Message: 1
Date: Fri, 21 Aug 2015 12:45:43 -0400
From: Chris Van Brederode <[email protected]>
To: [email protected]
Subject: [Simh] Odd Question on CP/M and sector sizes
Hello,
I'm working on coding my own 8080 emulator (as an exercise/hobby) and I've
gotten to the point where I need to build BIOS for CP/M. I'm looking at
using a SIMH based Altair emulator to build the CP/M for my system.
I have a weird question though. All the documentation I can find on CP/M
and floppies from that era point to a 128 byte sector size. But the source
code in simh/Altair/altair_dsk.c (and also in the AltairZ80 source), the
sector size is given as 137 bytes.
Wow, I've never run into that. CP/M used a logical sector
of 1024 bytes, IIRC. They mapped this onto however many
physical sectors the available media/controller provided.
The only thing I can think of is the sector header is in the
data portion (where usually it would be in a separate spot
on the track.) This would save space on the media, as you
would not have to have an erase gap before and after the
sector header, but it would require overwriting the sector
header+data sector any time you wrote a sector.
I've never come across that. I looked at the backup files
from my CPM system, and at least for the hard drive I added,
the logical sectors were definitely 1024, while the physical
sectors were 256. I couldn't quite dope out what the floppy
controller did.
Jon
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