This guide is part seven of the series, X86–64 Assembly Language Program.

This article is more of a reference guide for those learning x86–64 Assembly Language. I hope you find the information and links at the bottom helpful.

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 the case of nested functions.

rax is used as the return value from a function.

The registers rbx, rbp, and r12– r15 are preserved registers.

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

OPERATIONS

  • ADD- integer add
  • SUB- subtract
  • 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 rax.

Control Flow

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

Date Types

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

  • byte is eight bits
  • word is two bytes
  • doubleword is four bytes
  • quadword is eight bytes
  • double quadword is sixteen bytes (128 bits).

.DATA Directive

Directives are commands that are part of the assembler syntax but are not related to the x86 processor instruction set. All assembler directives begin with a period (.).

The .DATA directive is used for setting values in memory.

Syntax

The syntax within the .DATA directive is

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 byte |
+-----------+-------------------+-------------------+

For example:

choice		DB	'y'
number		DW	12345

GDB

Display the value of ecx register which is a char pointer (in other words, 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 this behavior:

undisplay 1

Note that the number can be two, three, or something else if there are multiple variables in display mode.

Further Resources

NASM

GDB

ASSEMBLY

Docker

NASM

LD Linker

Assembly

Call Stack

Utilities

Docker