B-10. Function Name: CLRCHN

Purpose: Clear I/O channels
Call address: $FFCC (hex) 65484 (decimal)
Communication registers:None
Preparatory routines: None
Error returns:
Stack requirements: 9
Registers affected: A, X

Description: This routine is called to clear all open channels and re- store the I/O channels to their original default values. It is usually called after opening other I/O channels (like a tape or disk drive) and using them for input/output operations. The default input device is 0 (keyboard). The default output device is 3 (the Commodore 64 screen).

If one of the channels to be closed is to the serial port, an UNTALK signal is sent first to clear the input channel or an UNLISTEN is sent to clear the output channel. By not calling this routine (and leaving lis- tener(s) active on the serial bus) several devices can receive the same data from the Commodore 64 at the same time. One way to take advantage of this would be to command the printer to TALK and the disk to LISTEN. This would allow direct printing of a disk file.

This routine is automatically called when the KERNAL CLALL routine is executed.

How to Use:

1. Call this routine using the JSR instruction.

EXAMPLE:

   JSR CLRCHN