Using Relative Files - Commodore 1541 User Manual

USING RELATIVE FILES
In order to OPEN a relative file once it exists, the format is simpler.
FORMAT FOR OPENING AN EXISTING RELATIVE FILE:
OPEN file#, device^, channels, "name"
In this case, the DOS automatically knows that it is a relative file. This syntax, and
the one shown in the above section, boih allow eilher reading or writing to the file.
In order to read or write, you must, before any operation, position the file pointer
to the correct record position.
FORMAT FOR POSITION COMMAND:
PRINTSfileS, "P" CHRS{channel» + 96) CHRS(rec#lo) CHR$(rec#hi)
CHR$(position)
ECAMPLES OF POSITION COMMAND:
PRINTSI5, "P" CHR$(2'+96VCHRS(]) CHR$(O)
PRINTSI5, "P" CHR$(CH + 96))CHR$(R1) CHRS(R2)
tg—^ *r ~^^ i^~l Position ^
PR1NTS15, "P" CHRS(4+96) CHRS(Rl) CHRS(R2) CHRS(P) (within Record
The 2-byte format for the record number is needed because one byte can only hold
256 different numbers, and we can have over 700 records in the file. The rccfl lo con
tains the least significant part of the address, and the recS hi is the most significant
part. This could be translated to the actual record number by the formula
REC# = REC HI • 256 + REC LO.
If the record number is known, the high and low bytes can be determined as
follows:
RECfl HI = INT{REC#/256)
REC# LO=REC#-REC# HI'256
EXAMPLE:
PR1NTS15, "P" CHRS(4 + 96) CHR$(R1) CHR$(R2) CHR$(0)
If REC# = 54O: R2= INT(54O/256) . . . so R2 = 2
R1 = 54O-R2'256 . . . so R]=28
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