Commodore 1541 - Random Files; Track Number; Total Block

6.
RANDOM FILES
Sequential files are Tine when you're just working with a continuous stream of
data, but some jobs require more ihan that. For example, with a large mailing list, you
would not want to have to scan through the entire list to Find a person's record. For
this you need some kind of random access method, some way to get to any record in
side a Tile without having to read through the entire file first.
There arc actually two different types of random access files an the Commodore
disk drive. The relative files discussed in the next chapter are more convenient for data
handling operations, although the random files in this chapter have uses of their own,
especially when working with machine language.
Random files on the Commodore disk drive reach the individual 256-byic blocks
of
data stored on the disk. As was mentioned in the first chapter, there are a total of
6S3 blocks on the diskette, of which 664 are free on a blank diskette.
The diskette 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 3 5 at the center, Track 18 is used for the directory, and ihe DOS
fills up the diskette from the center outward.
Each track is subdivided into blocks. Because there is more room on the outer
tracks, theie are more blocks there. The outer tracks contain 21 blocks each, white the
inner ones only have 17 blocks each. The table below shows the number of blocks per
track.
Table 6.1: Track and Block Format
TRACK NUMBER
1 SO 17
18 to 24
25 to 30
31 to 35
BLOCK RANGE
0to20
Ota 18
Oto 17
Oto 16
TOTAL BLOCK
2]
19
IS
17
The DOS contains commands for reading and writing directly lo any track and
block on the diskette. There are also commands for checking to see which blocks are
available, and for marking off used blocks.
These commands arc transmitted through the command channel (channel^ ]5),
and tell the disk what to do with the data. The data must be read later through one of
the open data channels.