Commodore 2040 User Manual page 65

Not to scale
NOTE:
POINTERS TO LINK
TOGETHER ALL BLOCKS
WITHIN A FILE
/
/
/
/
/
+ +
~
0
,...,
CHECK-
"" ""
254 BYTES CHECK- GAP
SYNC
08 IDI
ID2 TRACK SECTOR GAP I SYNC 07
t-< t-<
SUM
>< ><
OF DATA SUM
2
I:Q
I:Q
Figure 5 - 8050 Format: Expanded View of a Single Sector
Figure
5
illustrates an expanded view of a single sector on a diskette formatted for the
8050.
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
8050
maintains system files on tracks
38
and
39
which contain the BAM, diskette
name, ID, and file directory. The BAM, resident in the first
255
bytes of sectors
0
and
3
of
track
38,
monitors available and occupied storage locations on diskette. The file directory
header block, beginning on sector 0 of track
39,
contains the diskette name and ID within
the first 30 bytes.
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