The IBM System 34 double density format is a structure used by NEC765 compatible disc controllers when writing in MFM mode.
The data has this form:
The values below are unencoded data bytes. The data is encoded using MFM when written to the disc.
Gap 0 (80 bytes of data 4E) Sync (12 bytes of 0) Index mark IDAM which contains 3 bytes of C2 (which have a missing clock) followed by 1 byte FC. Gap 1 (50 bytes of data 4e)
Then follows the data for each sector:
Sync (12 bytes of 0) ID mark IDAM which contains 3 bytes of A1 (which have a missing clock) followed by 1 byte FE. 1 byte 'C' (defined by FDC format command) 1 byte 'H' (defined by FDC format command) 1 byte 'R' (defined by FDC format command) 1 byte 'N' (defined by FDC format command) 2 bytes CRC (stored big endian, i.e. high 8 bits, then low 8 bits). Gap 2 (22 bytes of 4E)
The CRC for the ID field is computed from the 3 bytes of A1, the 1 byte FE, C,H,R and N. When verifying include the CRC and if the result is 0, then the CRC is correct.
Then comes the data field for the sector:
Sync (12 bytes of 0)
Data mark IDAM which contains 3 bytes of A1 (which have a missing clock) followed by 1 byte FB for Data or F8 for Deleted Data.
'n' bytes of sector data (defined by format when formatting, or by ID field when reading/writing)
2 bytes CRC (stored big endian)
Gap 3 ('n' bytes of data 4E) (defined by FDC format command)
After the last sector:
Gap 4 ('n' bytes of 4E all the way to the end of the track).
NOTE: A1/C2 are the only MFM-words which have a missing clock. All the others are written like normal data bytes.