PCI Express

Guided tour · 1 of 8

One road for each device

PCI Express connects the CPU to the chips and cards around it: graphics cards, SSDs, network cards. The old PCI bus made every card share one set of wires; PCIe gives each device a link of its own, point to point. Data travels on it in packets, the coloured trains here, slowed down about 25 million times.

Look forPackets leave the CPU on the left and reach the device on the right, while short messages come back the other way.

Link

Generation

Lanes

Gen 3 (2010) · 8 GT/s per lane · 128b/130b · 4.00 GB/s raw each way

Packets

Payload 256 B

Header

90.0% of the line rate is data · serialization per packet 71.1 ns

Round trip

Latency, one way 100 ns

Device time per packet 40 ns

Packet slots 8

A slot returns no sooner than 311 ns. 5 slots cover this round trip.

Playback

Speed 40 ns/s

Each second shows 40 ns of the link: 25 million times slower than real

Key

start of packet + sequence number
header · header of a resent copy
payload: the data
LCRC: the error check
damaged by noise
ACK · NAK · credit, coming back