Interdata_v6/usr/doc/man/man2/signal.2

.th SIGNAL II 8/12/77
.sh NAME
signal, csignal - catch or ignore signals
.sh DESCRIPTION
The
.it signal
mechanism is as described in The Book, with the following addition:

Since the Interdata has no
.it rti
instruction, a special
.it signal
0 call allows signal routines to return to the
point of interruption without disturbing registers or the PSW.

When a caught signal occurs, UNIX first checks whether the process has
a stack (see below); if not, the stack pointer is initialized to the top of
the stack segment - 512. UNIX then stacks the
.it old
SP value, program counter, and PSW status (in reverse order) and transfers
to the process's signal routine.

An assembler signal routine can return to the point of interruption by
executing
.it signal
0. UNIX then restores the PSW status, program counter, and previous
SP value from the stack. (The PSW status is verified to
prevent processes getting into an "unauthorized" state.)

If the high-order bit of the restored PSW is
.it on,
the process is to be resumed in "no stack" mode.
This is normally used only in "foreign" programs running under the
OS SVC interpreter, which do not observe the UNIX stack discipline.

A C signal function normally cannot return - it must only clean up and exit,
or call
.it reset
to re-initialize the stack. However, a special version of
.it signal
exists (as \fIcsignal\fR()) which includes a pseudo-\c
.it rti.
Using this version, the process's signal function is called with the
interrupt-time register values as arguments; i.e.

.ti +10
func(r0, r1, r2, ... , re, rf, ps, pc);

When the function returns,
.it csgnal
restores registers and PSW from the arguments (which may have been
altered) and resumes execution at address
.it pc.
.sh "SEE ALSO"
rti(II)