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dos part 9
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2022-08-26
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Dos 'n' Don'ts --- Part 9
by Joel Ellis Rea
--------------------------------------
Last issue, we covered the use of
'wild card' characters in several DOS
Wedge commands to enable them to work
on more than one file at once. That
concluded our discussion of the DOS
File Maintenance, or houskeeping type
commands.
This issue, we will learn how to
actually use the disk drive under
program control. Our first topic will
be the Command/Error channel.
The 1541 Disk Drive supports 16
different 'channels', which can be
considered as separate bi-directional
data paths between the disk drive and
the computer. Think of them like
channels on a CB radio. With the CB,
you can talk to and listen to another
individual by using a channel. You
can only use one channel at any one
time, but by switching your attention
from one to another, you could have
several conversations going simul-
taneously although you could only be
actively talking or listening to one
at a time.
The 1541 has 16 channels, numbered
0 through 15. Channel #15 is a
special channel. Think of it as a
channel to the FCC on a CB. Channel
#15 acts to 'police' the operation of
the disk drive. In fact, whenever you
use the DOS Wedge to send a
maintenance command, it goes through
channel #15. When you ask Dos Wedge
for the disk drive status, the message
comes from the drive through channel
#15. Thus, channel #15 is called the
Command/Error channel.
The 1541 does not allow the use of
all 16 channels at a time. Only three
sequential data channels can be open.
The Command/Error channel does not
count as one of the three.
Channels #0 and #1, although used as
data channels, are special. Channel
#0 is used by BASIC's LOAD command,
and #1 is used by the SAVE command.
The point is, only channels 2-14 are
for general use with data files.
-------- continued in Part 10 --------