README.md

Resources

Most of these resources were learnt from kupala's YouTube playlist:
https://www.youtube.com/playlist?list=PLetF-YjXm-sCH6FrTz4AQhfH6INDQvQSn

X86 Assembly Wiki:
https://en.wikibooks.org/wiki/X86_Assembly/X86_Architecture

Registers:

General Purpose Registers:

Symbol	Meaning	Use
RAX	Accumulator	Arithemtic Operations
RBX	Base	Pointer to Data
RCX	Counter	Shift/Rotate Instructions or Loops
RDX	Data	Arithemtic or I/O Operations
RSI	Source Index	Pointer to Source in Stream Operations
RDI	Destination Index	Pointer to Destination in Stream Operations
RSP	Stack Pointer	Pointer to the Top of the Stack
RBP	Stack Base Pointer	Point to the Base of the Stack

System Call:

A very useful Linux system call table: https://blog.rchapman.org/posts/Linux_System_Call_Table_for_x86_64/

rax	System call	rdi	rsi	rdx
0	sys_read	#filedescriptor	$buffer	#count
1	sys_write	#filedescriptor	$buffer	#count
2	sys_open	$filename	#flag	#mode
...	...	...	...	...

Flags:

• Registers that hold data (1 bit each, true or refalse)
• Each flag is part of a larger register

Symbol	Meaning
CF	Carry
PF	Parity
AF	Adjust
ZF	Zero
SF	Sign
TF	Trap
IF	Interrupt
DF	Direction
OF	Overflow
...	...

Pointers:

• Like flags, are registers that hold data
• They hold the memory address of some data

Name	Meaning	Description
rip (eip, ip)	Instruction Pointer	Points to next address to be executed in control flow
rsp (esp, sp)	Stack Pointer	Points to the top address of the stack
rbp (ebp, bp)	Stack Base Pointer	Points to the bottom of the stack

Instructions:

List of instructions: https://www.felixcloutier.com/x86/

Comparisons:

• Allow programs to take different paths based on certain conditions
• They are done on registers
• The format of cmp is typically cmp register, value/register
• After a comparison, certain flags are set

Example:

cmp rax, 23
cmp rax, rbx

Conditional Jumps:

• After a comparison, a conditional jump can be made
• They are based on the status of the flags

Symbol (signed)	Symbol (unsigned)	Result of cmp a, b
je	-	a = b
jne	-	a ≠ b
jg	ja	a > b
jge	jae	a ≥ b
jl	jb	a < b
jle	jbe	a ≤ b
jz	-	a = 0
jnz	-	a ≠ 0
jo	-	Overflow
jno	-	Not Overflow
js	-	Jump If Signed
jns	-	Jump If Not Signed

Examples:

cmp rax, 23
je _doThis

cmp rax, rbx
jg _doThis

Math Operations:

• used to mathematically manipulate registers
• The format of a math operation is typically oper register, value/register
• The result of the operation is stored in the first register

Name (unsigned)	Name (signed)	Description
add a, b	-	a = a + b
sub a, b	-	a = a - b
mul reg	imul reg	rax = rax * reg
div reg	idiv reg	rax = rax / reg
neg reg	-	reg = -reg
inc reg	-	reg = reg + 1
dec reg	-	reg = reg - 1
adc a, b	-	a = a + b + CF
sbb a, b	-	a = a - b - CF

Stack Operations:

Operation	Effect
push reg/value	Pushes a value onto the stack
pop reg	Pops a value off the stack and stores it in 'reg'
mov reg, [rsp]	Stores the peek value in 'reg'

Macros:

• An instruction that expands into a predefined set of instructions to perform a particular task

Defining Macros:

%macro <name> <argc>
    ...
    <macro body>
    ...
%endmacro

<name>
    Name of macro
<argc>
    Number of arguments
<macro body>
    Definition of macro

Example:

; a NASM macro to add two numbers and store it in rax
%macro sumTwoNumbers 2
    mov rax, %1        ; move argument 1 into rax
    add rax, %2        ; add rax and argument 2 and store it in rax
%endmacro

File Permissions:

• We need to get the permission value (octal) first
• r = read, w = write, x = execute, sst = special attributes

Write to File:

Example:

section .data
    filename db "myfile.txt", 0                ; null-terminated file string
    text     db "Here's some text."            ; text to write to file
    textLen  equ $ - text                      ; length of text to write
 
section .text
    global _start
    
_start:
    mov rax, 2                                 ; argument to call 'sys_open'
    mov rdi, filename                          ; argument for file name
    mov rsi, 64+1                              ; argument for create (64) and write (1) flag
    mov rdx, 0644o                             ; argument for file permission value, suffix 'o' for octal
    syscall
    
    mov rdi, rax                               ; argument for file descriptor (stored in rax)
    mov rax, 1                                 ; argument to call 'sys_write'
    mov rsi, text                              ; argument for text to write to file
    mov rdx, textLen                           ; argument for text length (textLen)
    syscall
    
    mov rax, 3                                 ; argument to call 'sys_close'
    syscall
    
    mov rax, 60                                ; argument to call 'sys_exit'
    mov rdi, 0                                 ; argument for error code (0)
    syscall

Size Operands:

Size (bits)	Operands
8	byte, DB, RESB
16	word, DW, RESW
32	dword, DD, RESD
64	qword, DQ, RESQ
80	tword, DT, REST
128	oword, DO, DDQ, RESO, RESDQ
256	yword, DY, RESY
512	zword, DZ, RESZ

Miscellanous:

• Use %include 'file.h' to include other NASM files
• When using mul or div, the result is stored int rdx:rax, where rdx is the high bits, and rax is the low bits