Completing Relative File Creation - Commodore 1551 User Manual

450 INPUT"RECORD # DESIRED";RE
460 IF RE< lOR RE>65535 THEN 450
470 RH = INT(RE/256)
480 RL= RE-256*RH
490 PRINT#15, "P" +CHR$ (98) +CHR$ (RL) +CHR$ (RH)
Assuming RH and RL are calculated as in the previous example, programs may also use
variables for the channel, record, and offset required:
570 INPUT "CHANNEL, RECORD,
&
OFFSET DESIRED";CH,RE,OF
630 PRINT# 15, "P" + CHR$ (CH + 96) + CHR$ (RL) + CHR$ (RH) + CHR$ (OF)
ANOTHER RECORD# COMMAND
Basic 4.0 on Commodore's PET and CBM models includes a Basic Record#
command not found in any of the serial bus computers. However, some available
utility programs for these models include it. It serves the same function as the
Record# command explained above, but has a simplified syntax:
RECORD#file # ,record # ,offset
where "file #" is the relative file number being used, not the command channel's
file, "record #" is the desired record number, and "offset" is as above.
If you see a Record# command written in Basic 4 form in a program you want
to use, simply convert it into the usual form for both Basic 2 and 3.5 described in
this section.
COMPLETING RELATIVE FILE CREATION
Now that we have learned how to use both the Open and Record# commands, we are
almost ready to properly create a relative file. The only additional fact we need to know is
that CHR$(255) is a special character in a relative file. It is the character used by the DOS
to fill relative records as they are created, before a program fills them with other
information. Thus, if we want to write the last record we expect to need in our file with
dummy data that will not interfere with our later work, CHR$(255) is the obvious choice.
Here is how it works in an actual program which you may copy for use in your own
relative file programs.
1020 OPEN 15,8,15
1380 INPUT"ENTER RELATIVE FILE NAME";FI$
1390 INPUT"ENTER MAX. # OF RECORDS";NR
1400 INPUT"ENTER RECORD LENGTH";RL
48
Open command channel
Select file parameters