Alien Queen Comments - Answer Overflow

Alien Queen

•Created by Sterling on 9/26/2024 in #firmware-and-baremetal

Ensuring 16-bit THUMB Instruction Encoding for Code Execution from SRAM on STM32F103

@Sterling You’re seeing 32-bit instructions in your THUMB assembly code disassembly for an STM32F103, even though you expect only 16-bit THUMB instructions. This discrepancy arises because your assembler defaults to using THUMB-2, which includes both 16-bit and 32-bit instructions. you can try this out if your intent on 16bit

.syntax unified
.thumb

    movs r0, #40         // 16-bit immediate move
    movs r1, #2          // 16-bit immediate move
    adds r2, r0, r1      // 16-bit add
    mvns r0, #0x28       // 16-bit mvn with 8-bit immediate
    bx r0                // Branch exchange

.syntax unified
.thumb

    movs r0, #40         // 16-bit immediate move
    movs r1, #2          // 16-bit immediate move
    adds r2, r0, r1      // 16-bit add
    mvns r0, #0x28       // 16-bit mvn with 8-bit immediate
    bx r0                // Branch exchange

arm-none-eabi-as -mthumb -mcpu=cortex-m3 -o main.o main.S

arm-none-eabi-as -mthumb -mcpu=cortex-m3 -o main.o main.S

arm-none-eabi-objdump -d -m thumb main.o

arm-none-eabi-objdump -d -m thumb main.o

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