On a CBM disk the structure of a sector is as follows:
1) "sync" bytes
2) header block
3) "sync" bytes (same as 1)
4) data block
5) inter-sector gap
1) consists of 10 of more 1's in a row (usually 40). These are not GCR encoded.
2) a) HEADER BLOCK ID: this is always 8 and indicates that this is a header.
b) HEADER BLOCK CHECKSUM: this is an exclusive OR of tracknumber, sector-
number and the 2 id bytes
c) SECTOR NUMBER
d) TRACK NUMBER
e) first ID BYTE
f) second ID BYTE
g) two so called OFF bytes, value is 0F hex . These are for padding and
never read by the drive.
h) HEADER GAP: consists of eight 55 hex bytes, not GCR encoded. This is
breathing room for the drive, they are never read
3) a) DATA BLOCK ID: this is always 7 and indicates that this is a data block
b) 256 data bytes
c) DATA BLOCK CHECKSUM: an exclusive OR of all data bytes
d) two so called OFF bytes, value is 0. They are for padding and never read
by the drive.
4) The gap has a varying length and is never read (the drive just waits for the
next SYNC character.
The GCR table used is as follows:
nybble in memory 5-bit value on drive
Please E-mail me directly, since I'm not a member of this list. My address is:
Jos Grote Punt
Dept. of Geography
University of Nymegen
U267105 AT HNYKUN11.BITNET
Thanks in advance,