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

[Chris Van Brederode]

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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