MDOS uses single sided or double sided 8-inch soft sectored single
density diskettes with the IBM 3740 format: 77 cylinders of 26 sectors of
128 bytes. 2002 sectors for a single sided diskette and 4004 sectors for a
double sided diskette.
Space is allocated in clusters. Each cluster has four sectors and is 512
Total clusters on a single sided diskette is 500, with two unused sectors
left over at the end of the disk. Double-sided diskettes have exactly 1001
These sectors are reserved:
- Sector 0
- Diskette ID block
- Sector 1
- Cluster Allocation Table (or CAT) bitmap. 1 means
allocated. Bit 7 of byte 0 is the first cluster.
- Sector 2
- Lock-out cluster allocation table
- Sectors 3 - 22
- Directory - 20 sectors
- Sectors 23 - 24
- Boot block and MDOS RIB
Each directory entry is 16 bytes, so there can be up to 160 files on a
diskette. Each directory entry looks like this:
- 0 - 7
- File name
- 8 - 9
- 10 - 11
- Sector number of first cluster
- 12 - 13
- File attributes
- 14 - 15
Sectors within a file are addressed by LSN- Logical Sector Number. LSN
$FFFF is the first sector of the first cluster. It contains the the file's
RIB- Retrieval Information Block. The first data sector is LSN $0000.
Files are composed of up to 57 contiguous "extents", where each extent
can have up to 32 clusters. This works out to a maximum file size of 912 KB
(but the largest disk is 500.5 KB).
The RIB contains a list of SDWs- 16-bit words which give the starting
cluster number and size of each extent. The RIB also contains the load
address and starting execution address of binary files.