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

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

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)

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

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

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

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

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AssemblyGuide

Registers

The Stack

General Commands

Setting values

Jump

Labels

Calls

Return

Define Byte

Define Word

Define Doubleword

Define Quadword

Define Ten Bytes

Times

Increment

Decrement

Addition

Subtraction

BIOS Interrupts

Comparison

Conditional Jumps

XOR

Extra Details

Bits

MBR small details/notes

Pointers

Interrupts Usage

INT 10h (Graphics)

AH 00h (Enable Video Mode)

AH 01h (Set Text Mode Cursor Shape)

AH 02h (Set Cursor Position)

AH 03h (Get Cursor Information)

AH 08h (Get Character and Color from Cursor)

AH 09h (Write Character on the Cursor Position)

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

AH 0Bh (Change Background Color)

AH 0Ch (Set Graphic Pixel)

AH 0Eh (Teletype Output)

AH 0Dh (Get Graphic Pixel)

AH 13h (Write String)

INT 12h (Get Memory Size)

INT 13h (Disk Access)

AH 42h (Reading from a Sector with LBA)

AH 43h (Writing to a Sector with LBA)

INT 16h (Keyboard)

AH 00h (Get Keystroke)

INT 1Ah (System Time)

AH 00h (Get System Time)

AH 01h (Set System Time)