[Coco] 64K Coco mode and the MC6883

[gene heskett]

On 3/28/23 01:24, RETRO Innovations via Coco wrote:
> On 3/28/2023 12:17 AM, RETRO Innovations via Coco wrote:
> 
> 
> I also think S=7 is the "don't select anything" choice.  Routing another 
> pin to disable the 74ls138 would have pushed over the pin limit, and if 
> they only needed 7 decodes, the 8th could be the "don't use this" decode.
> 
> Jim
> 
> 
And that comes full circle, to the bitch I've had for 30 some years Jim.

All of the peripheral stuff we've ever used on the coco has a max 
requirement of 4 address registers, The fact that the ls138 only decoded 
to a $20 byte equ, 32 bytes wide, has crippled the number of i/o 
accessories usable. Just think of the i/o possibility's available had 
the decoding been down to the 4 byte wide level. Yet no one, in their 
wildest dreams about a coco4, has proposed a fix for that huge, glaring 
lack of adequate i/o decoding.

Is there a reason for that I've never understood? If there is, please 
explain it to me.

The only thing I can imagine is a rewrite of os9 to include a full 
remapping of its page 0 use, using up all of lsn0 on the disk.  With 
modern SSD disks, I don't see that as a problem.  Traffic on spinning 
rust, particularly on floppies where heads actually touch the disk, yes, 
a huge multipass erasure problem.

Cheers, Gene Heskett.
-- 
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