Quiz Entry - updated: 2026.07.14
Why does a computer need multiple buses instead of just one?
Because devices differ enormously in speed: a dedicated fast path serves CPU↔RAM, while slower shared buses serve peripherals without dragging memory traffic down.
| Bus | Speed | Bandwidth | Purpose |
|---|---|---|---|
| Memory bus | ~3200 MHz | ~50 GB/s | CPU ↔ RAM |
| PCIe (I/O) | ~16 GT/s | ~32 GB/s | Graphics, NVMe |
| USB 3.0 | 5 Gbps | ~500 MB/s | Peripherals |
| USB 2.0 | 480 Mbps | ~60 MB/s | Keyboard, mouse |
The problem with one bus:
- CPU needs to access memory billions of times per second
- A mouse sends a few bytes per millisecond
- If they shared one bus, the mouse would slow down CPU-memory traffic!
The solution - bus hierarchy:
- Memory bus: Dedicated, ultra-fast path for CPU ↔ RAM
- System bus: Fast interconnect for CPU ↔ chipset
- I/O bus: Slower, standardized bus shared by many devices
Tip: Think of it like roads - you don't drive your bicycle on the freeway, and you don't need a 6-lane highway to your mailbox.
Go deeper:
Front-side bus (Wikipedia) — a concrete bus hierarchy: fast back-side (cache) vs slower front-side (memory) vs peripheral buses, exactly the speed-mismatch reason for splitting them.- Bus (computing) (Wikipedia) — why modern systems evolved from one shared backplane bus to several clock-independent buses.