A packet arrives, a key is pressed, a timer fires — and the CPU drops everything, mid-instruction-stream, to run kernel code. How interrupts work, why handlers are split in half, and how NAPI killed the interrupt storm.
Flash can't overwrite in place, erases in giant blocks, and wears out per cell — so every SSD runs a translation layer that remaps, garbage-collects, and levels wear behind your back. Write amplification, TRIM, and the cliff at 90% full, explained.
Signed integers could have been encoded a dozen ways. Hardware settled on two's complement because it makes the adder ignorant of sign — one circuit for everything. The same choice explains INT_MIN, sign extension bugs, and why abs() can return a negative number.
RAID levels are three ideas — stripe, mirror, parity — composed. The interesting parts are the failure math: why RAID 5 rebuilds fail at modern disk sizes, what the write hole is, and why RAID is not backup, mathematically.
The same loop over the same data runs 5x faster when the array is sorted. The instructions are identical. The difference is a gambler inside your CPU that's either winning or losing.
At reset, an x86 CPU wakes up pretending it's 1978 — 16-bit mode, 1 MB address space. Everything after that is bootstrapping: firmware, boot sector or EFI application, protected mode, and finally a kernel. I wrote a bootloader to learn the path; here's the map.
DRAM is ~100ns away; your CPU can do ~400 arithmetic operations in that time. Caches exist to hide that gap, and whether they succeed is decided by how your code lays out and walks its data.