Craft a Bootloader

This is a simple bootloader for x86_64 arch, for learning purposes.

Prerequisites (Arch Linux)

Install the following packages:

sudo pacman -S nasm qemu-system-x86 tigervnc

Package	Purpose	Minimum Version
nasm	x86 assembler for boot.s	2.16+
qemu-system-x86	QEMU emulator (provides qemu-system-x86_64)	8.0+
tigervnc	Provides vncviewer for graphical QEMU output	1.14+

Build & Run

Build bootloader.bin

make

Launch in QEMU with VNC output (opens vncviewer automatically and closes QEMU on exit)

make run

Clean build artifacts

make clean

Learning Material

• https://www.joe-bergeron.com/posts/Writing%20a%20Tiny%20x86%20Bootloader/
• https://pdos.csail.mit.edu/6.828/2018/xv6/book-rev11.pdf

Notes

The code enters 16-bit Real mode, which is the simplest x86 CPU operating mode, active at startup (reset). It uses 16-bit registers, a 20-bit address bus (limiting accessible memory to 1 MB), and addresses memory via segment:offset pairs. There's no memory protection, no privilege levels, and no virtual memory — any code can access any memory or I/O port. Real mode is what DOS and legacy BIOS bootloaders run in.

We need to start with 0x07C0 as this address is the starting point for The last 2 bytes of the 512 bytes sector are 0xAA55, which represent the boot signature.

In x86 Real mode the following registers are used:

Category	Register(s)	Purpose
General-purpose	AX	Accumulator, commonly used for arithmetic and I/O.
General-purpose	BX	Base register, often used for memory addressing.
General-purpose	CX	Count register, used in loops and shifts.
General-purpose	DX	Data register, used with AX in some operations and I/O.
General-purpose	SI	Source index, used in string and memory operations.
General-purpose	DI	Destination index, used in string and memory operations.
General-purpose	BP	Base pointer, typically used to access stack-based data.
General-purpose	SP	Stack pointer, points to the top of the stack.
Segment	CS	Code segment, used with IP to locate instructions.
Segment	DS	Data segment, default segment for many data accesses.
Segment	ES	Extra segment, often used in string instructions.
Segment	SS	Stack segment, used with SP/BP.
Segment	FS	Extra segment register on later x86 CPUs.
Segment	GS	Extra segment register on later x86 CPUs.
Instruction pointer	IP	Offset of the next instruction within CS.
Flags	FLAGS	Status and control bits for CPU operations.
8-bit subregisters	AH / AL	High and low 8-bit parts of AX.
8-bit subregisters	BH / BL	High and low 8-bit parts of BX.
8-bit subregisters	CH / CL	High and low 8-bit parts of CX.
8-bit subregisters	DH / DL	High and low 8-bit parts of DX.

Notes on nasm assembly:

Instruction / Directive	What it does
bits 16	Tells NASM to assemble 16-bit code (Real mode).
mov dest, src	Copies src into dest. Can move between registers, memory, and immediates.
call label	Pushes the return address onto the stack and jumps to label.
ret	Pops the return address from the stack and jumps back after a call.
push val	Decrements SP and stores val on the stack.
pop reg	Loads a value from the stack into reg and increments SP.
pusha	Pushes all 16-bit general-purpose registers (AX, CX, DX, BX, SP, BP, SI, DI) onto the stack.
popa	Restores all registers saved by pusha (in reverse order).
add dest, src	Adds src to dest and stores the result in dest. Used here to clean arguments off the stack (add sp, 2).
cli	Clears the interrupt flag — CPU ignores maskable hardware interrupts.
hlt	Stops the CPU until the next external interrupt (or reset).
int n	Triggers software interrupt number n. BIOS provides services via int 0x10 (video), int 0x13 (disk), etc.
or dest, src	Performs a bitwise OR. Used here as or al, 0 to set the zero flag without changing the value (tests for zero).
je label	Jumps to label if the zero flag is set (i.e. if the previous operation resulted in zero).
jmp label	Unconditionally jumps to label.
db val	Defines a byte (or bytes) of data. msg: db "Hello", 0 stores the string followed by a null terminator.
dw val	Defines a word (2 bytes) of data. dw 0xAA55 places the boot signature at the end of the sector.
times count directive	Repeats directive count times. times 510-($-$$) db 0 fills the rest of the 512-byte sector with zeros.