System information

The following field abbreviations are used in Figure 6-4:
-SPT is the total number of sectors per track.
-BSH is the data allocation block shift factor, determined by the data block allocation
size.
-BLM is the data allocation block mask (2[BSH-1]).
-EXM is the extent mask, determined by the data block allocation size and the number of
disk blocks.
-DSM determines the total storage capacity of the disk drive.
-DRM determines the total number of directory entries that can be stored on this drive.
-AL0, AL1 determine reserved directory blocks.
-CKS is the size of the directory check vector.
-OFF is the number of reserved tracks at the beginning of the (logical) disk.
The values of BSH and BLM determine the data allocation size BLS, which is not an entry in the
DPB. Given that the designer has selected a value for BLS, the values of BSH and BLM are
shown in Table 6-8.
Table 6-8. BSH and BLM Values
BLS BSH BLM
1,024 3 7
2,048 4 15
4,096 5 31
8,192 6 63
16,384 7 127
where all values are in decimal. The value of EXM depends upon both the BLS and whether the
DSM value is less than 256 or greater than 255, as shown in Table 6-9.
6.10 Disk Parameter Tables CP/M Operating System Manual
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