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TechnologyComputer systems
CPU & memory hierarchy
How a CPU runs instructions and the memory hierarchy from registers to disk.
Inside the CPU
- The control unit fetches and decodes instructions; the ALU does arithmetic and logic; registers hold the values being worked on right now.
- The fetch–decode–execute cycle repeats for every instruction, paced by the clock (measured in Hz).
Memory hierarchy
Faster memory is smaller and costlier, so machines layer it (fastest first):
- Registers (inside the CPU)
- Cache — L1, L2, L3
- Main memory (RAM)
- SSD / hard disk
Ideas that speed things up
- Cache keeps recently-used data close to the CPU, exploiting *locality of reference*.
- Pipelining overlaps the stages of consecutive instructions like an assembly line.
- Interrupts let a device signal the CPU when it's ready, instead of the CPU wastefully polling.