AssemblyGuide

Guide for programming in Assembly. More stuff added as i learn.

Registers

Assembly has pre-made variables called 'Registers' that can be used for interrupts, memory access, store information, etc

Some registers have other versions of themselves, for example, AX is a 16-bit register and AL (8-bit register) is the lower bytes of AX

Here's a list of general purpose registers for 16, 32 and 64 bits:

8-bits	16-bits	32-bits	64-bits
AL(low)	AX(low)	EAX(low)	RAX
AH(high)	AX(low)	EAX(low)	RAX
BL(low)	BX(low)	EBX(low)	RBX
BH(high)	BX(low)	EBX(low)	RBX
CL(low)	CX(low)	ECX(low)	RCX
CH(high)	CX(low)	ECX(low)	RCX
DL(low)	DX(low)	EDX(low)	RDX
DH(high)	DX(low)	EDX(low)	RDX
(null)	BP(low)	EBP(low)	RBP
(null)	SI(low)	ESI(low)	RSI
(null)	DI(low)	EDI(low)	RDI

There's also another thing to notice, when you are in 16-bits you can only use your registers and the 8-bit ones,

On 32-bit you can access 16-bit registers but not 8-bit, similar in 64-bits you can access 32-bit but not 16-bit or lower

The Stack

The stack is used for storing temporal information and calling functions

The way on how it works is that it starts at a certain address and it adds or removes bytes as needed (like a pile of books)

There are certain commands to interact with the stack:

• push

  Adds data at the top of the stack

  Example:

  push AX ; Stores the data inside AX into the stack
  push 30 ; Stores the number 30 into the stack
  push dword 50 ; Stores the number 50 but as a dword (16-bits/2 bytes) into the stack

• pusha

  Adds all the registers' data into the stack

  Its like using lots of 'push' commands with the registers (its not always recommended cuz it takes quite a bit of space in the stack)

• pop

  Retrieves and removes data from the stack

  Example:

  pop AX ; Stores the last 2 bytes added to the stack into AX (16-bits)
  pop dword [0x8000] ; Stores the last 2 bytes added to the stack into the memory address 0x8000

• popa

  Retrieves all the last registers' data added into the stack

Now, when the 'call' function uses the stack, it pushes the current memory address, and the 'ret' function jumps to the last address stored in the stack

We can do something similar to jump to an specific address, like in this example:

push 0x8000 ; We push the address we want to jump to
ret

There's something important to notice when using the stack, when you 'pop' data you have to do it the reversed way of how you 'pushed' them.

For example, if i pushed AX and then CX, i have to pop CX and then AX, otherwise it won't retrieve the right data

General Commands

• Setting values

  It can be used to set variables

  Usage:

  mov destiny, source

  Example:

  mov ax, 0 ; this sets ax with 0
  mov ax, 0 + 1 ; this sets ax with 1 (0 + 1)
  mov ax, bx ; this sets ax with the data stored in bx

• Jump

  Jumps to a label, but code after it gets "lost" (basically, it dosen't get executed)

  Usage:

  jmp labelName

  Example:

  jmp mylabel ; Jumps to mylabel
  mov ax, 0 ; this won't be executed since the computer is running stuff from mylabel's code and onwards
  
  mylabel:
  mov ax, bx ; sets ax with bx
  ; code after this will be executed

• Labels

  These can be used to create points in code that can be accessed later.

  Usage :

  labelName:

  Example:

  jmp mylabel ; jump to mylabel
  
  ; more code
  
  mylabel: ; the label
  mov ax, 0 ; sets ax to 0

• Calls

  Jumps to a label but after the function/label ended execution its capable of returning to where it was originally

  Usage:

  call labelname

  Example:

  call mylabel ; call mylabel
  mov ax, bx ; unlike jmp, this code WILL be executed after mylabel returns
  
  ; more code
  
  mylabel: ; the label
  mov ax, 0 ; sets ax to 0
  ret ; return to where it was before

• Return

  Exits an ongoing label/function.

