PintOS

PintOS is the OS project from Berkeley's CS 162. You're handed a skeleton kernel and told to build the rest of it. By the end, you have a working operating system that can run user programs, manage memory, and persist files to disk.

What I implemented

Threads: Priority scheduling with priority donation to handle priority inversion. Implemented the thread lifecycle — creation, blocking, unblocking, and termination — along with synchronization primitives: locks, semaphores, and condition variables.

User programs: System call interface between user space and the kernel. Argument passing, process execution via exec, parent-child relationships with wait, and proper memory validation to prevent user programs from crashing the kernel.

Virtual memory: Demand paging with a supplemental page table. Pages are loaded lazily from disk on fault. Implemented a clock algorithm for page eviction, a swap partition for evicted pages, and memory-mapped files.

File system: Extended the base file system with a buffer cache (LRU eviction), extensible file growth, and subdirectories. Added synchronization throughout to support concurrent file operations.

Why it matters

PintOS is the project that made systems real for me. It's one thing to understand that an OS manages memory. It's another to spend three hours debugging a page fault handler at 2am, only to realize you forgot to acquire a lock. The gap between knowing and building is where the actual learning happens.