NonStop S-Series Server Description Manual (G06.24+)

TNS Execution Modes

HP NonStop S-Series Server Description Manual—520331-003

6-44

Actions of the EXIT Instruction

The EXIT instruction uses the three-word stack marker to restore the caller’s

environment. The sequence is as follows, with reference to Figure 6-25. (For

simplicity, and continuity with the preceding PCAL description, this sequence assumes

the return is from a procedure that was called with PCAL rather than XCAL.)

1. The S register setting is moved below the local area, the stack marker, and any

parameters to the exiting procedure.

The S^decrement value (which is specified in the EXIT instruction) is subtracted

from the current L register setting and placed in the S register. The value of

S^decrement is 3 (for the stack marker) plus the number of words of parameter

and mask information passed to the exiting procedure.

2. The P register is set with the P register value saved in the stack marker at L[–2].

The next instruction to be executed will be the one following the PCAL instruction.

3. The Environment register is restored from a combination of the current

Environment register setting and the Environment register value saved in the

L[–1].

The mode (privileged or nonprivileged) and data area bits are reestablished to be

the lesser of the caller’s and the current settings. This ensures that a

nonprivileged user cannot exit with privileged capability. The caller’s CS (code

space), LS (library space), T (traps), V (overflow), and K (carry) bits are

reestablished from L[-1]. Z and N (Condition Code) are left at their current settings

to reflect the results of the call. RP is left at its current setting so that a value in the

4. The L register is restored from the L register value saved in the stack marker at

L[0]. This moves L back to point to the preceding stack marker, thereby

reestablishing the preceding local data area.

The instruction following the PCAL instruction then executes.