Commodore Plus 4 - Page 327

The Operating System 315
OPEN (SFFCO) and CHKIN ($FFC6) must precede this call. The channel
remains open following the call to this routine.
The system I/O status byte may be checked for errors. It is read using
READSS ($FFB7).
For the keyboard, this routine gets 1 byte from the queue. If the queue is empty,
a zero is returned in .A.
$FFE7 CLALL
Registers Altered: .A, .X
This routine is vectored through $032A-$032B. This routine clears all channels
with a call to CLRCH ($FFCC) and closes all logical files.
The system I/O status byte may be checked for errors. It is read using
READSS ($FFB7).
SFFEA UDTIM
Registers Altered: .A, .X
Increments the system clock ($A3-$A5) and compares it with S4F1A01 to
determine when to reset to zero. Also, this routine checks for the STOP key and
sets the STOP key flag ($91), which is used by STOP ($FFE1). This routine is
normally called by the system IRQ service routine. See SETTIM ($FFDB) for a
description of the system clock.
$FFED SCRORG
Registers Altered: .X, .Y
Returns $28 (the screen width) in .X and $19 (the screen height) in .Y to
indicate the physical size of the Plus/4 screen. This subroutine is useful only for
programs intended to run on other Commodore computers with a different
screen size (e.g., the VIC-20).
$FFF0 PLOT
Registers Altered: .A, .X, .Y
When called with the carry flag clear, sets the cursor position. The new cursor
line number should be in .X, and the new cursor column should be in .Y. The
screen window is returned to its default (the whole screen). When called with the
carry flag set, this routine returns the current cursor position. The cursor line
number is returned in .X and the cursor column in .Y.