Why CPU Time ≠ Wall Clock Time! - Computerphile
The speaker explains how computers estimate time, from old systems that counted screen interrupts to modern CPUs with high-frequency counters, and how operating systems combine hardware clocks, boot-time offsets, and calibration to turn raw ticks into usable wall-clock time and precise timing measurements.
MAIN POINTS FROM TRANSCRIPT
- Early computers tracked elapsed time by counting regular interrupts tied to video refresh.
- Battery-backed clocks later provided real-world time at startup.
- Modern CPUs use very fast internal counters, often around gigahertz frequencies.
- Operating systems convert counter ticks into time using boot-time offsets and clock calibration.
TAKEAWAYS
- Timekeeping in computers is fundamentally about counting events and translating counts into seconds.
- Higher-resolution timing became necessary as computing tasks demanded nanosecond-level measurement.
- Raw CPU clock rates are not automatically trustworthy without calibration or correction.
- Accurate wall-clock time depends on combining hardware counters with external or persistent time sources.