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PART TWO:
GUIDE TO ADVANCED OPERATION
AND PROGRAMMING
COMMAND CHANNEL
CHAPTER4
COMMANDS
Commodore disk drives expect to receive many of their instructions over what is
known as a command
channel.
Although we will not explain the concepts behind it until
Chapter 6, we will learn it to use it now, so you can give your 1541 disk the commands it
needs
to do some essential chores.
To instruct the command channel, we use a Basic Open statement to the disk, with a
secondary address of 15. The usual form of this statement is:
OPEN 15,8,15
The first 15 is a file number, and could be any number from 1 to 255. It is used to
match
the secondary address (the last number on the line), which is ·also 15
.
The middle
number
is the primary address, better known as the device number, and is normally 8
when
talking with the 1541. A
second
disk drive would usually be 9, a third, 10 and so on.
Once the
command channel
has been opened, use the Basic Print# command to send
information
to the disk drive, and Basic's Input# command to receive information back
from
the disk drive. These two commands
are
like Basic's Print
and
Input statements,
except
that they use the device number specified in the preceding Open statement instead
of defaulting
to the screen and keyboard respectively.
In Basic 2, you'll use both Print#l5 and Input#l5 extensively, to send housekeep-
ing commands
to the disk and to
check
its error status. Basic 3.5 has built-in commands
for most
of these chores. Even so, it will be good for those of yo'u with Basic 3.5 to see
how
such commands are sent.
Sending
a Command via the Command Channel
Here is the way we send the Initialize command to the disk via the command
channel.
PRINT#l5
,
"IO"
This command assumes we have already opened the file 15 to the command
channel.
"10"
can
be replaced with any string expression that is a valid disk com-
mand.
If
file 15 isn't already open, we can combine the Open and the Print#
in
a
single
statement:
OPEN 15,8, 15,
"IO"
However, this only works for the first disk command given. After that, file 15 is
already open, and Opening it again would cause a
"ALE
OPEN" error. Added com-
mands
are
sent
via Print# instead.
25
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