Series Contents

      1. 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

    1. 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.

    1. 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

    1. 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).

  1. .data section
      1. 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
  1. 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. --- [[Series Contents|(Series) X86–64 Assembly Language Guide]] [[Next >>>|X86_64 Resources]]