System information

Position 00 corresponds to the high-order bit of the byte labeled AL0 and 15 corresponds to the
low-order bit of the byte labeled AL1. Each bit position reserves a data block for number of
directory entries, thus allowing a total of 16 data blocks to be assigned for directory entries (bits
are assigned starting at 00 and filled to the right until position 15). Each directory entry occupies
32 bytes, resulting in the following tabulation:
Table 6-10. BLS Tabulation
BLS Directory Entries
1,024 32 times # bits
2,048 64 times # bits
4,096 128 times # bits
8,192 256 times # bits
16,384 512 times # bits
Thus, if DRM = 127 (128 directory entries) and BLS = 1024, there are 32 directory entries per
block, requiring 4 reserved blocks. In this case, the 4 high-order bits of AL0 are set, resulting in
the values AL0 = 0F0H and AL1 = 00H.
The CKS value is determined as follows: if the disk drive media is removable, then CKS =
(DRM + 1)/4, where DRM is the last directory entry number. If the media are fixed, then set
CKS = 0 (no directory records are checked in this case).
Finally, the OFF field determines the number of tracks that are skipped at the beginning of the
physical disk. This value is automatically added whenever SETTRK is called and can be used as
a mechanism for skipping reserved operating system tracks or for partitioning a large disk into
smaller segmented sections.
To complete the discussion of the DPB, several DPHs can address the same DPB if their drive
characteristics are identical. Further, the DPB can be dynamically changed when a new drive is
addressed by simply changing the pointer in the DPH; because the BDOS copies the DPB values
to a local area whenever the SELDSK function is invoked.
Returning back to DPH for a particular drive, the two address values CSV and ALV remain.
Both addresses reference an area of uninitialized memory following the BIOS. The areas must be
unique for each drive, and the size of each area is determined by the values in the DPB.
6.10 Disk Parameter Tables CP/M Operating System Manual
6-33