Como usar
- Copie o comando e verifique o pacote antes de instalar no seu ambiente de agente.
- Consulte no original as ferramentas, credenciais e ambientes necessários.
- Descreva os dados de entrada e a entrega esperada; confira a saída com as fontes.
npx skills add https://github.com/mohitmishra786/low-level-dev-skills --skill os-dev-scratchAntes de começar
- A instalação não inclui assinatura de modelo, franquia API ou acesso aos seus arquivos.
- Estrelas e instalações indicam atividade, não um teste independente de qualidade ou segurança.
Documento original do Skill
Renderizado do repositório de origem, preservando títulos, exemplos, código, tabelas, links e imagens.
OS Development from Scratch
Purpose
Guide agents through building a minimal operating system from scratch: bootloader stages (BIOS/GRUB vs UEFI/limine), 64-bit long mode setup with GDT and page tables, IDT and interrupt handlers, PIC/APIC configuration, basic keyboard and serial drivers, physical and virtual memory managers, context switching, with xv6-RISC-V as a reference architecture.
When to Use
- Learning how an OS boots from power-on to
main() - Implementing protected/long mode transitions on x86-64
- Writing a physical memory allocator (bitmap) and page table manager
- Handling timer, keyboard, and page fault interrupts
- Testing with QEMU
-kerneland cross-compilerx86_64-elf-gcc - Porting concepts from xv6 to a custom x86 or RISC-V kernel
Workflow
1. Boot stages overview
BIOS path (legacy)
├── BIOS POST
├── MBR (512 bytes) → boot sector loads stage2
├── GRUB/multiboot → loads kernel ELF
└── kernel entry (_start)
UEFI path (modern)
├── UEFI firmware
├── EFI bootloader (limine, systemd-boot)
├── Loads kernel + initrd from ESP
└── kernel entry (handoff with memory map)2. Toolchain setup
bash
# Cross-compiler for bare metal
brew install x86_64-elf-gcc x86_64-elf-binutils # macOS
# or build from source / apt install gcc-x86-64-elf
x86_64-elf-gcc --version
# QEMU for testing
qemu-system-x86_64 --versionLinker script essentials:
ld
/* linker.ld */
ENTRY(_start)
SECTIONS {
. = 0x100000; /* 1MB — typical kernel load address */
.text : { *(.text .text.*) }
.rodata : { *(.rodata .rodata.*) }
.data : { *(.data .data.*) }
.bss : { *(.bss .bss.*) }
}bash
x86_64-elf-gcc -ffreestanding -nostdlib -c kernel.c -o kernel.o
x86_64-elf-ld -T linker.ld kernel.o -o kernel.elf3. Multiboot/limine boot
bash
# QEMU direct kernel boot (no disk)
qemu-system-x86_64 \
-kernel kernel.elf \
-serial stdio \
-m 128M \
-no-reboot -no-shutdown
# With limine (UEFI)
qemu-system-x86_64 \
-bios /usr/share/ovmf/OVMF.fd \
-drive file=disk.img,format=raw \
-serial stdio4. Long mode setup
Protected mode (32-bit) → enable PAE → setup 4-level page tables → enable long modec
// Minimal GDT entry (64-bit flat segments)
struct gdt_entry {
uint16_t limit_low;
uint16_t base_low;
uint8_t base_mid;
uint8_t access;
uint8_t granularity;
uint8_t base_high;
} __attribute__((packed));
// Page table setup (4KB pages, identity map first 1GB)
uint64_t pml4[512] __attribute__((aligned(4096)));
uint64_t pdpt[512] __attribute__((aligned(4096)));
uint64_t pd[512] __attribute__((aligned(4096)));
void setup_paging(void) {
for (int i = 0; i < 512; i++)
pd[i] = (i * 0x200000) | 0x83; // 2MB huge pages
pdpt[0] = (uint64_t)pd | 0x03;
pml4[0] = (uint64_t)pdpt | 0x03;
__asm__ volatile("mov %0, %%cr3" :: "r"(pml4));
}5. IDT and interrupt handlers
c
struct idt_entry {
uint16_t offset_low;
uint16_t selector;
uint8_t ist;
uint8_t type_attr;
uint16_t offset_mid;
uint32_t offset_high;
uint32_t zero;
} __attribute__((packed));
// ISR stub (assembly) → common handler → dispatch by vector
void interrupt_handler(struct trap_frame *frame) {
if (frame->vector == 14) // page fault
handle_page_fault(frame->cr2, frame->error_code);
else if (frame->vector == 33) // keyboard IRQ remapped
keyboard_handler();
}bash
# Test page fault
# QEMU monitor: info registers6. PIC and APIC
c
// Legacy PIC remapping (8259)
// Remap IRQ 0-15 to vectors 32-47
outb(0x20, 0x11); outb(0xA0, 0x11);
outb(0x21, 0x20); outb(0xA1, 0x28); // vector offsets
// ...
