mboot

This repository contains a minimal x86 bootloader and a simple protected-mode kernel. The bootloader loads the kernel from disk into memory, sets up a basic GDT, switches the CPU to 32-bit protected mode, and jumps to the kernel's entry point.

Build

# dependencies
sudo pacman -S base-devel gcc nasm qemu 

# build
make clean && make all 
# run the image using qemu
make run 

Boot Sector Memory Map

Physical Address	Size	Content
0x00000–0x07BFF	31 KB	BIOS data areas: IVT, BDA, EBDA
Boot Sector 0x07C00–0x07DFF	512 B	Loaded by BIOS as the first sector of the disk (MBR)
0x07C00–...	...	Bootloader code: sets up stack, loads kernel, sets up GDT
0x07C00–...	...	Includes GDT, GDT descriptor, and real-mode to protected-mode switch
0x07DFF	1 B	Padding up to offset 510
0x07FFE–0x07FFF	2 B	Boot signature (0xAA55)
Kernel 0x1000–…	2 × 512 B + …	Kernel flat binary (entry: _start/start_kernel)

Global Descriptor Table

Source: https://wiki.osdev.org/Global_Descriptor_Table

Access Byte

Bit 7	Bits 6–5	Bit 4	Bit 3	Bit 2	Bit 1	Bit 0
P	DPL	S	E	DC	RW	A

Field	Bit(s)	Description
P	7	Present — Must be 1 for a valid segment.
DPL	6–5	Descriptor Privilege Level — 0 = kernel (highest), 3 = user (lowest).
S	4	Descriptor Type — 0 = system segment (e.g. TSS), 1 = code/data segment.
E	3	Executable — 0 = data segment, 1 = code segment (executable).
DC	2	Direction/Conforming — For data segments: 0 = grows up, 1 = grows down; for code segments: 0 = non-conforming (only same DPL), 1 = conforming (callable from ≥ DPL).
RW	1	Read/Write — For data segments: 0 = read-only, 1 = writable; for code segments: 0 = non-readable, 1 = readable.
A	0	Accessed — Set by the CPU on first access; initialize to 1 if needed to avoid a fault on first access.

Flags

Bit 3	Bit 2	Bit 1	Bit 0
G	DB	L	Reserved

Flag	Bit	Description
G	3	Granularity: 0 = limit in 1 B blocks; 1 = limit in 4 KiB blocks
DB	2	Default size: 0 = 16-bit protected mode segment; 1 = 32-bit protected mode segment
L	1	Long-mode code: 1 = 64-bit code segment (DB must be 0); 0 = other segment types

BIOS Common functions

source: https://wiki.osdev.org/BIOS

• INT 0x10, AH = 1 -- set up the cursor
• INT 0x10, AH = 3 -- cursor position
• INT 0x10, AH = 0xE -- display char
• INT 0x10, AH = 0xF -- get video page and mode
• INT 0x10, AH = 0x11 -- set 8x8 font
• INT 0x10, AH = 0x12 -- detect EGA/VGA
• INT 0x10, AH = 0x13 -- display string
• INT 0x10, AH = 0x1200 -- Alternate print screen
• INT 0x10, AH = 0x1201 -- turn off cursor emulation
• INT 0x10, AX = 0x4F00 -- video memory size
• INT 0x10, AX = 0x4F01 -- VESA get mode information call
• INT 0x10, AX = 0x4F02 -- select VESA video modes
• INT 0x10, AX = 0x4F0A -- VESA 2.0 protected mode interface