[Simh] Simh Digest, Vol 139, Issue 23

[Jon Elson]

>
>
> Message: 1
> Date: Fri, 21 Aug 2015 12:45:43 -0400
> From: Chris Van Brederode <cvanbrederode at gmail.com>
> To: simh at trailing-edge.com
> 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