SectorZero-64

0x7C00: The iconic BIOS load address (64-bit)

A bare-metal x86_64 bootloader and freestanding 64-bit C kernel implementation built from scratch without external libraries, runtimes, or modern firmware abstraction layers (UEFI).

SectorZero-64 boots directly on bare silicon or an emulator (QEMU) in 16-bit Real Mode via the Master Boot Record (MBR), loads kernel sectors from disk, constructs a 4-level paging hierarchy (PML4), transitions into 64-bit Long Mode via PAE and the EFER Model-Specific Register (MSR), and displays text using the memory-mapped VGA frame buffer (0xB8000).

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Technical Overview

• MBR Stage (boot.asm):
  • Initializes segments, registers, and the stack at 0x7C00.
  • Executes BIOS INT 0x13, AH=0x02 to load kernel sectors from the boot medium into RAM starting at physical address 0x10000.
  • Identity maps the lower 2 MB of physical memory using a 2 MB huge page across three tables located from 0x1000 to 0x4000:
    • PML4 (Page Map Level 4) at 0x1000
    • PDPT (Page Directory Pointer Table) at 0x2000
    • PD (Page Directory) at 0x3000 with the huge page flag (0x83) set
  • Enables Physical Address Extension (PAE) via bit 5 of CR4.
  • Activates Long Mode Enable (LME) by reading and updating the extended feature enable register (EFER MSR 0xC0000080) using rdmsr / wrmsr.
  • Enables paging and protected mode simultaneously by writing to CR0 (bits 31 and 0).
  • Performs a far jump using a 64-bit code segment descriptor (0x00209A0000000000) to flush the execution pipeline and start 64-bit instruction decoding.
• Entry Shim (kernel_entry.asm):
  • Sets up the 64-bit ABI stack frame (rsp initialized at 0x90000), calls the C entry point main(), and traps execution in a cli; hlt loop upon return.
• Freestanding C Kernel (kernel.c):
  • Built with -ffreestanding, -mno-red-zone, and SIMD disabled (-mno-sse).
  • Directly addresses the 16-bit character/attribute memory cells at 0xB8000.
• Linker Specification (linker.ld):
  • Aligns sections (.text, .rodata, .data, .bss) on 4 KB page boundaries, mapped linearly starting at address 0x10000.

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Directory Structure

.
├── LICENSE           # GPLv3 license
├── Makefile          # Build scripts, emulator runners, and debug targets
├── README.md         # Architecture documentation and usage guide
├── boot.asm          # 16-bit MBR bootloader, paging table setup, and Long Mode jump
├── kernel.c          # Freestanding 64-bit C kernel driving VGA text memory
├── kernel_entry.asm  # 64-bit assembly wrapper calling C main()
└── linker.ld         # Linker script locating code at 0x10000

Prerequisites

Building requires standard x86_64 development utilities, NASM, and QEMU.

Debian / Ubuntu / Mint

sudo apt update
sudo apt install build-essential nasm qemu-system-x86

Arch Linux

sudo pacman -S base-devel nasm qemu-system-x86

Fedora

sudo dnf install gcc nasm qemu-system-x86

Building and Running

1. Build the Disk Image

Compile all assembly shims and C files into flat binaries, concatenate them, and pad the resulting image:

make

Output files in build/:

build/boot.bin: 512-byte MBR boot sector terminated by 0xAA55.

build/kernel.bin: 64-bit flat kernel binary.

build/0x7C00-64.img: 16 KB raw bootable image.

2. Boot in QEMU

Launch QEMU with the raw image attached as a drive:

make run

3. Debug with GDB

To inspect the boot sequence and transition from real mode to long mode step by step:

Launch QEMU with a halted CPU listening on port 1234:

make debug

In a separate terminal, connect GDB:

gdb -ex "target remote localhost:1234" \
    -ex "set architecture i8086" \
    -ex "break *0x7C00" \
    -ex "continue"

(Switch architectures via set architecture i386:x86-64 after the long mode transition).

4. Clean Build Files

make clean

Memory Map

Physical Address Range	Usage / Mapped Region
0x00000 - 0x003FF	Real Mode Interrupt Vector Table (IVT)
0x00400 - 0x004FF	BIOS Data Area (BDA)
0x01000 - 0x01FFF	Page Map Level 4 (PML4)
0x02000 - 0x02FFF	Page Directory Pointer Table (PDPT)
0x03000 - 0x03FFF	Page Directory (PD) with 2MB Identity Page
0x07C00 - 0x07DFF	MBR Bootloader Binary (boot.bin)
0x10000 - 0x13FFF	Loaded 64-Bit C Kernel (kernel.bin)
0x80000 - 0x90000	64-bit Stack Space (Top at 0x90000)
0xB8000 - 0xB8FA0	Color Text VGA Buffer (80 columns × 25 rows)

License

This project is open-source software licensed under the GNU General Public License v3.0 (GPLv3). See the LICENSE file for terms and conditions.

Copyright (C) k!M/pizslacker 2026