ARBOS - A Custom Operating System

Documentation

1. Introduction

ARBOS is a 32-bit x86 operating system built from scratch, designed to run on real hardware or emulators like QEMU. It includes essential OS components such as:

• Bootloader (Multiboot-compliant)
• Memory Management (PMM & Paging)
• Interrupt Handling (IDT, PIC, Exceptions, IRQs)
• Process Scheduling (Basic Round-Robin Scheduler)
• System Calls (Syscall Interface)
• Device Drivers (Keyboard, VGA, Timer)

This document explains the key components and their functionality step-by-step.

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2. Boot Process

2.1. Bootloader (boot/boot.asm)

• The OS boots using a Multiboot-compliant bootloader.
• The bootloader:
  1. Sets up a stack for the kernel.
  2. Calls idt_setup() to initialize interrupts.
  3. Jumps to the kernel’s kmain() function.

2.2. Kernel Entry (kernel/kernel.c - kmain())

• Initializes core OS components:
  • Physical Memory Manager (PMM)
  • Paging (Virtual Memory)
  • Timer (PIT)
  • Task Scheduler
• Creates two test tasks (task_a() and task_b()) and starts the scheduler.

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3. Memory Management

3.1. Physical Memory Manager (kernel/pmm.c)

• Tracks free and used memory pages (4KB each).
• Uses a bitmap to manage allocations.
• Key Functions:
  • pmm_init(): Marks low memory (0-1MB) and kernel space as used.
  • pmm_alloc_page(): Allocates a free page.
  • pmm_free_page(): Releases a page back to the free pool.

3.2. Paging (kernel/paging.c)

• Implements identity paging (maps physical addresses 1:1 to virtual).
• Sets up a Page Directory (PD) and Page Tables (PT).
• Enables paging by setting the CR0 and CR3 registers.

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4. Interrupts & Exceptions

4.1. Interrupt Descriptor Table (kernel/interrupts.c)

• The IDT defines how the CPU handles interrupts.
• idt_setup():
  • Configures exception handlers (ISRs 0-31).
  • Remaps the PIC (Programmable Interrupt Controller) to avoid conflicts.
  • Sets up IRQ handlers (Timer, Keyboard).
  • Configures Syscall handler (int 0x80).

4.2. Exception Handling (kernel/exceptions.c)

• Handles CPU exceptions (e.g., Divide Error, Page Fault).
• Prints debug info (exception number, error code).

4.3. Hardware Interrupts

1. Timer (kernel/timer.c)

   • Uses the PIT (Programmable Interval Timer).
   • init_timer(): Sets the timer frequency (e.g., 100Hz).
   • timer_handler(): Increments a tick counter.

2. Keyboard (drivers/keyboard.c)

   • Reads scancodes from PS/2 keyboard.
   • Converts scancodes to ASCII and prints them via VGA.

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5. Process Management & Scheduling

5.1. Task Structure (include/process.h)

• Each task has:
  • PID (Process ID)
  • State (Running, Ready, Blocked, etc.)
  • Stack Pointer (Saved ESP)
  • Stack Page (4KB allocated memory)

5.2. Scheduler (kernel/process.c)

• Round-Robin Scheduling: Tasks yield CPU voluntarily.
• Key Functions:
  • task_create(): Allocates a stack and initializes a task.
  • task_yield(): Switches to the next ready task.
  • scheduler_start(): Begins task execution.

5.3. Context Switching (kernel/context_switch.S)

• Saves CPU registers of the current task.
• Loads registers of the next task.
• Uses pushad/popad for register preservation.

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6. System Calls (Syscalls)

6.1. Syscall Mechanism (kernel/syscall.c)

• int 0x80 triggers a syscall.
• Syscall Numbers:
  • SYS_WRITE (1): Writes data to the screen.
  • SYS_GET_TICKS (2): Returns timer ticks.

6.2. Syscall Handling (kernel/isr_syscall.S)

• The syscall stub forwards arguments to syscall_handler_regparm().
• Return values are stored in syscall_ret.

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7. Device Drivers

7.1. VGA Text Mode (drivers/vga.c)

• Writes text to 0xB8000 (VGA buffer).
• Functions:
  • vga_putc(): Prints a character.
  • vga_puts(): Prints a string.
  • vga_clear(): Clears the screen.

7.2. Keyboard (drivers/keyboard.asm & keyboard.c)

• IRQ1 handler reads scancodes from port 0x60.
• Converts scancodes to ASCII (simplified mapping).

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8. Build & Execution

8.1. Compilation (build.sh)

• Uses NASM for assembly files.
• Uses Clang for C files (cross-compiled for i686-elf).
• Links objects into kernel.bin.

8.2. Running in QEMU

qemu-system-i386 -kernel dist/kernel.bin

• Boots the OS in QEMU with keyboard support.

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9. Future Improvements

1. User-mode separation (Ring 3).
2. File system support.
3. Dynamic memory allocation (Heap management).
4. Multi-core CPU support.
5. Better shell/command interface.

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Conclusion

ARBOS is a simple but functional 32-bit OS with:
✔ Bootloader
✔ Memory Management
✔ Interrupts & Syscalls
✔ Task Scheduling
✔ Basic Drivers