IO access differences in MD mode vs SMS

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Enforcer
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IO access differences in MD mode vs SMS

Post by Enforcer » Mon Oct 01, 2018 1:42 am

I’m in the midst of a project and was wondering if the bus
Arbiter / IO chip disables PORT access to the various peripherals from the Z80 or if it leaves that access available in addition to the memory mapped access? If it leaves it enabled could you get in a weird situation where the 68K is reading or writing the controller direction register while the Z80 side could be modifying it via PORT?

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Re: IO access differences in MD mode vs SMS

Post by TmEE co.(TM) » Mon Oct 01, 2018 8:10 am

Since it would require bus crossing, abitration will take place and one will wait while other finishes, there will not be a bus conflict happening.
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Re: IO access differences in MD mode vs SMS

Post by notaz » Mon Oct 01, 2018 11:24 am

AFAIK PORT accesses are not doing anything in MD mode.

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Re: IO access differences in MD mode vs SMS

Post by Sik » Mon Oct 01, 2018 4:26 pm

Yeah, I/O port accesses don't do anything. The memory map is completely changed between MD and SMS modes (Z80 outright uses the 68000 bus in SMS mode, that should give a good idea of how heavily it changes - I assume this is so mappers can see accesses to their registers?).

Also if I recall correctly, if you try to access the 68000 $Axxxxx region from the Z80 (as opposed to using IN/OUT), expect the whole thing to lock up.
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Enforcer
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Re: IO access differences in MD mode vs SMS

Post by Enforcer » Mon Oct 01, 2018 11:54 pm

Thanks for the info. I don’t suppose there are any die shots of the IO chip and Arbiter? That may make things a little easier on me.

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