Introduction

The ARM architecture is widely used in modern devices, offering a powerful and efficient instruction set. In this blog post, we will explore the top 10 ARM instructions that can be used to store a signed byte into memory. Whether you are a beginner or an experienced developer, this guide will help you master these instructions and enhance your programming skills.
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Section 1Understanding Signed Bytes

Before diving into the instructions, it is crucial to understand what a signed byte is. In ARM, a signed byte is represented by 8 bits, where the most significant bit (MSB) determines the sign of the value. If the MSB is 0, the value is positive, and if it is 1, the value is negative.
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Section 2STRB - Store Byte

The first instruction on our list is the STRB instruction. This instruction stores a byte from a register into memory. To store a signed byte, we need to ensure that the register contains the correct value. If the value is negative, the MSB should be set to 1. Here's an example of how to use the STRB instruction:

    MOV R0, #127     ; Load positive value into R0
    STRB R0, [R1]    ; Store the byte from R0 into memory at address R1

Section 3LDRSB - Load Signed Byte

To load a signed byte from memory into a register, we can use the LDRSB instruction. This instruction extends the sign of the loaded value based on the MSB. Here's an example:

    LDRSB R0, [R1]   ; Load the signed byte from memory at address R1 into R0

Section 4MOV - Move Immediate

The MOV instruction is not specifically designed for storing signed bytes, but it can be useful in certain scenarios. To store a positive signed byte, we can use the MOV instruction with an immediate value. However, for negative values, we need to ensure that the MSB is set to 1. Here's an example:

    MOV R0, #127     ; Load positive value into R0
    MOV R1, #-128    ; Load negative value into R1

Section 5STMDB - Store Multiple Registers Decrement Before

In some cases, you may need to store multiple signed bytes consecutively in memory. The STMDB instruction allows you to store multiple registers in a single operation. Here's an example:

    STMDB R1!, {R2-R5}   ; Store signed bytes from R2 to R5 into memory starting at address R1

Section 6LDMIA - Load Multiple Registers Increment After

To load multiple signed bytes from memory into registers, we can use the LDMIA instruction. This instruction works in a similar way to STMDB but with a different addressing mode. Here's an example:

    LDMIA R1!, {R2-R5}   ; Load signed bytes from memory starting at address R1 into registers R2 to R5

Section 7STREXB - Store Byte Exclusive

The STREXB instruction is used in exclusive access scenarios where atomicity is required. Although it is not specifically designed for storing signed bytes, it can be used as an alternative. Here's an example:

    MOV R0, #127          ; Load positive value into R0
    STREXB R0, R1, [R2]   ; Store the byte from R0 into memory at address R2 with exclusive access using R1 as a temporary register

Section 8LDREXB - Load Byte Exclusive

Similar to STREXB, the LDREXB instruction is used for exclusive access scenarios but for loading bytes from memory into registers. Here's an example:

    LDREXB R0, [R1]   ; Load the byte from memory at address R1 into R0 with exclusive access

Section 9STRH - Store Halfword

Although this instruction is designed for storing halfwords (16 bits), it can also be used to store signed bytes by manipulating the values in registers. Here's an example:

    MOV R0, #127      ; Load positive value into R0
    STRH R0, [R1]     ; Store the halfword from R0 into memory at address R1 (the upper byte will be ignored)

Section 10LDRH - Load Halfword

Similar to STRH, the LDRH instruction can be used to load signed bytes by manipulating the values in registers. Here's an example:

    LDRH R0, [R1]   ; Load the halfword from memory at address R1 into R0 (the upper byte will be ignored)

Conclusion

Storing signed bytes in memory is a common task in ARM programming. By mastering these top 10 instructions, you will have a solid foundation to work with signed bytes effectively. Remember to pay attention to sign extension and setting the MSB correctly for negative values. With practice and experimentation, you will become proficient in handling signed bytes in ARM assembly language.

Mastering ARM InstructionsThe Top 10 Ways to Store a Signed Byte into Memory