  Usage:

  ret

  Example:

  call mylabel ; calls mylabel
  ; after 'ret' it continues execution here
  mov bx, ax ; set bx with ax
  
  ; more code
  
  mylabel: ; the label or function
  
  mov ax, 0 ; sets ax to 0
  ret ; returns

• Define Byte

  Used to define 1 or multiple bytes

  Usages:

  db value
  db value1, value2
  mybyte db value

  Example:

  mytext db "Hello World!", 0 ; 'Hello World!' text with a NULL byte at the end. its memory address can be found by accessing 'mytext'

• Define Word

  Used to define 1 or multiple words (2 bytes, equivalent to a 'short'/'ushort' type in C# or a 16-Bit integer)

  Usages:

  dw value
  dw value1, value2
  wordname dw value

  Example:

  dw 0AA55h ; MBR signature in hexadecimal (bytes 55 and AA)
  dw 0FF02h, 0AA55h ; 2 bytes and the MBR signature (first 02 and FF, finally 55 and AA)

• Define Doubleword

  Used to define 1 or multiple doublewords (4 bytes, equivalent to a 'int'/'uint' type in C# or a 32-Bit integer)

  Usages:

  dd value
  dd value1, value2
  ddname dd value

  Example:

  dd 100000 ; 32-bit integer, in 16-bit this number would not be possible 

• Define Quadword

  Used to define 1 or multiple quadwords (8 bytes, equivalent to a 'long'/'ulong' type in C# or a 64-Bit integer)

  Usages:

  dq value
  dq value1, value2
  dqname dq value

  Example:

  dq 10345879094 ; 64-bit integer, in 32-bit it wouldn't be possible

• Define Ten Bytes

  Used to define sets of 10 bytes

  Usages:

  dt value
  dt value1, value 2
  dtname dt value

  Example:

  hello dt "HelloWorld" ; "HelloWorld" text. its 10 ASCII characters (10 bytes) which means it can be used with 'dt'

• Times

  Used to fill data a certain amount of times

  Usages:

  times number db data
  variableName times number db data

  Example:

  myArray times 10 db 0 ; Creates an empty "array/variable" called myArray with a size of 10 bytes
  times 510 - ($ - $$) db 0 ; Fills with zeroes since the end of the program until it reaches 510 (used for MBR/bootloaders)

• Increment

  Increments 1 from a variable (Equivalent in C# to variable++;)

  Usage:

  inc variable

  Example:

  aNumber db 14 ; Declare a byte with a value of 14
  inc byte [aNumber] ; Increments 1 (14 + 1 = 15)
  
  mov AX, 20
  inc AX ; We should get 21 (20 + 1 = 20)

• Decrement

  Decrements 1 from a register or memory space (Equivalent in C# to variable--;)

  Usage:

  dec variable

  Example:

  aNumber db 15 ; Declare a byte with a value of 15
  dec byte [aNumber] ; Decrements 1 (15 - 1 = 14) | You need to specify the size of the variable, in this case its a byte
  
  mov AX, 10
  dec AX ; We get 9 (10 - 1 = 9)

• Addition

  Basic Addition function (Equivalent in C# to variable += number;)

  Usages:

  add destination, source
  add destination, number

  Example:

  firstNumber db 1 ; Declare a byte with a value of 1
  secondNumber db 2 ; Declare a byte with a value of 2
  mov AL, byte [firstNumber] ; We can't directly use firstNumber + secondNumber, we have to store at least one of them into a register
  add AL, secondNumber ; Add secondNumber with firstNumber and store it in AL (2 + 1 = 3)

• Subtraction

  Basic Subtraction function (Equivalent in C# to variable -= number;)

  Usages:

  sub destination, source
  sub destination, number

  Example:

  firstNumber db 1 ; Declare a byte with a value of 1
  secondNumber db 2 ; Declare a byte with a value of 2
  mov AL, byte [firstNumber] ; We can't directly use firstNumber - secondNumber, we have to store at least one of them into a register
  sub AL, secondNumber ; Subtract secondNumber with firstNumber and store it in AL (2 - 1 = 1)

• BIOS Interrupts

  Used to send an interrupt

  Usage:

  int value

  Example:

  int 13h ; BIOS Interrupt used for disk access

• Comparison

  Used to compare 2 values to later use a conditional jump.

