5.7.3. Implementing stacks with LDM and STM

The load and store multiple instructions can update the base register. For stack operations, the base register is usually the stack pointer, r13. This means that you can use load and store multiple instructions to implement push and pop operations for any number of registers in a single instruction.

The Load and Store Multiple Instructions can be used with several types of stack:

descending or ascending

The stack grows downwards, starting with a high address and progressing to a lower one (a descending stack), or upwards, starting from a low address and progressing to a higher address (an ascending stack).

full or empty

The stack pointer can either point to the last item in the stack (a full stack), or the next free space on the stack (an empty stack).

In practice stacks are almost always full, descending. The C compilers produce full, descending stacks.

To make it easier for the programmer, stack oriented suffixes can be used instead of the Increment/Decrement and Before/After suffixes. See Table 5.6 for a list of stack oriented suffixes.

Table 5.6. Suffixes for load and store multiple instructions

Stack typePushPop
Full DescendingSTMFD (DB)LDMFD (IA)
Full AscendingSTMFA (IB)LDMFA (DA)
Empty DescendingSTMED (DA)LDMED (IB)
Empty AscendingSTMEA (IA)LDMEA (DB)

For example:

	STMFD 		r13!, {r0-r5}				; Push onto a Full Descending Stack.
	LDMFD 		r13!, {r0-r5}				; Pop from a Full Descending Stack.
	STMFA 		r13!, {r0-r5}				; Push onto a Full Ascending Stack.
	LDMFA 		r13!, {r0-r5}				; Pop from a Full Ascending Stack.
	STMED 		r13!, {r0-r5}				; Push onto Empty Descending Stack.
	LDMED 		r13!, {r0-r5}				; Pop from Empty Descending Stack.
	STMEA 		r13!, {r0-r5}				; Push onto Empty Ascending Stack.
	LDMEA 		r13!, {r0-r5}				; Pop from Empty Ascending Stack.

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. You can return from a subroutine by popping the pc off the stack at exit, rather than by 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

Warning

Use this with care in mixed ARM/Thumb systems. You cannot return to Thumb code by popping directly into the program counter.

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