THUMB Epilog (Windows CE 5.0)

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The THUMB epilog is a contiguous sequence of instructions that does the following:

• Restores the saved permanent registers
• Resets the stack pointer to its value on function entry
• Returns to the function's calling function

The following list shows important information about an epilog.

• If a frame pointer has been set up by the prolog, the epilog restores the stack pointer from the frame pointer. It uses a single mov instruction to copy R7 to R13. This copy restores R13 to the value it had just after the prolog linked the stack.
  Unlinking the stack restores R13.

• If {R4-R7} are to be restored, and if high registers are not to be restored, the epilog must contain an instruction to pop {R4-R7}. This instruction updates the stack pointer. If the Unwinder restores high registers, it also restores R4-R7.

• The epilog contains a sequence of zero or two instructions that pop the link register from the stack into R3.

  If the routine is a leaf or if the routine restores high registers, the epilog omits this sequence.

  If the epilog restores high registers, it uses a branch and link to call a restore-register routine that restores R4 through R7 and the link register. It also restores the stack pointer to the value it held just after the prolog saved {R0-R3}, or to the value the stack pointer held on entry of the function if {R0-R3} were saved.

• In a non-interworking routine that does not need to save registers other than {R4-R7, LR}, the epilog ends with pop {R4-R7, PC} or pop {PC} if {R4-R7} do not need to be saved. If a noninterworking routine needs to save other registers, the epilog ends with a copy of R3 to the PC.

  In an interworking routine, the epilog ends with a branch and exchange (BX) to R3.

Example

The following examples show a variety of THUMB epilogs.

• THUMB Epilog with no frame.

  ADD    SP, SP, #4    ; Stack link (as needed)
  POP    {r4-r7}      
  POP    {r3}        ; POP link register into r3
  ADD    SP, SP, #16    ; POP r0-r3 as needed
  MOV    PC, r3
  

• THUMB Epilog with frame in R7.

  MOV    r13, r7
  ADD    SP, SP, #4    ; Stack unlink (as needed)
  POP    {r4-r7}      
  POP    {r3}        ; POP link register into r3
  ADD    SP, SP, #16    ; POP r0-r3 as needed
  MOV    PC, r3
  

• THUMB Epilog restoring high registers.

  ADD    SP, SP, #4    ; Stack link (as needed)
  BL    __restgpr_9
  ADD    SP, SP, #16    ; POP r0-r3 as needed
  BX    LR
  

• THUMB Epilog with a large stack frame.

  LDR    r7, [PC, #4]
  ADD    SP, r7
  POP    {r7}
  MOV    PC, LR
  

See Also

SEH in RISC Environments | THUMB Prolog

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