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X86_64 Assembly and GDB Quick Reference

30th December, 2023

Series Contents

Registers

General-purpose registers - there are 16 general-purpose registers - rax, rbx, rcx, rdx, rbp, rsp, rsi, rdi, r8, r9, r10, r11, r12, r13, r14, r15

  • data- section is used for declaring initialized data or constants
  • bss- section is used for declaring non initialized variables
  • text- section is used for code
  • rdi- first argument
  • rsi- second argument
  • rdx- third argument
  • rcx- fourth argument
  • r8- fifth argument
  • r9- sixth

The first six integer or pointer arguments are passed in registers RDI, RSI, RDX, RCX, R8, R9 (R10 is used as a static chain pointer in case of nested functions.

RAX is used as the return value from a function.

Preserved registers: RBX, RBP, and R12–R15

All other registers must be saved by the caller if it wishes to preserve their values

Operations

  • ADD- integer add
  • SUB- substract
  • MUL- unsigned multiply
  • IMUL- signed multiply
  • DIV- unsigned divide
  • IDIV- signed divide
  • INC- increment
  • DEC- decrement
  • NEG- negate

The initial number must be stored in rax. rax can be multiplied by a value in any of the other registers. The result will be stored in rdx:rax.

Control Flow

JE - if equal JZ - if zero JNE - if not equal JNZ - if not zero JG - if first operand is greater than second JGE - if first operand is greater or equal to second JA - the same that JG, but performs unsigned comparison JAE - the same that JGE, but performs unsigned comparison

Data Types

The fundamental data types are bytes, words, doublewords, quadwords, and double quadwords.

  • byte is eight bits
  • word is 2 bytes
  • doubleword is 4 bytes
  • quadword is 8 bytes
  • double quadword is 16 bytes (128 bits).

.data section

syntax

[variable-name] define-directive initial-value

There are five basic forms of the define directive −

Directive Purpose Storage Space
DB Define Byte allocates 1 byte
DW Define Word allocates 2 bytes
DD Define Doubleword allocates 4 bytes
DQ Define Quadword allocates 8 bytes
DT Define Ten Bytes allocates 10 bytes

for example 

choice		DB	'y'
number		DW	12345

GDB

Display the value of ECX register which is a char pointer, e.g. print the string referred to: 

display (char *) $ecx

Note, this will display the value at every break of the program execution, including each step if you are stepping through. To stop the behaviour: 

undisplay 1

Note that the number may be 2, 3 or something else if you have multiple variables on display mode.

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