Commodore 1581 - Chapter 10: 1581 Internal Operation; Logical Versus Physical Disk Format; Track Cache Buffer

CHAPTER 10
1581 INTERNAL OPERATION
This chapter describes some of the internal operations of the
1581—how things work on the 'other side of the fence' of the host
computer. Experienced programmers may find this information use
ful. The information learned in previous chapters (especially Chapter
7) combined with that presented in this chapter provides a wide realm
of possibilities for the creative and persistent programmer. Please be
reminded, however, of the NOTE presented at the beginning of Chap
ter 7 regarding future compatibility.
Logical versus Physical Disk Format
All DOS operations of the 1581 are done in 256 byte blocks. These
blocks appear as individually numbered! sectors on the disk. By going
through the DOS interface via the cofrimands outlined in this manual,
the logical disk format is as follows:
single sided
80 tracks (track 1 through track 80)
40 256 byte sectors per track (sector 0 through sector 39)
Internally, however, the 1581 has a different view of things. The
disk is actually formatted as follows:
double sided (side 0 and side 1)
80 tracks per side (track 0 through track 79)
10 512 byte sectors per track (sector 1 through sector 10)
That is the physical disk format created whenever a HEADER
(NEW) command is sent to the 1581 from the host computer. The phys
ical format is different from the logical so that more data can be
squeezed onto each disk.
All commands sent from the host computer are parsed through
the DOS (except the BURST commands) and refer to the logical format.
Software inside the 1581 automatically takes care of the logical to phys
ical translations necessary to retrieve the data properly from the disk.
Track Cache Buffer
One of the improvements in the 1581 design over previous Com
modore disk drives which makes the device more efficient is the Track
101 \