Commodore 1581 - Using Relative Files: RECORD# Command

BASIC 2.0: 200 INPUT'WHICH FILE";FI$
210 OPEN 5,8,5,FI$
USING RELATIVE FILES: RECORD# COMMAND
When a relative file is opened for the first time, it is not quite
ready for use. Both to save time when using the file later, and to assure
that the file will work reliably, it is necessary to create several records
before closing the file for the first time. At a minimum, enough
records to fill more than two disk sectors (512 bytes) should be
written. In practice, most programs go ahead and create as many
records as the program is eventually expected to use. That approach
has the additional benefit of avoiding such problems as running out of
room on the diskette before the entire file is completed.
If you simply begin writing data to a just-opened relative file, it
will act much like a sequential file, putting the data elements written
by the first PRINT# statement in Record #1, those written by the
second PRINT# statement in record #2 and so on. This means each
record must be written by a single PRINT# statement, using embed
ded carriage returns within the data to separate fields that will be read
in via one or more INPUT# statements later. However, it is far better
to explicitly specify which record number is desired via a RECORD#
command to the disk. This allows you to access records in any desired
order, hopping anywhere in a file with equal ease.
FORMAT FOR THE RECORD# COMMAND:
BASIC 7.0: RECORD # file #, record number [,offset]
BASIC 2.0: PRINT#15, "P" + CHR$ (channel # + 96) + CHR$
(<record #) + CHR$(>record #) + CHR$(offset)
where "file #" is the file # specified in the current DOPEN statement
for the specified file, "record number" is the desired record number,
"channel #" is the channel number specified in the current OPEN
statement for the specified file, "<record #" is the low byte of the
desired record number, expressed as a two-byte integer, ">record #"
is the high byte of the desired record number, and an optional "offset"
value, if present, is the byte within the record at which a following
Read or Write should begin.
To fully understand this command, you must understand how
most integers are stored in computers based on the 6502 and related
microprocessors. In the binary arithmetic used by the microprocessor,
it is possible to express any unsigned integer from 0-255 in a single
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