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Assembly Crash Course

pwn.college Assembly Crash Course dojo.

GNU Assembler - GAS

The dojo specifies using as to work through the exercises using Intel format.

The base asm.s file should look like this,

.intel_syntax noprefix

.global _start

.text
_start:
    mov rdi, 0x1337

To assemble and submit it for the exercises,

as -o asm.o asm.s
objcopy -O binary --only-section=.text ./asm.o ./asm.bin
cat ./asm.bin | /challenge/run

NASM

I prefer using NASM for x86 assembly.

The base asm.asm file should look like this,

bits 64
section .text

_start:
    mov rdi, 0x1337

To assemble and submit it for the exercises,

nasm -f bin -o asm.bin asm.asm
cat ./asm.bin | /challenge/run

Registers

64-bit register Lowest 32-bits Lowest 16-bits 2nd Lowest 8-bits Lowest 8-bits
rax eax ax ah al
rbx ebx bx bh bl
rcx ecx cx ch cl
rdx edx dx dh dl
rsi esi si sil
rdi edi di dil
rbp ebp bp bpl
rsp esp sp spl
r8 r8d r8w r8b
r9 r9d r9w r9b
r10 r10d r10w r10b
r11 r11d r11w r11b
r12 r12d r12w r12b
r13 r13d r13w r13b
r14 r14d r14w r14b
r15 r15d r15w r15b

Flag Registers

Name Symbol Bit Use
Carry CF 0 Used to indicate if the previous operation resulted in a carry.
Parity PF 2 Used to indicate if the last byte has an even number of 1's (i.e., even parity).
Adjust AF 4 Used to support Binary Coded Decimal operations.
Zero ZF 6 Used to indicate if the previous operation resulted in a zero result.
Sign SF 7 Used to indicate if the result of the previous operation resulted in a 1 in the most significant bit (indicating negative in the context of signed data).
Direction DF 10 Used to specify the direction (increment or decrement) for some string operations.
Overflow OF 11 Used to indicate if the previous operation resulted in an overflow.

Memory Sizes

Size NASM Bytes Bits
Quad Word dq 8 Bytes 64 bits
Double Word dd 4 bytes 32 bits
Word dw 2 bytes 16 bits
Byte db 1 byte 8 bits

In x86_64, you can access each of these sizes when dereferencing an address, just like using bigger or smaller register accesses:

Instruction Description
mov al, [address] moves the least significant byte from address to rax
mov ax, [address] moves the least significant word from address to rax
mov eax, [address] moves the least significant double word from address to rax
mov rax, [address] moves the full quad word from address to rax

📖 Further Reading