// Modern: use APIC/IOAPIC (ACPI MADT parsing)
// LAPIC timer for preemption7. Serial and keyboard drivers
c
// COM1 serial (0x3F8)
void serial_putc(char c) {
while ((inb(0x3F8 + 5) & 0x20) == 0);
outb(0x3F8, c);
}
// PS/2 keyboard scancode → ASCII lookup table
void keyboard_handler(void) {
uint8_t scancode = inb(0x60);
char c = scancode_to_ascii[scancode];
if (c) serial_putc(c);
outb(0x20, 0x20); // EOI to PIC
}bash
qemu-system-x86_64 -kernel kernel.elf -serial stdio
# printk output appears in terminal8. Physical memory manager
c
// Bitmap allocator over usable RAM regions
// From multiboot memory map or UEFI memory map
#define PAGE_SIZE 4096
uint8_t *frame_bitmap;
uint64_t total_frames;
uint64_t alloc_frame(void) {
for (uint64_t i = 0; i < total_frames; i++) {
if (!test_bit(frame_bitmap, i)) {
set_bit(frame_bitmap, i);
return i * PAGE_SIZE;
}
}
return 0; // OOM
}9. Context switching
c
struct context {
uint64_t rax, rbx, rcx, rdx, rsi, rdi, rbp, rsp;
uint64_t r8, r9, r10, r11, r12, r13, r14, r15;
uint64_t rip;
};
void switch_context(struct context *old, struct context *new);
// Assembly: save callee-saved regs to old, restore from new, ret to new->ripCooperative scheduling first; add timer IRQ preemption later.
10. xv6-RISC-V reference
bash
git clone https://github.com/mit-pdos/xv6-riscv
cd xv6-riscv && make qemu| xv6 component | x86 equivalent |
|---|---|
kernel/vm.c | Page table management |
kernel/trap.c | IDT/interrupt dispatch |
kernel/proc.c | Context switch, scheduler |
kernel/plic.c | PIC/APIC interrupt controller |
user/usys.pl | System call stubs |
Common Problems
| Symptom | Cause | Fix |
|---|---|---|
| Triple fault on boot | Invalid GDT/IDT or stack | Set up stack before enabling interrupts |
| QEMU black screen | No serial output configured | -serial stdio; early serial_init |
| Page fault in kernel | Unmapped address | Identity-map kernel; check CR3 |
| IRQ never fires | PIC mask or IDT not loaded | lidt; unmask IRQ in PIC |
| Timer doesn't tick | LAPIC not initialized | Parse ACPI; calibrate LAPIC timer |
| Linker relocation error | Wrong load address | Match linker.ld with bootloader expectation |
Related Skills
skills/low-level-programming/assembly-x86— x86-64 assembly for ISR stubsskills/low-level-programming/assembly-riscv— xv6-RISC-V reference ISAskills/platform/riscv-privileged— RISC-V trap handling and page tablesskills/virtualization/qemu-kvm— QEMU flags for kernel developmentskills/kernel/kernel-internals— Linux implementation of these conceptsskills/low-level-programming/linux-kernel-modules— graduate to Linux once basics work
