[Simh] Odd Question on CP/M and sector sizes

[Ton van Overbeek]

Chris,

A bit of Googling showed that the 137 byte physical sector size came from
The Altair 88 disk controller.
Search for 'Altair 88 dcdd disk controller 137¹

HTH
Ton

From:  Simh <simh-bounces at trailing-edge.com> on behalf of Chris Van
Brederode <cvanbrederode at gmail.com>
Date:  Friday 21 August 2015 18:45
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.

Looking at this makes me cock my head like a dog.  What is up with the extra
9 bytes?  Where do they go when the fdc reads a 128 sector to memory for
CP/M 3?

Sorry if I sound ignorant, but I've actually tried to research this, but the
only place I can find that mentions 137 byte sectors is the source file, and
one post on this list that mentions that it is 137 bytes but doesn't say
anything beyond that.

Thanks for any help,

Chris
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