[Coco] 64K Coco mode and the MC6883
gene heskett
gheskett at shentel.net
Tue Mar 28 10:45:02 EDT 2023
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.
--
"There are four boxes to be used in defense of liberty:
soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author, 1940)
If we desire respect for the law, we must first make the law respectable.
- Louis D. Brandeis
Genes Web page <http://geneslinuxbox.net:6309/>
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