  Usage:

  cmp firstValue, secondValue

  Example:

  counter db 0 ; Create a counter and start from 0
  
  loop: ; The test loop
  inc counter ; Increment the counter by 1
  cmp byte [counter], 10 ; Compare the counter with 10 (We have to specify the size here, cuz its not a register)
  jle loop ; Jump to 'loop' if the first value (counter) is less or equal to the second value (10) from the last comparison

• Conditional Jumps

  Here is a list of jump commands that can be used with comparison (cmp)

  Usage:

  cmp firstValue, secondValue ; First make a comparison between two values
  ; Then use one of the commands from the list below

  Name for signed data	Name for UNsigned data	Description
  je or jz	je or jz	Jumps if both values are equal
  jne or jnz	jne or jnz	Jumps if both values are NOT equal
  jg or jnle	ja or jnbe	Jump if the first value is greater than the second value
  jge or jnl	jae or jnb	Jump if the first value is greater or equal than the second value
  jl or jnge	jb or jnae	Jump if the first value is less than the second value
  jle or jng	jbe or jna	Jump if the first value is less or equal than the second value

• XOR

  Can be used to set a value as 0 without using mov (Its useful cuz it uses sighly less space)

  Plus, it works for Bitwise operations.

  Usages:

  xor firstValue, secondValue ; used for Bitwise operations
  xor value, value ; better alternative to mov value, 0

  Example:

  xor ax, ax ; set ax to 0

Extra Details

• Real Mode 1MB of RAM

  Sometimes in Real Mode (16-bits) you are limited to 1MB of RAM even when valid 16-bit addresses exceed 1MB

  There is something called 'A20 Line' which is a method that allows the code to bypass the 1MB restriction

  I'll probably make someday a tutorial on that either here or somewhere else

• Bits

  This is used at the start of a program to specify the bits that its going to work with

  (Usually, bootloaders use Real mode and Operating systems use either Protected or Long mode)

  BITS 16 = Real mode BITS 32 = Protected mode BITS 64 = Long mode

• MBR small details/notes

  [BITS 16] ; The bootloader uses 16-bit Real Mode
  [org 0x7C00] ; This is to specify that the code starts at memory address 0x7C00

  must be at the start of the program

• Pointers

  Brackets ( [ & ] ) are used for pointers (equivalent to * in C# and & in C)

  Example:

  mov AL, [0x8000] ; This will take the value from 0x8000 and store it in AL, it can also be used with registers but not all of them
  mov AX, [BX] ; Example using a register (BX), it takes the value inside BX as a memory address
  mov [0x8000], AL ; We can also store things into memory

Interrupts Usage

When using BIOS Interrupts you'll need some information stored in registers before calling the interrupt.

(Some interrupts store result values)

Take a look at the data tables to know what you need to specify for an specific interrupt.

Note: this section is not complete

• INT 10h (Graphics)

  • AH 00h (Enable Video Mode)

    Main interrupt:

    Register	Data
    AH	00h
    AL	Video mode

    Returns:

    Register	Data
    AL	Video mode flag (30h if mode is 0-5 and 7 3Fh if mode = 6 20h if mode is > 7)

  • AH 01h (Set Text Mode Cursor Shape)

    Note: This function changes the shape of text mode cursor.

    Each character usually uses 8 scan lines.

    CX = 0607h is a regular cursor

    CX = 0007h is a full cursor

    CX = 2607h is an invisible cursor

    Main Interrupt:

    Register	Data
    AH	01h
    CH	Initial Scan Line
    CL	Final Scan Line

  • AH 02h (Set Cursor Position)

    Main Interrupt:

    Register	Data
    AH	02h
    BH	Page (0-3 in modes 2 & 3 0 - 7 in modes 0 & 1 0 on graphics modes)
    DH	Row (X)
    DL	Column (Y)

  • AH 03h (Get Cursor Information)

    Main Interrupt:

    Register	Data
    AH	03h
    BH	Page (0-3 in modes 2 & 3 0 - 7 in modes 0 & 1 0 on graphics modes)

    Returns:

    Register	Data
    CH	Initial Scan Line
    CL	Final Scan Line
    DH	Row (X)
    DL	Column (Y)

  • AH 08h (Get Character and Color from Cursor)

    Main Interrupt:

    Register	Data
    AH	08h
    BH	Page (0-3 in modes 2 & 3 0 - 7 in modes 0 & 1 0 on graphics modes)

    Returns:

