ARM Data Transfer: Pre/Post-Index and STR/LDR

Store and load instructions

• ARM does not allow memory-to-memory data processing.
• You move data with register ↔ memory transfer instructions.

Load

• Memory → Register

Store

• Register → Memory

Single-register data transfer

<LDR | STR> {<cond>} {<size>} Rd, <address>

• LDR, STR, LDRB, STRB (size: WORD, BYTE)

Pre- and post-index addressing

Pre-index method

• After the transfer, the base register is unchanged.
• Append ! to write the computed address back into the base register.

Post-index method

• After the transfer, the base register is updated with base + offset.

• In the example, base register r1 becomes 0x100 + 0xc.