Operating Systems Internals Experiments

A personal sandbox for exploring OS internals below the abstraction layers that most developers take for granted. This repository sits alongside the formal IN3000 coursework but goes further into personal experiments and notes.

Areas explored

Boot process and x86 architecture

• Writing a minimal bootloader in assembly that loads a second-stage binary from disk
• Understanding the real mode → protected mode transition, the A20 line, and the Global Descriptor Table (GDT)
• Reading the BIOS interrupt table and using INT 13h for disk I/O before any OS is loaded
• Notes on the x86 register set, calling conventions (cdecl vs System V AMD64 ABI), and stack layout

Process and memory management in C

• Implementing a minimal round-robin scheduler with context switching via setjmp/longjmp
• Manual memory allocator experiments — malloc/free with a free-list, coalescing adjacent blocks, and alignment
• Playing with mmap, mprotect, and /proc/self/maps to understand virtual address space layout

Debugging at the metal

• GDB scripting for automated debugging sessions: breakpoints on function entry, examining registers, disassembling at runtime
• Bochs emulator for running bare-metal code with hardware-accurate CPU simulation
• Using objdump, readelf, and nm to understand ELF binaries

Linux kernel interfaces

• ptrace experiments — attaching to a process, reading/writing memory, intercepting syscalls
• Writing character device drivers as loadable kernel modules (LKM) to understand the kernel module lifecycle
• /proc filesystem — reading /proc/PID/status, /proc/meminfo, and parsing /proc/self/maps

Key insight

The line between “application programmer” and “systems programmer” is mostly familiarity. Once you understand that a process is just a virtual address space + a set of registers, and that the kernel is just code that runs with higher privilege, a lot of previously mysterious behavior becomes obvious.