Commodore 2040 User Manual page 64

NOTE:
Not to scale
POINTERS TO LINK
TOGETHER ALL BLOCKS
WITHIN A FILE
/
/
/
/
/
• t
~
0
,...;
CHECK-
r..:I r..:I
254 BYTES CHECK- GAP
SYNC
08 IDl
ID2 TRACK SECTOR
GAP 1 SYNC 07 E-<
E-<
SUM
><
><
OF DATA SUM 2
P=I P=I
Figure 4 - 2040,3040,4040 Format: Expanded View of a Single Sector
Figure 4 illustrates an expanded view of a single sector on a diskette formatted for the 2040.
In addition to other information, each sector contains a data block consisting of 256 stored
characters. Blocks within the same file are linked together by means of a two character
block pointer. By pointing to the location of the next data block, block pointers enable the
system to retrieve data from non-contiguous blocks. Retrieving the first data block within a
file triggers a search for the next data block which, in turn, utilizes block pointers to locate
related blocks until the entire file is assembled and made available for display. All PRG,
SEQ, and USR files utilize this format.
A data block is addressed by track and sector. A 2040 diskette contains 35 tracks (or rings)
numbered 1 to 35 while an 8050 diskette contains 77 tracks numbered 1 to 77. The number
of sectors per track will vary (as illustrated in Table 6) due to differences in track circum-
ference and recording frequency.
The 2040 maintains a system file on track 18 which contains the BAM, diskette name, ID,
and file directory. The BAM, resident in the first 128 bytes of sector 0, monitors available
and occupied storage locations on diskette. The last 128 bytes of sector 0 are used to store
the diskette name and ID. The file directory begins on the next sector, sector 1.
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