Communicator e3000 MPE/iX Release 7.5 (Software Release C75.00): HP e3000 MPE/iX Computer Systems > Chapter 7 Catalog of User Documentation

7.7 An Advanced Example: Multiple Unwind Regions

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In the following example function, there is one entry point $$foo. A frame of 64 bytes is allocated, and there is an instruction that will cause a trap to be taken. Notice that this is a millicode routine, which uses non-standard entry/exit sequences. Please read through the assembler manual and familiarize yourself with assembler directives before reading the example.


  ;-----------------------------------------------------------------

  ;; Control does not start here. This is not the entry point to the
  ;; example. See below for entry point $$foo.
  ;; Note that callinfo, tells that this is a millicode region, and
  ;;   that stack has gr3 stored into it.
  ;; First Unwind region, Stack has a 64 byte frame.
          .PROC 
          .CALLINFO   millicode, entry_gr=3, frame=60 
  r3_zero_trap 

          ldwm        -64(sp),r3
 
          .PROCEND 
  ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
  ;; This not the entry point. Control falls through here from the
  ;; region defined above. The only change here, is that the stack
  ;;   has been killed.
  ;; This is communicated by a callinfo without any frame
  ;;   information.
  ;; Second Unwind region. Stack is empty.
          .PROC 
          .CALLINFO   millicode
region_two

          be          0(sr0,r31)
          addito,=    0,r0,r0

          .EXIT 
          .PROCEND 

  ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
  ;;-------------------MILLICODE ENTRY POINT, $$foo
  ;; Control starts here. The callinfo tells that the stack has gr3
  ;;   in it.
  ;; By default this means that the stack has been bumped up by 64
  ;;   bytes.
          .PROC 
          .CALLINFO   millicode, entry_gr=3, frame=60 
          .ENTRY
$$foo

  ;; Store r3 on stack.
          stwm        r3,64(sp)
  ;;
  ;; Assume a benign body for this routine, i.e neither the stack,
  ;; nor RP, DP get changed.
  ;;

  ;; Trap if r3 is zero.
          comb,=      0,r3,r3_zero_trap
          nop

  ;; Restore r3 and exit.
          be          0(sr0,r31)
          ldwm        -64(sp),r3

          .EXIT
          .PROCEND
  ;;----------------------------------------------------------------

This example illustrates:

Non-standard entry-exit sequences, notice .ENTER and .LEAVE are not used.
A need to create multiple unwind regions to maintain unwindability.

Control enters at $$f00. The non-standard entry code bumps the stack up by 64 bytes and stores gr3 in the stack. The .CALLINFO indicates that that there is a 64 byte stack frame. If the conditional branch (COMB) is not taken, then gr3 is restored from the stack and the stack is decremented properly. Note that in this region the stack always has a frame. If the condition succeeds, then we branch to r3_zero_trap. This region is physically above the entry point and is also marked as a region with a 64 byte frame. The stack is decremented in this region, and control flows into the third region. Note that in this region, the stack will be empty. To indicate this change to the state of the stack, another .CALLINFO directive is used. When the ADDITO instruction traps, the trap handler can successfully complete a stack trace or resume execution in the context of a procedure somewhere below the current stack pointer.

7.6 A Simple Example

Appendix A Standard Procedure Calls

7.7 An Advanced Example: Multiple Unwind Regions

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