30 May
2000
30 May
'00
10:07 a.m.
I always figured the reason for the restriction was
dumb boot PROMs, which
only know how to do programmed I/O to the FDC, and 8" DD comes in too fast
for typical 8-bit CPUs of the time to handle with PIO. If the boot PROM on
a particular system is smart enough to set up DMA, no need to require SD.
it's an artifact of how people though the DDmedia was speced by IBM and
a lack of knowledge of CP/M boot as everyone just followed the book blindly.
Not all required {or even had it!} DMA to do DD, CCS didn't.
I'd add that DMA was mostly uncommon save for the more refined or robust
systems.
Allison
30 May
30 May
12:20 p.m.
See comments embedded below, plz.
----- Original Message -----
From: allisonp <allisonp(a)world.std.com
To: <classiccmp(a)classiccmp.org
Sent: Tuesday, May 30, 2000 4:07 PM
Subject: Re: Defining Disk Image Dump Standard
blindly.
>I always figured the reason for the restriction
was dumb boot PROMs,
which
>only know how to do programmed I/O to the FDC, and
8" DD comes in too
fast
>for typical 8-bit CPUs of the time to handle with
PIO. If the boot PROM
on
a particular
system is smart enough to set up DMA, no need to require SD.
it's an artifact of how people though the DDmedia was speced by IBM and
a lack of knowledge of CP/M boot as everyone just followed the book
Not all required {or even had it!} DMA to do DD, CCS didn't.
That's proof that DMA was unnecessary! A DMAC cost quite a bit back then.
I'd add that DMA was mostly uncommon save for the
more refined or robust
systems.
Allison
I don't see how adding an unnecessary part makes a system more robust or
refined. It was quite straightforward to write and run FDC handlers that
used programmed I/O quite adquately and since CP/M and other simple OS'
seldom did anything useful with the small segments of saved time, (12
u-sec's per byte) it didn't help. Some other devices might have justified
DMA, but floppy drives actually didn't unless they were used with an 8080 or
other slow processor.
8973
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8973
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