Commodore Plus 4 - Page 370

358 Using Peripheral Devices
Programming the Serial Bus Occasionally it is desirable to program directly to
the serial bus. The family of operating subroutines used for this purpose consists
of LISTN ($FFB1), SECND ($FF93), CIOUT (SFFA8), UNLSN ($FFAE),
TALK ($FFB4), TKSA ($FF96), ACPTR ($FFA5), and UNTLK (SFFAB).
Programming at this level eliminates the file structure used with OPEN and
CLOSE. Instead of thinking in terms of logical file numbers, think in terms of the
channels of the disk drive. A channel may be opened and told to send data to the
computer (to talk) or to receive data from the computer (to listen). The error
checking must be done using the status variable for device not present, end-of-file,
and such, and by reading the error channel. When these operations are done
depends to a large extent on what assumptions the particular application allows.
When in doubt, check for an error.
Opening a File
Here is an outline of a procedure:
1. Set the status variable ($90) to zero.
2. Load .A with the device number of the disk drive (and store in the current
device number at $AE if this has not already been done). Call LISTN ($FFB1).
Check for a device not present error in the status variable.
3. Load .A with the disk drive channel number you want to use ORed with $F0
(to indicate open up this channel). Call SECND ($FF93).
4. Send the file name (if any) through .A to the drive using CIOUT ($FFA8).
5. Call UNLSN ($FFAE).
The channel specified is now opened to the file specified.
Getting Data from an Open Channel
1. Load .A with the device number of the disk drive and call TALK ($FFB4).
2. Load .A with the channel number ORed with $60 (to tell the drive this is a
secondary address). Call TKSA ($FF96).
3. Call ACPTR ($FFA5) to receive bytes in .A. When bit 6 of the status variable is
set, the end-of-file has been reached.
4. Call UNTLK (SFFAB).
Sending Data to an Open Channel
1. Load .A with the device number of the disk drive and call LISTN (SFFB1).
2. Load .A with the channel number ORed with $60 (to tell the drive this is a
secondary address). Call SECND ($FF93).