The OS protects the hardware from misuse and gives programs simple, uniform abstractions for it — processes, virtual memory, and files.

It sits between programs and the machine so that no single buggy or malicious program can monopolize the CPU, corrupt another program's memory, or talk to devices directly — everything risky is funneled through the kernel.

Two main roles:

1. Protect hardware from misuse by buggy/malicious programs
2. Provide simple, uniform mechanisms for hardware manipulation

Three fundamental OS abstractions:

Why these abstractions exist:

• Without processes, one buggy program could crash the whole system
• Without virtual memory, programs would fight over memory addresses
• Without file abstraction, every program would need disk/network/device drivers!

Protection Rings (x86):

• Ring 0 (Kernel): Full hardware access - can execute ANY instruction
• Ring 1-2 (Device drivers): Limited access (rarely used in modern OSes)
• Ring 3 (Applications): Most restricted - can only execute "safe" instructions

Why rings matter for security:

• Malware in Ring 3 can't directly access hardware or other processes
• Even if your browser is compromised, it can't read kernel memory
• Kernel bugs (Ring 0) are much more dangerous than userspace bugs

Tip: Ring 0 = "Root/God mode", Ring 3 = "Restricted/User mode". Lower ring number = more power.

Go deeper:

• Operating system (Wikipedia) — managing and protecting hardware, isolating programs, and abstracting devices behind common services.

It zeroes %eax (anything XORed with itself is 0) — smaller and faster than mov $0, %eax, and it also clears the upper half of %rax.

xor %eax, %eax

Any value XORed with itself is zero. The CPU recognizes this special case and:

• Executes it with zero latency (register renaming)
• Clears dependency chains
• Is only 2 bytes (vs 5 bytes for mov $0, %eax)

Bonus: On x86_64, writing to %eax automatically zeros the upper 32 bits of %rax. So xor %eax, %eax also zeros %rax.

Where you'll see it: Function prologues, before a loop accumulator, before call to set %eax = 0 (indicates no floating-point varargs to printf/sscanf).

Go deeper:

• Intel XOR reference — DEST := DEST XOR SRC, so a register XORed with itself becomes zero
• Wikipedia: Bitwise operation — why x XOR x = 0