NonStop S-Series Server Description Manual (G06.27+)

Native Execution Mode

HP NonStop S-Series Server Description Manual—520331-004

7-12

Stack Switching for Native Privilege Transition

If a nonprivileged process, operating in nonprivileged native mode, calls a callable

procedure, the process must switch its operation from the main stack to the privileged

stack. This switch is performed as part of the transition to privileged mode (discussed

in Invoking Privilege Requires Taking an Exception on page 7-10).

The stack switch is accomplished by these steps. The step numbers correspond to the

numbered callouts in Figure 7-6.

1. The call in nonprivileged code to a callable procedure causes a privilege exception,

invoking the exception handler in millicode. (See Invoking Privilege Requires

Taking an Exception on page 7-10.)

2. Upon verifying this exception as an invocation of a legitimate callable procedure,

the exception handler passes control to the Enter_Priv routine. This routine builds

a special stack frame on the privileged stack, beginning with the caller’s return

address (ra) and existing stack pointer address (sp). These items are followed

with space for the maximum number of arguments.

3. The Enter_Priv routine now moves sp to the privileged stack (above the parameter

area) and invokes the called callable procedure. The return address in register ra

now designates the Exit_Priv millicode for use in Step 6.

4. The called procedure allocates its frame on the privileged stack.

5. The called procedure transfers parameters from the caller’s frame on the main

stack to the new frame on the privileged stack. (Parameters passed in registers a0

through a3 are undisturbed by Enter_Priv and so need no copying.) The

procedure executes on the privileged stack until ready to return, placing any return

values in processor registers.

6. The called procedure exits normally to the address it finds in the ra register.

Because this register was set to point at the Exit_Priv routine (Step 2), control

passes to that routine.

7. The Exit_Priv routine switches the mode back to nonprivileged and retrieves the ra

and sp values that were stored at the beginning of the privileged stack. These are

the caller’s values. Loading these values into the ra and sp registers, the millicode

returns control to the caller. Any values returned in v0 and v1 by the callable

procedure are undisturbed by the Exit_Priv operation.