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Basic Disk Operating System (EDOS) Functions
4-28
SET RANDOM RECORD
ENTRY
CL: 24H
DX: FCB
Offset
FUNCTION 36
SET RANDOM
RECORD
RETURN
Random Record
Field Set
The Set Random Record function returns the random record position of the next
record to be accessed from a file which has been read or written sequentially to a
particular point. The function can be useful in two ways.
First, it can be used to create a table of key field values and the corresponding record
addresses.
It
is often necessary to first read and scan a sequential file to extract the
positions of various "key" fields. As each key is encountered, function 36 is called to
compute the random record position for the data corresponding to this key.
If
the
data unit size is 128 bytes, the resulting record position minus one is placed into a
table with the key for later retrieval. After scanning the entire file and tabularizing
the keys and their record numbers, programs can move instantly to a particular
keyed record by performing a random read using the saved random record number.
The scheme is easily generalized when variable record lengths are involved since the
program need only store the buffer-relative byte position along with the key and
record number in order to find the exact starting position of the keyed data at a later
time.
A second use of function 36 occurs when switching from sequential read (or write)
to random read (or write). A file is sequentially accessed to a particular point,
function 36 is called to set the record number, and subsequent random read (write)
operations continue from the next record in the file.
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