ARM VERSION 1.2

Writing ARM and Thumb Assembly Language

Stacking registers for nested subroutines

Stack operations are very useful at subroutine entry and exit. 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 safely be made 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 lr and then moving that value into the pc. For example:

subroutine STMFD sp!, {r5-r7,lr} ; Push work registers and lr

; code

BL somewhere_else

; code

LDMFD sp!, {r5-r7,pc} ; Pop work registers and pc

Note

Use this with care in mixed ARM and Thumb systems. In ARM architecture v4T

systems, you cannot change state by popping directly into the program counter.

In ARM architecture v5T and above, you can change state in this way.

See the Interworking ARM and Thumb chapter in ADS Developer Guide for further

information on mixing ARM and Thumb.