Commodore 1541 - Page 35

OPENING A DATA CHANNEL FOR RANDOM ACCESS
When working with random access files, you need to have 2 channels open to the
disk: one for the commands, and the other for the data. The command channel is
OPENed to channel 15, just Uke other disk commands you've encountered so far. The
data channel for random access files is OPENed by selecting the pound sign (#) as the
file name.
FORMAT FOR OPEN STATEMENT FOR RANDOM ACCESS DATA:
OPEN fileS, device*, channel*. "#"
or optionally
OPEN file#, devke#, channel*, "#buffer#"
EXAMPLES OF OPENING RANDOM ACCESS DATA CHANNEL:
OPEN 5, 8, 5, "#" ^(dON'T CARE WHICH BUFFER
OPEN A, B, C, "#2" _^/F
BLOCK-READ
FORMAT FOR BLOCK-READ COMMAND:
PRINTSfiktf, "BLOCK-READ:" channel; drive; track; block
or abbreviated as
PRINT#file#, "B-R:" channel; drive; track; block
This command will move one block of data from the diskette into the selected
channel. Once this operation has been performed, the INPUTS and GET# statements
can read the information.
SAMPLE PROGRAM TO READ BLOCK 2 FROM TRACK 18: (stores contents in
BS)
DRIVE
(track)
10 OPEN15.8.15 \ >—ZLs-S /~^~^
20 OPEN5,8,5,"r \b/jS^ f BLOCK J
30 PRINT#15."B-R:"5;O;18;2 V^v_—-
40 BS-""
50 FOR L = 0TO255
60 GET«,AS
70 IF ST = 0 THENBS - BS + AS:NEXTL
SO PRINT "FINISHED"
90 CLOSES:CLOSE 15 \COLLECT ENTIRE
BLOCK:BYTE
BY BYTE