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Stack operations for nested subroutines

Stack operations can be very useful at subroutine entry and exit to avoid losing register contents if other subroutines are called.

At the start of a subroutine, any working registers required can be stored on the stack, and at exit they can be popped off again.

In addition, if the link register is pushed onto the stack at entry, additional subroutine calls can be made safely without causing the return address to be lost. If you do this, you can also return from a subroutine by popping the PC off the stack at exit, instead of popping the LR and then moving that value into the PC. For example:

subroutine  PUSH    {r5-r7,lr} ; Push work registers and lr
            ; code
            BL      somewhere_else
            ; code
            POP     {r5-r7,pc} ; Pop work registers and pc

Use this with care in mixed ARM and Thumb systems. In ARMv4T systems, you cannot change state by popping directly into PC. In these cases you must pop the address into a temporary register and use the BX instruction.

In ARMv5T and later, you can change state in this way.