Commodore 1541-II - Chapter 7: Direct Access Commands

CHAPTER 7
DIRECT ACCESS COMMANDS
A TOOL FOR ADVANCED USERS
Direct access commands specify individual sectors on the diskette, reading and
writing information entirely under your direction. This gives them almost complete
flexibility in data-handling programs, but also imposes tremendous responsibilities on the
programmer, to be sure nothing goes awry. As a result, they are normally used only in
complex commercial programs able to properly organize data without help from the disk
drive itself.
A far more common use of direct access commands is in utility programs used to
view and alter parts of the diskette that are not normally seen directly. For instance, such
commands can be used to change the name of a diskette without erasing all of its
programs, to lock a program so it can't be erased, or hide your name in a location where it
won't be expected.
DISKETTE ORGANIZATION
There are a total of 683 blocks on a 1541 diskette, of which 664 are available for use,
with the rest reserved for the BAM (Block Availability Map) and the Directory.
The diskette's surface is divided into tracks, which are laid out as concentric circles
on the surface of the diskette. There are 35 different tracks, starting with track 1 at the
outside of the diskette to track 35 at the center. Track 18 is used for the directory, and the
DOS fills up the diskette from the center outward, alternately in both directions.
Each track is subdivided into sectors (also called blocks). Because there is more
room on the outer tracks, there are more sectors per track there. The outermost tracks
contain 21 sectors each, while the innermost ones only have 17 sectors each. The table
below shows the number of sectors per track.
Table 6.1: Track and Sector Format
TRACK NUMBER
1 to 17
18 to 24
25 to 30
31 to 35
SECTOR NUMBERS
0 through 20
0 through 18
0 through 17
0 through 16
TOTAL SECTORS
21
19
18
17
In this chapter we will describe the DOS commands for directly reading and writing
any track and block on the diskette, as well as the commands used to mark blocks as used
or unused. Unless otherwise notes, all direct access commands are the same in both Basic
2 and Basic 3.5.
OPENING A DATA CHANNEL FOR DIRECT ACCESS
When working with direct access data, you-need two channels open to the disk: the
command channel we've used throughout the book, and another for data. The command
channel is opened with the usual OPEN 15,8,15 or equivalent. A direct access data
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