverse of how the BUSREQ port works (0100h = REQUEST BUS; 0000h = LET GO OF BUS). Is the BUSREQ pin wired to the 68k backward or something?
- Do I need to check to make sure the Z80 regains control of the bus after I release control of it? If so, what should I do if the Z80 doesn't regain control? Write another release command to the port, or maybe reset the Z80?
- Is it necessary to write a word to these ports, or can I write bytes instead:
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move.b #1,($00a11100) ; Request bus move.b #0,($00a11200) ; Request reset
- The Z80 manual mentioned something about how lengthy DMA operations with BUSREQ active can prevent DRAM from being refreshed properly. Do I need to worry about that when using BUSREQ? (The Genesis Technical Overview mentions this DRAM refresh business, but it's talking about hardware dev kits or something like that.)
- Is there any reason besides accessing Z80 memory from the 68k that I would use BUSREQ?
- When/why would I need to reset the Z80 in the main body of my code? Every time the 68k writes to its memory?
- Can you read from the RESET port on real hardware? Or the BUSREQ port, for that matter (with a MOVE instruction, that is)?
Also, if you'd like to make suggestions about different debugging or dev tools I should try out, go right ahead. I'm happy with the ones I'm using so far, but I like hearing different opinions.