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PROCEDURE CALLS
7.8
RETURNS
The return (
) instruction provides a generalized return mechanism that can be used to return
ret
from any procedure that was entered by
executes, the processor uses the information from the return-type field in the PFP register
(Figure
7-5) to determine the type of return action to take.
Return Status
Return-Type Field - PFP.rt
Pre-Return-Trace Flag - PFP.p
Previous Frame Pointer
Address-PFP.a
a
3
1
28
24
31
Figure 7-5. Previous Frame Pointer Register (PFP) (r0)
return-type field indicates the type of call which was made.
encoding for the various calls: local, supervisor, interrupt and fault.
trace-on-return flag (PFP.rt0 or bit 0 of the return-type field) stores the trace enable bit value when
an explicit system-supervisor call is made from user mode. When the call is made, the PC register
trace enable bit is saved as the trace-on-return flag and then replaced by the trace controls bit in the
system procedure table. On a return, the trace enable bit's original value is restored. This
mechanism allows instruction tracing to be turned on or off when a supervisor mode switch
occurs. See
section 9.5.2.1, "Tracing on Explicit Call" (pg.
prereturn-trace flag (PFP.p) is used in conjunction with call-trace and prereturn-trace modes.
When call-trace mode is enabled when a call is made, the processor sets the prereturn-trace flag;
otherwise it clears the flag. Then, when this flag is set and prereturn-trace mode is enabled, a
prereturn trace event is generated on a return, before any actions associated with the return
operation are performed. See
interaction between call-trace and prereturn-trace modes with the prereturn-trace flag.
7-20
,
,
, an interrupt call or a fault call. When
call
calls
callx
20
16
12
9-13).
section 9.2, "TRACE MODES" (pg. 9-3)
r
r
r
a
t
t
t
p
4
2
1
0
8
4
0
Table 7-2
shows the return-type field
for a discussion of
ret
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