Assembly(x86)

Lesson 1:

Notes:

• Not really a language

• Basically Machine code

• Has more control over the processor

  • More code to write

• Code made for cpu architecture

• x86 cpus have registers

  • like working memory

  • Have random access

  • Different purpose registers

  • Have a fixed width

    • 64 -bit or 32-bit

      • 64-bit can run 32-bit but 64-bit cannot on 32-bit

• Has these things called Stacks

  • Stacks is a more permanent way of storing memory

    • like stacking books

  • Dysplayed as an array

  • Stack Pointer(like the register)

  • Have random access

  • Can alter the top of the stack

• Syntax can differ from assembler to assembler

  • Even on the same architecture

• Operations

  • Syntax: operation [operands, ...]

    • mov

      • Moves a value into a register

        • Syntax

          • register, value/register

    • add

      • Adds a value to a register

        • Syntax

          • register, value/register

    • sub

      • Subtracts a value from a register

        • Syntax

          • register, value/register

    • int

      • Interrupts the process

        • Syntax

          • Interrupt instruction written in hexadecimal

    • mul

      • Multiplies a value into eax/rax(32-bit/64-bit)

        • Syntax

          • value

    • div

      • divides eax/rax(32-bit/64-bit) by a value

        • Syntax

          • value

            • Note this value is the right hand operator

• Code

  • To tell the compiler what to your code it you can use the section .data and section .text

    • .data is for things like variables and structs

    • .text is where you put your assembly code

Lesson 2:

Notes:

• Instruction Pointer

  • This value is where the code in the code the cpu is processing

  • You can't assign this a value by the operations above

    • You have to use something called jump ops

    • Can be used for conditional operations like if x = y do z if not do a

      • For this you create a label and you can use the syntax jmp label to jump to a label in your code(refer to jump.asm in the repository)

        • For this example in the repo I use cmp all that means is that im comparing one value to another one

      • The point above brings the next point you can use cmp to make conditionals like if ecx is equal to 4 then run x code if not run y

• Operations

  • jmp

    • jumps to a label

      • Syntax:

        • jmp label

  • cmp

    • compares to values

      • cmp value/register(mostly a register), value/register

• Note: Everything below is used after cmp

  • je

    • see if the values are equal if so it will jump to a label

      • Syntax:

        • je label

  • jne

    • same as above but if they are not equal

      • jne

        • Syntax:

          • jne label

  • jg

    • jumps if the value is greater

      • Syntax:

        • jg label

  • jge

    • jumps if value is greater or equal

      • Syntax:

        • jge

          • jge label

  • jl

    • jumps if value is less

      • Syntax:

        • jl label

  • jle

    • jumps if less than or equal

      • Syntax:

        • jle label

Lesson 3:

Notes:

• Addresses

  • Used to store information

    • you can edit this code in the runtime

      • To do this do mov [variable] byte "a" edit the first byte

        • add a + then a number to move the position

• Data types

  • db = 1 byte

  • dw = 2 bytes

  • dd = 4 bytes

• The Stack (LIFO(Last in First Out))

  • Putting in something is like puting a book onto a stack

    • Popping is like taking that book off

    • Can use the pointer to tell where this is meaning its random access

  • In 32 bit the stack it stores 4 bytes in each point

  • Each stack entry is in 4s

  • Starts at the highest and works down

  • Can use the example about to work with the stack

• Operations

  • push

    • push a value to the stack

    • Syntax

      • push value

  • pop

    • moves the top value off the stack to the desination defined

    • Syntax

      • pop register/variable

Lesson 4:

Notes:

• Functions

  • Uses the operation call

    • pushes the instruction pointer onto the stack

      • Next instruction

    • Jumps to the location we are calling

    • A combination allows us to jump to code and then jump back without hard coding it in

    • To return back you can use the operation ret

      • returns the function back to the value of eip/rip(32 bit/64 bit) that was stored in the stack

• Operations

  • call

    • call a function

    • Syntax

      • call  label_name

  • ret

    • returns to the value of eip/rip(32 bit/64 bit) that as pushed to the stack

    • Syntax

      • ret

Lesson 5:

Notes:

• Functions Again

  • Arguments

    • Arguments are pushed to the stack in Assembly

      • You have to add 8 to the stack allowing you to access these arguments then after that if you have more than one you would add 4 to esp/rsp(32 Bit / 64 Bit)

  • The value returned from a function is stored in the eax/rax(32 bit / 64 Bit) register

  • To call functions from C just put extern and then name the function there

    • To link this code it is best to use gcc instead of ld

      • The reason is that gcc is used to compile C code so it will have the all the C libraries to use

• Operations

  • extern

    • Links an external function

    • Syntax

      • extern function_name