Commodore Plus 4 - Page 319

The Operating System 307
SFF90 SETMSG
Registers Altered: .A
Sets the system message flag to control output of messages. The value in .A
when the routine is called is stored in the message flag ($9A). A value of $00
means a program is running (and presumably handles message output). A value
of $80 flags output of direct BASIC mode messages. A value of $C0 flags output
of MONITOR messages.
SFF93 SECND
Registers Altered: .A
Sends the value in .A as a secondary address to a device following a call to
LISTN ($FFB1). The valid secondary addresses depend on the device.
The secondary address must be ORed with $60 before SECND is called. For
the 1541 disk operating system, the low nybble determines the channel ($0 is
reserved for LOAD, $1 for SAVE, and $F is the command channel). If the high
nybble is $F, a CLOSE is indicated. If the high nybble is $E, an OPEN is
indicated.
The system I/O status byte may be checked for errors. It is read using
READSS ($FFB7).
$FF96 TKSA
Registers Altered: .A
Sends the value in .A as a secondary address to a device following a call to
TALK ($FFB4). The valid secondary addresses depend on the device.
The secondary address must be ORed with $60 before TKSA is called. For the
1541 disk operating system, the low nybble determines the channel ($0 is reserved
for LOAD, $1 for SAVE, and $F is the command channel). If the high nybble is
$F, a CLOSE is indicated. If the high nybble is $E, an OPEN is indicated.
The system I/O status byte may be checked for errors. It is read using
READSS ($FFB7).
SFF99 MEMTOP
Registers Altered: .X, .Y
When called with the carry set, returns with .X equal to the low byte of the top
of memory, and. Y equal to the high byte of the top bf memory. When called with
the carry clear, the top of memory ($0533-$0534) is set. The low byte is from .X
and the high byte from. Y. The top of memory is the address plus one of the end of
a contiguous section of RAM for use by BASIC. It is initially set during the
power-on sequence.