x86 assembly

Create your object file: nasm -f elf64 file.asm -o files.o
Link your object files: ld file.o -o file
;s are comments.

Registers

Sub-registers	Description
al	Lowest 8 bits
ah	Highest 8 bits
ax	Lowest 16 bits
eax	Lowest 32 bits. Resets the top 32 bits.
rax	64 bits

Register name	Description
rax	Primary accumulator
rbx	Base register
rcx	Count register
rdx	Data register
r8-r15	General purpose
rbp	Base pointer reg
rsp	Stack pointer reg

Commands

Commands follow this syntax: cmd des, src

The source(src) goes into the destination(des).
The sources and destinations can either be a register, a memory address, or an immediate number/string.
- All immediate values are limited to 32 bits.
- If you have a negative immediate value, the sign is extended to the end of the 64 bits.

Moving and conditional codes

Commands	Description
mov des, src	Copies the value form source to destination. Dereferences pointers.
lea des, src	Copies the value form source to destination. Doesn’t dereferences pointers.
cmovCC des, src	Conditional move. cc is the conditional code.

Conditional Code(CC)	Description	Flags	Description
e	Equals	ZF = 1
ne	Not equal	ZF = 0
g	Greater than	SF = OF and ZF = 0
ge	Greater than or equals	SF = OF
l	Less than	SF != OF
le	Less than or equals	ZF = 1 or SF != OF
b	Below	CF = 1	Unsigned less than
nb	Not below	CF = 0	Unsigned greater than or equal to
a	Above	CF = 0 and ZF = 0	Unsigned greater than
na	Not above	CF = 1 or ZF = 1	Unsigned less than or equal to
s	Sign	SF = 1
ns	Not sign	SF = 0
p	Parity	PF = 1
np	Not parity	PF = 0

Flags and Comparisons

These commands are used to set the flags and don’t store the result.

Commands	Description
cmp des, src	Same as sub.
test des, src	Same as and.

Flags	Names	Description	Example
CF	Carry flag	Unsigned overflow	1111 + 0001 = 1 0000
ZF	Zero flag	If the result is 0
SF	Signed flag	If the result is signed
OF	Overflow flag	Signed overflow	0111 + 0001 = 1000
PF	Parity flag	Count the number of 1 bits in the last 8 bits of the result

Arithmetic

Commands	Description
add des, src
sub des, src
inc des	Increment. Doesn’t effect carry flag.
dec des	Decrement. Doesn’t effect carry flag.
div des	Unsigned division. Answer in RAX and remainder in RDX.
idiv des	Signed division. Answer in RAX and remainder in RDX.
mul des or mul des, src	Signed multiplication. Lower 128 bits(64 bits) in RAX and higher in RDX.
imul des or imul des, src	Unsigned multiplication. Lower 128 bits(64 bits) in RAX and higher in RDX.
shl des, src	Shift left. Multiplying by powers of 2.
shr des, src	Unsigned shift right. Dividing by powers of 2.
sar des, src	Signed shift right. Dividing by powers of 2. If odd then round up.
rol des, src	Rotate left.
ror des, src	Rotate right.
rcl des, src	Rotate left through the carry flag.
rcr des, src	Rotate right through the carry flag.

The src for shift commands can only be al or imm8.

Logic gate

Commands	Description
and des, src	anding a reg with itself updates the flags with no changes.
or des, src
not des	Flips bits.
neg des	Negates signed numbers(2s compliment).
xor des, src	xoring a reg with itself makes it 0.

Jump

Commands	Description
call label	Transfers control to a subroutine at the label. The return address is pushed on the stack.
ret	Returns from a subroutine. Pops the stack and uses the return address.
syscall	Makes a syscall to the operating system. The specific service is determined by values in certain registers.
jmp label	Jumps to a label.
jCC label	Conditional jump based on the conditional code.

Stack

Commands	Description
pop des
push des
pushfq	Push the contents of the rflags register onto the stack.
popfq	Pop the stack onto the rflags register.

Low
   +-----+ ^
	 +-----+ |
	 +-----+ |
	 +-----+ |
	 +-----+ |
	 +-----+ <- rsp
High

Floats

Sections and Start

Sections	Description
.data	Used to declare global variables with values. Initialized values are in the executable.
.bss	Used to declare global variable with 0s. It doesn’t store 0 in the executable.
.text	Contains the executable instruction of the program.

.bss is used to save 0s from being in the executable.

section .text
	global _start

_start:
	; Your code goes here
	; Syscall exit
	mov rax, 60
	xor rdi, rdi ; Exit code 0
	syscall

Variables

Variables take the form of: varName varSize immValue/?

The s in front means it’s signed.
Immediate floats need to have decimal points. Ex: 10.0

varSize	Description
byte/db/sbyte	8 bits
word/dw/sword	16 bits
dword/dd/sdword	32 bits
qword/dq/sqword	64 bits
real4	32 floating point bits
real8	64 floating point bits

Arrays

You can create arrays with a repeated patter using times.
- Ex: arrName times 5 db 0 creates an array of 5 bytes all initialized to 0.
You can create arrays by listing out each element.
- Ex: arrName db 1, 2, 3, 4
You can use quotes("s) to create arrays.
- Ex: str db "Hello, World!", 0xA
You can get teh size of an array using equ
- Ex: arrSize equ $ - arrName

Pointers

	Description
`var`	Gets the value of var
`[ptr]`	Dereferences the pointer
`[reg/mem +/- # * #]`	The `* #` is the bytes of each array element. The `+/- #` is the index.

If you want to get the address of var, then use the lea command.

Hello World

section .data
	msg db "Hello, world!", 0xA   ; Message with newline
	len equ $ - msg               ; Calculate length of the message

section .text
	global _start                 ; Entry point for the program

_start:
	; Syscall write
	mov rax, 1                    ; System call number for write
	mov rdi, 1                    ; File descriptor 1 (stdout)
	mov rsi, msg                  ; Address of the message
	mov rdx, len                  ; Length of the message
	syscall                       ; Invoke the system call

	; Syscall exit
	mov rax, 60                   ; System call number for exit
	xor rdi, rdi                  ; Exit code 0
	syscall                       ; Invoke the system call