The encyclopedia · Engineering & Operations · Technical decision · 2002–2003
PCI Express beat the shared bus by giving every device its own serial lanes
PCIe swapped a shared parallel bus for point-to-point serial lanes, so bandwidth scales by adding lanes and software still works.
Intel · Dell · Hewlett-Packard · IBM · PCI-SIG
the move
Peripheral and graphics cards had outgrown the shared PCI bus. Because every device shared the same parallel lines, the bus clocked to the slowest peripheral and only one master could transfer at a time.
PCI Express, designed by Intel, Dell, HP and IBM and managed by PCI-SIG, made each link a point-to-point serial connection. The same bus can run one narrow link for a low-cost card or a wide one for a graphics card, and it auto-negotiates the highest mutually supported lane count.
why it works
- Point-to-point links let many devices transfer data simultaneously instead of arbitrating for one bus.
- Lanes let throughput scale to the device, so a video card gets a wide link while a sound card gets a narrow one.
- Full-duplex serial lanes mean a link can send and receive at the same time, unlike the shared bus.
- Backward-compatible software meant the industry could adopt PCIe without rewriting PCI drivers.
what transfers
When a shared resource becomes the bottleneck, stop sharing it: give each heavy user its own scaled channel and keep the software interface unchanged.
what came after
PCI Express replaced PCI, PCI-X and AGP as the universal internal interconnect, becoming the standard for graphics cards, SSDs, networking and storage. It has continued to double speed across generations while keeping software compatibility.
references
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