Assembly

Assembly is currently in-progress. You are viewing an entry that is unfinished.

Assembly requires a basic understanding of bitwise math

Introduction

• assembler - An assembler is a program that compiles human-readable operations into instructions interpreted by the processor
• linker - A linker is a program that combines the compiled assembly objects into a binary. 'ld' is the standard linker on Linux platforms.

Compilers such as GCC/CC do both operations dynamically.

• Assemble-time: Assembly & operands -> Opcode Sequence
• Link-time: Flat binary of opcode sequence -> executable file format for OS
• Runtime: Opcode Sequence -> hardware gates (may interact with ram etc)

Binary

Main article: Bitwise Math

• counting
• endianness
• nybble - An uncommon unit of memory equivalent to 4 bits.
• byte - A byte is a unit of memory equivalent to 8 bits.
• word - 2 bytes
• dword - 4 bytes, also called a long
• qword - 8 bytes

Number handling

• signed - Signed values are required to represent negative numbers. Most languages by default assume values are signed. The range of numbers it can assign extends from -1 downwards, depending on the data type.
• unsigned - Despite not being able to assign negative numbers, unsigned values are particularly advantageous for positive ranges. The memory that would have been assigned to the negative range is instead added to the positive range (twice as many positive numbers).
• 2's compliment

Data storage

register

A location where memory can be stored temporarily. A register has the bit-width of a cpu's bit description. So for 32 bit systems, a register is 32 bits (4 bytes or a doubleword, also called long) whereas on a 64 bit system a register is 64 bits in length (8 bytes or a qword).

pointer

An address that points to another location in memory

sub-register

A portion of another register always divisible by 8 bits (1 byte) in size

cpu flag registers

• pflag
• zflag

architecture-specific registers and sub-registers

x86

32 bit general purpose

• eax

eax
		ax
		'ah'	'al'
##	##	##	##

• ebp

64 bit general purpose

• rax
• r8-15

Memory Addressing

Stack Pointer

Commonly known as the ESP in x86 Assembly, the stack pointer is a register that contains the location of the top of the stack.

Instruction Pointer

Commonly known as the EIP in x86 Assembly, the instruction pointer is a register that holds the address to the next instuction. When a return instruction is executed, the instruction pointer derives its address from the return address, which exists on the stack.

Base Pointer

Commonly known as the EBP in x86 Assembly, the base pointer is generally used to find local variables and parameters on the stack.

• addressing mode
• index

Instructions

Syntaxes

Primarily two syntaxes of assembly have been the most prominent to date. Intel assembly syntax is traditionally used for Microsoft Windows environments, whereas AT&T System V syntax is generally used on Linux and Unix machines.

Intel Syntax (dest, src)

Generally, in intel syntax, all instructions are applied to destination, source operands. For example, to move 8 into the eax register:

mov eax, 0x8

ATT Syntax (src, dest)

Data manipulation basic primitives

• mov
• push
• pop

Basic arithmetic

• add
• sub
• div
• mul

Bitwise mathematics operators

• and
• not
• or
• xor

Shifts and rotations

• shl
• shr
• rol
• ror

Control flow operators

• cmp
• jmp
• call
• ret

Taking it further

• kernel interrupt
• architecture - i386, i686, x86_64
• operating system