Intel 8272 based floppy controller & Missing Address Mark

Fred Cisin cisin at
Thu Nov 28 00:08:04 CST 2019

I'll agree with Chuck, that FM/MFM seems a likely possibility.

index blindness can prevent seeing the first sector, but it should see 
other sectors.

What is on the disk that you are trying to read?

You are using an 8" drive, but you mention settings for 720K. 
720K (MFM) is normally 250K bts per second (300 K at 360 RPM in a 1.2M)
8" (MFM) is normally 500K bits per second.
8" FM is 250K bits per second. 
Is the data transfer rate consistent with what you are trying to read?

Do you have a scratch disk that you can spare?
Set up the code to FORMAT a track (eventually appropriate loops to format 
the whole disk).
After formatting a track, and without doing any stepping to another track, 
try to read it.  FM/MFM and data transfer rates and even sector 
header (CHRN) should therefore be compatible.
If you have another machine, try reading THAT track on the other machine 
and check what it sees.

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