Commodore 2040 User Manual page 71

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pointers in the side sectors allow the DOS to move from one record to another within two
disk read commands-a considerable savings in the amount of time required to find a de-
sired data block when compared to sequential methods.
A file may contain up to six side sectors and each side sector may contain pointers to 120
data blocks. Therefore, the largest file on the 204 Dual Drive Floppy Disk contains
182,880 bytes (120 pointers/side sector
*
6 side sectors
*
254 bytes/block) which happens
to be greater than the total storage capacity of that particular disk.
Table 16. RELATIVE FILE FORMAT
DATA BLOCK
BYTE DEFINITION
0,1
Track and sector of next data block.
2-256
254 bytes of data. Empty records contain FF (all binary
ones) in the first byte followed by 00 (binary all zeros) to
the end of the record. Partially filled records are padded with
nulls (00).
SIDE SECTOR BLOCK
BYTE DEFINITION
0,1 Track and sector of next side sector block.
2 Side sector number. (0-5)
3
Record length.
4,5 Track and sector of first side sector (number 0)
6,7 Track and sector of second side sector (number 1)
8,9 Track and sector of third side sector (number 2)
10,11 Track and sector of fourth side sector (number 3)
12,13
Track and sector of fifth side sector (number 4)
14,15
Track and sector of sixth side sector (number 5)
16-256 Track and sector pointers to 120 data blocks.
To expand a relative file, a programmer may reference the last record number generated
through the RECORD command and print to that particular record. The intermediate
records from the point of the current end of the file to the reference record number will be
automatically generated by the DOS. This includes any side sectors and all data blocks
necessary to contain a file, regardless of size, but within the capacity limits of the diskette.
For example, if the current size of the relative record is one data block long and the record
number referenced would expand it to 125 blocks, then an additional side sector would be
generated by the DOS since one side sector can only represent 120 data blocks.