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BIOS Disk Definition Tables
7-6
Position 00 corresponds to the high order bit of the byte labeled ALO, and 15
corresponds to the low order bit of the byte labeled ALl. Each bit position reserves a
data block for a number of directory entries, thus allowing a total of 16 data blocks
to be assigned for directory entries. The bits are assigned starting at 00 and filled to
the right until position 15. Each directory entry occupies 32 bytes, as shown in Table
7-6.
Table 7-6 BLS and Number of Directory Entries
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 is 127 (128 directory entries), and BLS is 1024, there are 32 directory
entries per block, requiring four reserved blocks. In this case, the four high order bits
of ALO are set, resulting in the values ALO = OFOH and ALl = OOH.
The CKS value is determined as follows .
• If the disk drive medium is removable, CKS = (DRM+I)/4, where DRM is
the last directory entry number .
• If the medium is fixed, CKS=O, and no directory records need to be checked.
The OFF field determines the number of tracks which 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.
NOTES
• Several D PH's can address the same D PB if
their drive characteristics are identical.
• The DPB can be dynamically changed when a
new drive is addressed by changing the pointer
in the DPH. The BDOS copies the DPB values
to a local area whenever the SELDSK function
is invoked.
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