The processor communicates with other peripherals in the PC through a path of data called bus. Since the release of the first PC, in 1981, up to the present day, several types of bus have been developed in order to allow the communication between the processor and input and output peripherals. We can name the following buses already launched:
ISA
EISA
MCA
VLB
PCI
AGP
PCI Express
The main difference among the several types of bus is in the number of bits that can be transmitted at a time, and in the operating frequency used. Nowadays the two fastest types of PC expansion bus are the PCI and the AGP. We listed the transfer rate of those buses in the chart below. The PCI-X bus is an extension of the PCI bus designed to the market of network servers.
Bus
Clock
Number of bits
Data per Clock Cycle
Maximum Transfer Rate
PCI
33 MHz
32
1
133 MB/s
PCI
66 MHz
32
1
266 MB/s
PCI
33 MHz
64
1
266 MB/s
PCI
66 MHz
64
1
533 MB/s
PCI-X 64
66 MHz
64
1
533 MB/s
PCI-X 133
133 MHz
64
1
1,066 MB/s
PCI-X 266
133 MHz
64
2
2,132 MB/s
PCI-X 533
133 MHz
64
4
4,266 MB/s
AGP x1
66 MHz
32
1
266 MB/s
AGP x2
66 MHz
32
2
533 MB/s
AGP x4
66 MHz
32
4
1,066 MB/s
AGP x8
66 MHz
32
8
2,133 MB/s
The PCI bus was released by Intel in June, 1992. Since then, almost all PC expansion peripherals, such as hard disks, sound cards, LAN cards, and video cards have been using the PCI bus. The thing is, the PCI bus maximum transfer rate - 133 MB/s – proved to be insufficient for modern 3D applications and it represented a limitation to the development of more sophisticated video cards. In order to solve that issue, Intel created a new bus, called AGP, to increase the transfer rate of video cards – now they wouldn’t have to be installed in the PCI bus anymore, but in the AGP bus, which is faster. Then the PCI was not so “busy” anymore, since video cards were the great responsible for the intense traffic in the PCI bus.
With the coming of faster graphics chips and new network technologies, such as Gigabit Ethernet and RAID technology, once more the maximum transfer rate of the PCI bus proved to be insufficient to handle those new applications. Something needed to be done and the answer came with the launching of the PCI Express bus.
In the tutorial we will explain in details how the PCI Express bus works and how it differs from the PCI bus.
Obs: Technically speaking, PCI Express is not a bus. A bus is a data path where you can attach several devices at the same time, sharing this data path. PCI Express is a point-to-point connection, i.e. it connects only two devices and no other device can share this connection. Just to clarify, on a motherboard using standard PCI slots, all PCI slots are connected to the PCI bus and share the same data path. On a motherboard with PCI Express slots, each PCI Express slot is connected to the motherboard chipset using a dedicated lane, not sharing this lane (data path) with other PCI Express slots. In name of simplification, we are calling PCI Express as a "bus", since for laymen “bus” is easily recognized as “data path between devices”.