    Register	Data
    AH	Color
    AL	Character

  • AH 09h (Write Character on the Cursor Position)

    Main Interrupt:

    Register	Data
    AH	09h
    AL	Character
    BH	Page (0-3 in modes 2 & 3 0 - 7 in modes 0 & 1 0 on graphics modes)
    BL	Color
    CX	Number of times to write the character

  • AH 0Ah (Only Write Character on the Cursor Position)

    Main Interrupt:

    Register	Data
    AH	0Ah
    AL	Character
    BH	Page (0-3 in modes 2 & 3 0 - 7 in modes 0 & 1 0 on graphics modes)
    CX	Number of times to write the character

  • AH 0Bh (Change Background Color)

    Main Interrupt:

    Register	Data
    AH	0Bh
    BH	00h
    BL	Color

  • AH 0Ch (Set Graphic Pixel)

    Main Interrupt:

    Register	Data
    AH	0Ch
    AL	Color
    BH	Page (0-3 in modes 2 & 3 0 - 7 in modes 0 & 1 0 on graphics modes)
    CX	X
    DX	Y

  • AH 0Eh (Teletype Output)

    I recommend this command for displaying characters

    Main Interrupt:

    Register	Data
    AH	0Eh
    AL	Character to write
    BH	Page
    BL	Foreground color

  • AH 0Dh (Get Graphic Pixel)

    Main Interrupt:

    Register	Data
    AH	0Dh
    BH	Page (0-3 in modes 2 & 3 0 - 7 in modes 0 & 1 0 on graphics modes)
    CX	X
    DX	Y

    Returns:

    Register	Data
    AL	Color

  • AH 13h (Write String)

    Main Interrupt:

    Register	Data
    AH	13h
    AL	Write mode (Update cursor after writing = 80h Text uses alternating chars and attributes = 40h Both modes: C0h)
    BH	Page (0-3 in modes 2 & 3 0 - 7 in modes 0 & 1 0 on graphics modes)
    BL	Color
    CX	String Length
    DH	Row (X)
    DL	Column (Y)
    ES:BP	String position

• INT 12h (Get Memory Size)

  Returns:

  Register	Description
  AX	Number of Kilobytes of memory

• INT 13h (Disk Access)

  • AH 42h (Reading from a Sector with LBA)

    Main interrupt:

    Register	Data
    AH	42h
    DL	Drive number
    DS:SI	Disk address packet

    Disk address packet:

    Size	Description
    Byte	Size of packet (10h or 18h)
    Byte	Reserved (0h)
    Word	Number of blocks (LBA) to transfer
    DWord	Transfer buffer
    QWord	Starting block number (LBA)

    Returns:

    Register	Description
    CF	Clear if success
    AH	00h
    CF	Set on error
    AH	Error code

  • AH 43h (Writing to a Sector with LBA)

    Main interrupt:

    Register	Data
    AH	43h
    AL	Write flags
    DL	Drive number
    DS:SI	Disk address packet

    Disk address packet:

    Size	Description
    Byte	Size of packet (10h or 18h)
    Byte	Reserved (0h)
    Word	Number of blocks (LBA) to transfer
    DWord	Transfer buffer
    QWord	Starting block number (LBA)

    Returns:

    Register	Description
    CF	Clear if success
    AH	00h
    CF	Set on error
    AH	Error code

• INT 16h (Keyboard)

  • AH 00h (Get Keystroke)

    Main interrupt:

    Register	Data
    AH	00h

    Returns:

    Register	Description
    AH	BIOS Scan code
    AL	ASCII Character

• INT 1Ah (System Time)

  • AH 00h (Get System Time)

    Main interrupt:

    Register	Data
    AH	00h

    Returns:

    Register	Description
    CX	Higher bytes of the Number of Clock Ticks since Midnight
    DX	Lower bytes of the Number of Clock Ticks since Midnight
    AL	Midnight flag (If it isn't a value of 0, Midnight passed since the last time the interrupt was used)

    For reference: Each second is around 18.2 Clock Ticks

  • AH 01h (Set System Time)

    Main interrupt:

    Register	Data
    AH	01h
    CX	Higher bytes of the Number of Clock Ticks since Midnight
    DX	Lower bytes of the Number of Clock Ticks since Midnight

    For reference: Each second is around 18.2 Clock Ticks