Computer Architecture & Operating Systems

A deep-dive into the x86 boot sequence. We write a 512-byte MBR in NASM, load the kernel via INT 0x13 extended read, set up a minimal three-entry GDT, transition the CPU from 16-bit Real Mode to 32-bit Protected Mode by setting CR0 bit 0, execute the mandatory FAR JUMP to flush the pipeline, and land in kernel_main(). We then build a text-mode VGA driver writing directly to 0xB8000 and a PS/2 keyboard driver for a working line-editing shell.

Extension Ideas for Students

• Full ISO keyboard layout with Shift, CapsLock, and numeric keypad support in the scancode table
• VGA scrolling buffer — circular ring, Page-Up / Page-Down support
• ANSI escape-code colour support (ESC[31m…ESC[0m) in the VGA driver
• Command history (up/down arrow) with a 20-entry ring buffer
• Tab-completion scanning the command table for prefix matches
• Replace the custom bootloader with a GRUB2 multiboot header and load as a standard ELF binary

We design the pcb_t struct (state, stack pointer, entry point, PID), implement create_process(), program the i8253 PIT for 100 Hz clock interrupts on IRQ0, and write a PUSHAD/POPAD context-switch stub in NASM. The shell gains a ps command that displays all PCBs.

Extension Ideas for Students

• Multi-level feedback queue (MLFQ) with 3 priority bands and ageing to prevent starvation
• sleep(ms) syscall using the PIT tick counter and a wake-time-sorted sleep queue
• fork(): duplicate the PCB, copy 4 KB stack, return 0 to child and child PID to parent
• CFS-style virtual runtime (vruntime) with a sorted run-queue for proportional-share scheduling

We extend the kernel with lightweight kernel threads sharing a process's address space, implement a blocking mutex_t and a counting semaphore_t. The classic myglobal race condition demonstrates the problem — with and without a mutex. A bounded-buffer producer-consumer validates all three semaphores working together.

Extension Ideas for Students

• Priority inheritance in the mutex: boost the lock holder's priority to the highest waiter's level
• Read-write lock (rwlock_t): concurrent readers, exclusive writers with starvation prevention
• Deadlock detector: periodically walk the resource-allocation graph with DFS looking for cycles
• Implement the Readers-Writers problem with writer starvation prevention using a turn variable

We parse the BIOS E820 memory map from the boot stage, build a bitmap (1 bit per 4 KB frame), and implement pmm_alloc_frame() and pmm_free_frame(). The shell gains a meminfo command showing total / used / free frame counts. Kernel identity-mapping ensures physical == virtual addresses throughout.

Extension Ideas for Students

• Buddy allocator to reduce external fragmentation for multi-frame contiguous requests
• Virtual memory manager: software-managed page table + #PF handler for demand paging
• Slab allocator: kmalloc(n) / kfree(p) for variable-size kernel heap objects on top of the frame allocator
• Clock-algorithm page replacement when physical frames are exhausted — swap to the RAM disk from Stage 4

The final stage builds a POSIX-inspired file system on a 1 MB RAM disk. We implement a superblock, a bitmap for blocks and inodes, a flat directory structure, and a full open / read / write / close / unlink interface. Shell commands ls, touch, cat, and write validate the complete system.

Extension Ideas for Students

• Single-indirect block pointer in the inode — expand max file size from 32 KB to 4 MB + 32 KB
• Subdirectories: mkdir / cd / pwd with a per-process current-directory pointer stored in the PCB
• Write-ahead journaling (8 KB log) so the FS survives a simulated crash and can replay on next mount
• VFS abstraction layer: file_ops_t vtable shared by the RAM disk FS and a second ROM read-only FS