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Appendix I
Blocking and Deblocking
Algorithms
Upon each call to the BIOS WRITE entry point, the CP/M-86 BDOS includes
information that allows effective sector blocking and deblocking where the host disk
subsystem has a sector size which is a multiple of the basic 128-byte unit. This
appendix presents a general-purpose algorithm that is included in the CBIOS and
that uses the BDOS information to perform the operations automatically.
Upon each call to WRITE, the BDOS provides the following information in register
CL:
0= normal sector write
1
=
write to directory sector
2
=
write to the first sector
of a new data block
Condition 0 occurs whenever the next write operation is into a previously written
area, such as a random mode record update, when the write is to other than the first
sector of an unallocated block, or when the write is not into the directory area.
Condition 1 occurs when a write into the directory area is performed. Condition 2
occurs when the first record (only) of a newly allocated data block is written. In most
cases, application programs read or write mUltiple 128-byte sectors in sequence, and
thus there is little overhead involved in either operation when blocking and deblock-
ing records since pre-read operations can be avoided when writing records.
This appendix briefly describes the blocking and deblocking algorithm (the file is
included on your CP/M-86 system diskette). Generally, the algorithms map all
CP 1M sector read operations onto the host disk through an intermediate buffer
which is the size of the host disk sector. Throughout the program, values and
variables which relate to the CP 1M sector involved in a seek operation are prefixed
by "sek", while those related to the host disk system are prefixed by "hst".
1-1
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