Assembly
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| Assembly requires a basic understanding of bitwise math |
Contents
Introduction
Assembler
An assembler is a program that compiles relatively human-readable operations into instructions that can be directly interpreted by the processor. Assembly Language can be seen as a "mmemonic" for machine instructions - it consists of words that are easier for humans to remember than machine language instructions - for example, "int" for the interrupt operation rather than "0xcd". The assembler produces compiled "objects", translating these mmemonics into machine code - also known as opcodes or as "binary code" in popular culture. By convention, many assembly programmers use the .s extension for assembly code and the .o extension for compiled objects.
On Linux platforms, 'as' is the standard assembler.
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
mmx
sse
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, 8h |
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
