ARM VERSION 1.2

Writing ARM and Thumb Assembly Language

2.8.5 Thumb LDM and STM instructions

The Thumb instruction set contains two pairs of multiple-register transfer instructions:

•

LDM

and

STM

for block memory transfers

•

PUSH

and

POP

for stack operations.

LDM and STM

These instructions ca n be used to load or s tore any subset of th e low registers from or

to memory. The base register is always updated at the end of the multiple register

transfer instruction. You must specify the

character. The only valid suffix for these

instructions is

Examples of these instruct ions are:

LDMIA r1!, {r0,r2-r7}

STMIA r4!, {r0-r3}

PUSH and POP

These instructions can be used to push any subset of the low registers and (optionally)

the link register onto the stack, and to pop any subset of the low registers and

(optionally) the pc off the stack. The base address of the stack is held in r13. Examples

of these instructions are:

PUSH {r0-r3}

POP {r0-r3}

PUSH {r4-r7,lr}

POP {r4-r7,pc}

The optional addition of the lr or pc to the register list provides support for subroutine

entry and exit.

The stack is always full descending.

Thumb-state block copy example

The block copy example, Example2-11 on page 2-44, can be converted into Thumb

instructions (see Example2-13 on page 2-47).

Because the Thumb

LDM

and

STM

instructions can access only the low registers, the

number of words copied per iteration is reduced from eight to four. In addition, the

LDM

and

STM

instructions can be used to carry out the single word at a time copy, because they

update the base pointer a fter each access. I f

LDR

and

STR

were used for this, separate

ADD

instructions would be required to update each base pointer.