Commodore 1541-II - Using Relative Files: Record

Notes:
1. Do not precede the drive number with the "at" sign (@); there is no reason
to replace a relative file.
2. ,L ," + CHR$(record length) is only required when a relative file is first
created, though it may used later, so long as the "record length" is the same as
when the file was first created. Since relative files may be read from or written to
alternately and with equal ease, there is no need to specify Read or Write mode
when opening a relative file.
3. "file #", "device #" and "channel #" must be valid numeric constants,
variables or expressions. The rest of the command must be a valid string literal,
variable or expression.
4. Only 1 relative file can be open at a time on the 1541, although a sequential
file and the command channel may also be open at the same time.
EXAMPLES:
To create or re-open a relative file named "GRADES", of record length 100, use
OPEN 2,8,2,"GRADES,L," +CHR$(100)
To re-open an unknown relative file of the user's choice that has already been
created, we could use
200 INPUT"WHICH FILE";FI$
210 OPEN 5,8,5,FI$
USING RELATIVE FILES: RECORD#
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 2 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. (As this
implies, each relative 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. Properly used, it also
avoids a subtle error (bug) common to all Commodore disk drives.
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