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Z80 access to cart address space?
Posted: Sun Jul 18, 2010 11:03 pm
Hey guys, long time no see. I have a h/w question. Can the Z80 access the cartridge address space? If so, does the bus arbitration work to make Z80 reads look exactly like 68000 reads? Specifically is the timing with respect to the clock the same, and is !AS strobed as well as !C_CE and !C_OE? How is the arbitration done? Does the Z80 get priority and the 68000 waits? Or the other way around?
Z80 does not strobe !AS by the looks of it
Posted: Sun Jul 18, 2010 11:30 pm
Partly answering my own question: it looks like the Z80 can read from the cartridge address space, but I'm not sure what the Z80 read cycles look like in detail. They appear to strobe !C_CE and !C_OE just as 68000 reads do, but not !AS.
Posted: Mon Jul 19, 2010 6:46 am
Whichever side has an access in progress makes the other wait, 68K should get priority on simultaneous access and Z80 will wait until the 68K is done. Z80 will continue to run during DMA as long as it does not access the bus, when it does it will be stopped for the remaining DMA duration.