[sdiy] Re: PCSYNTH Web Page Up

[Scott Gravenhorst]

Helen Austin <gwenhwyfaer at gmail.com> wrote:
>On 6/8/05, Scott Gravenhorst <music.maker at gte.net> wrote:
>> There is also an explanation as to why I designed an 8 bit memory cycle ISA
>> card instead of a 16 bit memory cycle ISA card.
>
>Just a quick comment. You plan to decode the addresses so that
>$B8000 and $B8002 are the low and high byte of the DAC. Why not decode
>them so that $B8000 is the low byte and $B8001 is the high? 

Heh heh, I knew this would come up.  I think that it may end up that way.  If
I'm not mistaken, when writing to an 8 bit ISA card with a 16 bit transfer, it
automatically breaks it up into two 8 bit transfers both presented on data bus
bits <0:7>, so I can save a couple of bytes of code.  Part of this was simple
uncertainty over the whole big-endian vs. little-endian nature of the box. 
Does the odd address always translate to the high byte?  If so, I'll just make
that change and start from there instead of using address bit 1.

Please note, none of this is final yet.  I will build the interface in stages
and test to confirm the design.  

>That way
>you can use a single 16-bit MOV to write to the DAC; the ISA bus
>management should translate them automagically. Then the code
>becomes shorter, and you can get more of it into the CPU's cache.
>(It also becomes simpler and more direct to write.)

Yes yes.  As I said, it will _probably_ end up that way, unless it doesn't work.

>Helen (following this thread avidly!)

Uh oh, now I'm on the spot...  (c:

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