Exploring assembly with RPi 3B+

• Hardware: Raspberry Pi 3B+
• OS: Ubuntu 22.04.4 LTS

Install the following packages.

sudo apt install gcc-arm-linux-gnueabi gdb

Compile the file.

# Create obj file via assembler
arm-linux-gnueabi-as hello.s -o hello.o
# Create executable file
arm-linux-gnueabi-gcc hello.o -o hello -nostdlib

Run the executable.

./hello

Compile the file.

# Create obj file via assembler
arm-linux-gnueabi-as hello.s -g -o hello.o
# Create executable file
arm-linux-gnueabi-gcc hello.o -o hello -nostdlib

Run with gdb.

gdb ./hello

Display gdb in asm layout.

(gdb) layout asm
(gdb) break 1    # Breakpoint at line 1

• Could be upper or lower: add or ADD is okay.
• Use 'S' in the mnemonic to set the flags in the CPSR (Current Program Status Register) register.

• MOV: Move value from source to destination.
• ADD: Add values.
• SUB: Subtract values.
• MUL: Multiply values.

Only load & store instructions can access memory. (ARM uses load-store architecture)

• LDR: Load register.
• STR: Store value from memory to register.

• AND: AND operator.
• ORR: OR operator.
• EOR: XOR operator.
• MVN: NOT operator.

ARMv7 register length: 32 Bits

• r0 - r6: General purpose registers, could be used as required.
• r7: Special register, holds system call number.
• sp: Stack pointer, point to the next available location in the stack. Use to store variables like strings, arrays, etc. (Extra places to store values)

• Tutorial: https://www.youtube.com/watch?v=kKtWsuuJEDs&list=PLn_It163He32Ujm-l_czgEBhbJjOUgFhg
• ARMv7 emulator: https://cpulator.01xz.net/
• Linux system call table: https://www.chromium.org/chromium-os/developer-library/reference/linux-constants/syscalls/