[Coco] ML Question: MMU Block Switching

[William Mikrut]

No, I didn't/don't ... and I am still learning.

After reading Lomont_CoCoHardware.pdf P66 last night I began to suspect
that if I took $6C000 divided by $800 I ended up with D8... so I knew that
had to fit in and was going to try it today.

I Picked $800 because the text said the start of video was video location *
2048.
LSB was easy because it's zero.

So much to learn... and I sincerely appreciate all the guidance!




On Tue, Feb 17, 2015 at 8:47 AM, Robert Gault <robert.gault at att.net> wrote:

> William Mikrut wrote:
>
>> Ahhh.... FF9D and FF9E.
>>
>> I can find plenty of documentation that specifies what this offset should
>> be for video ram.
>> When I read several articles on these registers, I find that this is
>> MSB/LSB format.
>>
>> The reason I was swapping in page #36 was to be able to access the Hires
>> mem locations $6C000-$6DFFF through $2000-3FFF.
>>
>> Is there a way to set these registers to it allows me to access that Hiram
>> location directly from local addressing (meaning within the 64K Address
>> space) without the need to swap blocks?
>>
>> I can see setting $FF9D/$FF9E with $C0/$00 -- but how would it know it was
>> in the $60000 range?
>>
>>
>>
> William,
>
> Taking the above verbatim, it looks like you don't know the math for
> $FF9D-$FF9E. These registers tell the Coco3 in Coco3 mode where to point
> video memory. That memory being VMem/8=16-bit value in $FF9D-$FF9E. The MMU
> block numbers are VMem/$2000.
> Video Memory      16-bit $FF9D
> $6C000            $D800             MMU block $FFA1  normal value $36
> default
> $72000            $E400             MMU block $FFA1  normal value $39
> $74000            $E800                  "                        $3A
> etc.
>
> This is similar to the video addressing in Coco 1or2 mode controlled by
> $FFC8-$FFD3 where the VMem/$200=BIT value, even=OFF odd=ON
> Video Memory      bit value
> $2000             %10000 ie poke $FFD2, $FFD0, $FFCF, $FFCC, $FFCA, $FFC8,
> $FFC6
>                                    0      0      1       0      0     0
>   0
> $FE00           %1111111           1      1      1       1      1     1
>   1
>
>
> Robert
>
>
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