assembly

Assembly

Low-level language with a very strong correspondence between the instruction in the language and the architecture's machine code instructions.

When to Use

• Low-Level Hardware & OS Kernel Programming: Writing bootloaders, CPU context switches, interrupt handlers, and page table setup.
• Micro-Architectural SIMD Vector Optimization: Crafting hand-tuned AVX-512, Neon, or SSE assembly kernels for game physics and matrix multiplication.
• Reverse Engineering & Exploit Analysis: Disassembling binary executables to audit firmware, analyze malware, or debug compiler crashes.
• Embedded Bare-Metal Firmware: Direct CPU register manipulation on microcontrollers without operating system abstractions.

Quick Start

section .data
    msg db "Hello, World!", 0xa
    len equ $ - msg

section .text global _start

_start: mov rax, 1 ; write syscall mov rdi, 1 ; stdout mov rsi, msg ; buffer mov rdx, len ; length syscall

mov rax, 60     ; exit syscall
xor rdi, rdi    ; exit code 0
syscall

## Core Concepts

### CPU Registers & Memory Addressing Modes (x86-64 / ARM64)

Registers hold immediate state for execution; addressing modes compute memory operands:

```nasm
; x86-64 NASM: Moving data and base-index-scale-displacement addressing
mov rax, 42                     ; Immediate to register
mov rbx, [rsp + 16]             ; Base + displacement
mov rcx, [rdi + rsi * 8 + 32]   ; Base + index * scale + displacement

The System V AMD64 Calling Convention

Governs how functions receive parameters and return values in Linux and macOS:

Register	Argument Role
rdi	1st function argument
rsi	2nd function argument
rdx	3rd function argument
rcx	4th function argument
r8, r9	5th and 6th function arguments
rax	Function return value (and syscall number)

; Direct 64-bit Linux syscall (sys_write = 1)
mov rax, 1          ; syscall: sys_write
mov rdi, 1          ; file descriptor: stdout (1)
mov rsi, msg        ; pointer to buffer
mov rdx, 14         ; count (bytes)
syscall             ; invoke kernel

SIMD Vectorization (AVX-512 / AVX2)

Processes multiple data elements simultaneously across 256-bit or 512-bit vector registers:

; Add 8 single-precision floats in parallel using AVX
vmovups ymm0, [rdi]        ; Load 8 floats into YMM0
vmovups ymm1, [rsi]        ; Load 8 floats into YMM1
vaddps  ymm2, ymm0, ymm1   ; YMM2 = YMM0 + YMM1 in single clock cycle
vmovups [rdx], ymm2        ; Store result

Common Patterns

Syscall Invocation with Proper Register Arguments (Linux x86-64)

Problem: Invoking kernel system calls directly without C runtime dependencies.

Solution: Load syscall number into rax and arguments in rdi, rsi, rdx, r10, r8, r9:

section .text
global _start

_start:
    ; sys_write(fd: 1, buf: msg, len: 14)
    mov rax, 1
    mov rdi, 1
    mov rsi, msg
    mov rdx, 14
    syscall

    ; sys_exit(code: 0)
    mov rax, 60
    xor rdi, rdi
    syscall

section .data
msg db "Hello, Assembly", 10

Best Practices

Do:

• Preserve Callee-Saved Registers: Always preserve rbx, rsp, rbp, r12-r15 across function boundaries.
• Maintain 16-Byte Stack Alignment: Ensure rsp is 16-byte aligned before issuing call instructions to avoid segmentation faults in libc.
• Use Inline Assembly Sparingly in C/Rust: Rely on compiler intrinsic headers (<immintrin.h>) whenever possible instead of raw inline assembly.
• Comment Every Assembly Routine Thoroughly: Document input register assumptions, output registers, and modified flags.

Don't:

• Rewrite algorithms in assembly without profiling: Modern LLVM/GCC compilers generate near-optimal machine code for standard loops.
• Ignore processor architecture differences: x86-64 CISC code is incompatible with ARM64 RISC instruction sets.
• Bypass platform calling conventions: Mixing calling conventions causes silent stack corruption and elusive crashes.

Troubleshooting

Error	Cause	Solution
Segmentation fault (core dumped)	Dereferencing invalid pointer or misaligned stack before call.	Ensure stack pointer rsp is 16-byte aligned before function calls.
Undefined reference to '_start'	Linker entrypoint missing or global _start not exported.	Declare global _start or compile with gcc -nostartfiles.
relocation R_X86_64_32S against ... can not be used	Position-independent executable (PIE) requires RIP-relative addressing.	Use [rel symbol] addressing in NASM.

References

• x86 Assembly Guide