Commodore Plus 4 - Page 326

314 Machine Language on the Commodore Plus/4
$00 = The routine was terminated by the STOP key.
$05 = The specified device is not present.
$08 = The file name was missing.
$09 = The specified device is illegal for this purpose.
The system I/O status byte may also be checked for errors. It is read using
READSS ($FFB7).
SFFDB SETTIM
Registers Altered: none
Sets the system clock. This clock consists of 3 bytes (24 bits) on zero page
($A3-$A5). The low byte of the desired setting must be in .A, the middle byte in
.X, and the high byte in .Y. The clock is updated by the normal system IRQ
service routine. If the system IRQ service is not done, calls to UDTIM ($FFEA)
may be used to update it. The normal system interrupt is a raster interrupt that
occurs every 1 / 60th of a second (on NTSC systems). The clock is incremented by
1 until it reaches $4F1 A00, which is 24 hours; then it is reset to zero. During some
I/O operations, interrupts are disabled. This affects the clock.
SFFDE RDTIM
Registers Altered: .A, .X, .Y
Reads the system clock. The low byte of the clock setting is returned in .A, the
middle byte in .X, and the high byte in .Y. See SETTtM ($FFDB) for a descrip
tion of the system clock.
$FFE1 STOP
Registers Altered: .A, .X
This routine is vectored through $0326-$0327. This routine looks at a zero
page location ($91) to determine if the STOP key was pressed. This location is
updated by the system IRQ service routine or UDTIM ($FFEA). If the STOP key
was detected, this routine calls CLRCH ($FFCC) and clears the keyboard queue.
If the Z flag is set on exit, the STOP key was detected; if it is clear, the STOP key
was not detected.
SFFE4 GETIN
Registers Altered: .A, .X, .Y
This routine is vectored through $0328-$0329. This subroutine returns 1 byte
in .A from the current input channel. For all devices except the keyboard, calls to