Commodore 1581 - Chapter 6 Direct Access Commands; Opening a Data Channel for Direct Access

CHAPTER 6
DIRECT ACCESS COMMANDS
Direct access commands specify individual sectors on the dis
kette, reading and writing information entirely under your direction.
This gives them almost complete flexibility in data-handling programs,
but imposes tremendous responsibilities on the programmer. As a
result, they are normally used only in complex commercial programs
able to properly organize data without help from the disk drive itself.
A far more common use of direct access commands is in utility
programs used to view and alter parts of the diskette that are not
normally seen directly. For instance, such commands can be used to
change the name of a diskette without erasing all of its programs, to
lock a program so it can't be erased, or hide your name in a location
where it won't be expected.
OPENING A DATA CHANNEL FOR DIRECT ACCESS
When working with direct access data, you need two channels
open to the disk: the command channel we've used throughout the
book, and another for data. The command channel is opened with the
usual OPEN 15,8,15 or equivalent. A direct access data channel is
opened much like other files, except that the pound sign (#), option
ally followed by a memory buffer number, is used as a file name.
FORMAT FOR DIRECT ACCESS FILE OPEN STATEMENTS:
OPEN file #,device #, channel #, "#buffer #"
where "file #" is the file number, "device #" is the disk's device num
ber, normally 8; "channel #" is the channel number, a number be
tween 2 and 14 not used by other files open at the same time; and
"buffer #," if present, is a 0,1, 2, 3, 4, 5, or 6, specifying the memory
buffer within the 1581 to use for this file's data.
EXAMPLES:
To specify which disk buffer to use:
OPEN 4,8,4,"#2" -
If you don't specify which to use (OPEN 5,8,5,"#"), the 1581 selects
one.
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