ARM VERSION 1.2 Thumb LDM and STM instructions, Push and POP, Thumb-state block copy example

Writing ARM and Thumb Assembly Language

2.8.5Thumb 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 can be used to load or store any subset of the 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 IA.

Examples of these instructions 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, Example 2-11 on page 2-44, can be converted into Thumb instructions (see Example 2-13 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 after each access. If LDR and STR were used for this, separate ADD instructions would be required to update each base pointer.

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