Intel DD format differences

[Eric Smith]

Steve Thatcher wrote about Intel double density (M2FM) disk format:
> I referred to the 1793 technical spec that outlined what the 3740 and
> system 34 formats are. Those manuals are also out on bitsavers under
> Western Digital. I saw no reference to PLO sync bytes or the special gap
> bytes in the SD format.

The gaps (PLO sync) and marks look different to me:


                IBM 3740 (FM)     IBM System 34 (MFM)    Intel DD (M2FM)

gap 1:          40 * 00 or FF       80 * 4E                46 * ??
                 6 * 00             12 * 00

index mark:      1 * FC (Ind Mk)     3 * C2                 1 * 0C
                                     1 * FC

gap 2:          26 * 00 or FF       50 * 4E                18 * 00
                 6 * 00             12 * 00                10 * FF

addr mark:       1 * FE (IDAM)       3 * A1                 1 * 0E
                                     1 * FE

                 1 * trk num         1 * trk num            1 * trk num
                 1 * 00 (side num)   1 * side num           1 * 00
                 1 * sector num      1 * sector num         1 * sector num
                 1 * 00 (sect len)   1 * sector len         1 * 00
                 2 * CRC             2 * CRC                2 * CRC

gap 3:          11 * 00 or FF       22 * 4E                 2 * 00
                 6 * 00             12 * 00                10 * FF

data mark:       1 * FB (DAM)        3 * A1                 1 * 0B
                                     1 * FB
               128 * data          256 * data             256 * data
                 2 * CRC             2 * CRC                3 * CRC

gap 4:        ~247 * 00 or FF     ~598 * 4E                 ? * ??