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Memory

4. Memory Instructions: Load and Store

  • pre-indexed address mode STR
  • post-indexed address mode LDR
.data
var1: .word 3
var2: .word 4

.text
.global _start

_start:
    ldr r0, adr_var1         @ load the memory address of var1 via label adr_var1 to R0
    ldr r1, adr_var2         @ load the memory address of var2 via label adr_var2 to R1
    ldr r2, [r0]             @ load the value (0x03) at memory address found in R0 to R2
    str r2, [r1, r2, LSL#2]  @ address mode: offset. Store the value found in R2 (0x03) to the memory address found in R1 with the offset R2 left-shifted by 2. Base register (R1) unmodified.
    str r2, [r1, r2, LSL#2]! @ address mode: pre-indexed. Store the value found in R2 (0x03) to the memory address found in R1 with the offset R2 left-shifted by 2. Base register modified: R1 = R1 + R2<<2
    ldr r3, [r1], r2, LSL#2  @ address mode: post-indexed. Load value at memory address found in R1 to the register R3. Then modifiy base register: R1 = R1 + R2<<2
    bkpt

adr_var1: .word var1
adr_var2: .word var2

LDR for PC-Relative Addressing

.section .text
.global _start

_start:
   ldr r0, =jump        /* load the address of the function label jump into R0 */
   ldr r1, =0x68DB00AD  /* load the value 0x68DB00AD into R1 */
jump:
   ldr r2, =511         /* load the value 511 into R2 */ 
   bkpt

immediate values on ARM

  • immediate value v: v = n ror 2*r, n 8-bit, r 4-bit.
  • ways to bypass this restrictions:
  1. Construct a larger value out of smaller parts
  • Instead of using MOV r0, #511
  • Split 511 into two parts: MOV r0, #256, and ADD r0, #255
  1. Use a load construct ‘ldr r1,=value’ which the assembler will happily convert into a MOV, or a PC-relative load if that is not possible.
  • LDR r1, =511

5. Load and Store Multiple

  • LDM / STM variations: -IA (increase after), -IB (increase before), -DA (decrease after), -DB (decrease before)

  • PUSH is a synonym for STMDB sp!

    1. the address in SP gets DECREASED by 4.
    2. information gets stored to the new address pointed by SP.

  • POP is a synonym for LDMIA sp!

    1. The value at the current SP address is loaded into a certain register,
    2. Address in SP gets INCREASED by 4.

miscellaneous