Mini OS Kernel Development

The CPU starts in 16-bit real mode, loads sector 0 (MBR) at 0x7C00 and jumps to it. Your bootloader: enables A20 line (allows access beyond 1MB), sets up GDT (Global Descriptor Table) defining memory segments, switches to 32-bit protected mode, loads kernel from disk using BIOS INT 13h, jumps to kernel entry point. Alternatively, use GRUB as bootloader and implement Multiboot specification — vastly simpler.

Kernel entry in Assembly: set up stack pointer, clear BSS segment (zero-initialize global variables), call the main kernel function in C. Linker script (kernel.ld) places code at 0x100000 (1MB) in the virtual address space. The kernel runs in ring 0 (highest privilege). Before any C code runs: disable interrupts, set up stack frame, ensure alignment. From this point, write kernel in C.

VGA text mode provides an 80×25 character buffer at physical address 0xB8000. Each character = 2 bytes: ASCII value + attribute byte (foreground/background color, blink). To print a character: write to buffer[row × 80 + col]. Implement putchar(), puts(), printf() equivalent. Implement cursor movement using VGA port 0x3D4/0x3D5. This is your kernel

IDT maps interrupt vectors to handler functions. Set up 256 entries. For hardware interrupts (IRQ): remap PIC (Programmable Interrupt Controller) to vectors 32–47 (avoid conflict with CPU exceptions at 0–31). CPU exceptions: divide by zero (0), page fault (14), general protection fault (13) — implement handlers that print diagnostic info and halt. Timer IRQ (IRQ0) triggers scheduler. Keyboard IRQ (IRQ1) handles keyboard input.

Detect available RAM using BIOS E820 memory map. Implement a bitmap allocator: each bit represents a 4KB page frame. Initially mark all memory as used, then free pages according to E820 available regions minus kernel code. alloc_frame(): scan bitmap for first

Each process has a PCB (Process Control Block): PID, state (running/ready/blocked), saved register context, stack pointer, address space. Implement round-robin scheduler: maintain a ready queue. On each timer interrupt (every 10ms), save current process context (all CPU registers) to its PCB, pop next process from queue, restore its context, switch stack pointer, execute IRET to resume it. This is context switching — the core of multitasking.

User programs communicate with kernel via software interrupts: INT 0x80 (Linux convention). In the IDT, vector 128 (0x80) points to syscall handler. User program places syscall number in EAX, parameters in EBX/ECX/EDX, executes INT 0x80. Handler saves registers, calls appropriate kernel function based on EAX, stores return value in EAX, restores registers, returns. Implement syscalls: sys_write (print string), sys_read (keyboard input), sys_exit (terminate process).