[Coco] CoCo 3 RAM mapping question.

[Dave Philipsen]

I imagine you’ll have to turn on all-RAM mode ($FFDF) and if your diagnostics is running out of ROM that will have to be copied into RAM and run from there.

I *believe* that when you’re in ROM-RAM mode the MMU blocks $3C-3F refer to the ROMs instead of RAM.

Dave

> On Apr 26, 2020, at 8:26 AM, Phill Harvey-Smith <afra at ramoth.org.uk> wrote:
> 
> Hi All,
> 
> I'm currently working on getting the Dragon and CoCo Diag cart working with the CoCo3.
> 
> This has mainly consisted of making sure the appropreate GIME registers are programmed correctly at startup. Note this cart pulls SLENB / DSD so that it's ROM can take over straight away at reset time, so that the tests can still be run if for example the internal ROM is dead / corrupt.
> 
> I'm currently working on the RAM test, to do this I am sequentially paging in the 8K block under test at $4000-$5FFF, first checking for mirrors so that I start at block $30 on a 128K machine, block $00 on a 512K. This seems to work without problems until I page in block $3C, at which point the test fails! Tracing in MAME it seems that this has paged in the ECB ROM,  not what I want or expected, I had assumed that the DAT registers only applied to the RAM?
> 
> The tests are currently operating in 32K RAM, 16K internal rom, 16K external ROM (though the external is actually 8K ROM, 8K RAM but my card handles that). The Init1 register contans $C4, so CoCo 1/2 compatible mode, MMU enabled and standard SCS.
> 
> Anyone know how to resolve this?
> 
> Cheers.
> 
> Phill.
> 
> -- 
> Phill Harvey-Smith, Programmer, Hardware hacker, and general eccentric !
> 
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