Deciphering an odd floppy disk format.

[Fred Cisin]

On Mon, 15 Feb 2021, Mattis Lind via cctalk wrote:
> My guess is that the data that follows the sector ID is some kind of
> checksum.

yes.  well, sorta. 16 bit CRC


A typical IBM/WD style format has:

a gap
Index Address Mark
a gap   (note that WD can use a shorter post index gap than NEC can)

and then, repeated for each sector:
{
a gap
ID Address Mark, 
C Cylinder number
H Head number    (0,1)
R Sector number
N Sector Length  (0:128, 1:256, 2:512, 3:1024)
16 bit CRC of the sector header
a gap
Data Address Mark
Sector data  (128, 256, 512, or 1024 bytes.  Larger is possible, but unlikely)
16 bit CRC of the data
a gap
}

(This is from memory (error prone?).  The WD1791 datasheet should have 
more detail, including CRC algorithm?, the specific requirements for the 
address marks, and gap contents (write splice, synchronization, etc.))


--
Grumpy Ol' Fred     		cisin at xenosoft.com