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Basic Disk Operating System (BDOS) Functions
4-24
READ RANDOM
ENTRY
CL: 2lH
DX: FCB
Offset
RETURN
FUNCTION 33
AL: Return Code
READ RANDOM
The Read Random function reads a record at a particular record number. The
record number is selected by the 24-bit value constructed from the three-byte field
(rO, rl, r2) in the FCB at byte positions 33, 34, and 35. The sequence of 24 bits is
stored with least significant byte first (rO), middle byte next (rl), and high byte last
(r2). CPIM does not reference byte r2, except in computing the size of a file
(function 35). Byte r2 must be zero, however, since a non-zero value indicates
overflow past the end of file.
The rO, rl byte pair is treated as a double-byte, or "word" value, which contains the
number of the record to read. This value ranges from 0 to 65535, providing access to
any particular record of any size file. In order to access a file using the Read
Random function, the base extent (extent 0) must first be opened. Although the base
extent mayor may not contain allocated data, this ensures that the FCB is properly
initialized for subsequent random access operations. The selected record number is
then stored into the random record field (rO, r 1), and the BDOS is called to read the
record. Register AL returns either an error code (see Table 4-4) or the value 00
indicating the operation was successful. In the latter case, the buffer at the current
DMA address contains the randomly accessed record. Unlike the sequential read
operation, the Random Read function does not advance the record number. Thus,
subsequent random read operations continue to read the same record.
Each random read operation automatically sets the logical extent and current
record values. Thus, the file can be sequentially read or written starting from the
current randomly accessed position. Note, however, that as a program switches
from random mode to sequential, the last randomly read record is reread. Similarly,
the last record is rewritten as a program switches from a random to sequential write
operation. To obtain the effect of a sequential 1/0 operation, advance the random
record position following each random read or write.
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