Hardware Reference
In-Depth Information
architectures. As an aside, please note that ISA stands for Instruction Set Architec-
ture in the context of machine levels, whereas it stands for Industry Standard Ar-
chitecture in the context of buses.
The PCI and PCIe Buses
Nevertheless, despite the market pressure not to change anything, the old bus
really was too slow, so something had to be done. This situation led to other com-
panies developing machines with multiple buses, one of which was the old ISA
bus, or its backward-compatible successor, the
EISA
(
Extended ISA
) bus. The
winner was the
PCI
(
Peripheral Component Interconnect
) bus. It was designed
by Intel, but Intel decided to put all the patents in the public domain, to encourage
the entire industry (including its competitors) to adopt it.
The PCI bus can be used in many configurations, but a typical one is illustrated
in Fig. 2-31. Here the CPU talks to a memory controller over a dedicated high-
speed connection. The controller talks to the memory and to the PCI bus directly,
so CPU-memory traffic does not go over the PCI bus. Other peripherals connect to
the PCI bus directly. A machine of this design would typically contain two or
three empty PCI slots to allow customers to plug in PCI I/O cards for new periph-
erals.
Memory bus
PCI
bridge
Main
memory
CPU
cache
SCSI
bus
Cache
SCSI
scanner
SCSI
disk
SCSI
controller
Video
controller
Network
controller
PCI bus
Figure 2-31.
A typical PC built around the PCI bus. The SCSI controller is a
PCI device.
No matter how fast something is in the computer world, a lot of people think it
is too slow. This fate also befell the PCI bus, which is being replaced by
PCI
Express
, abbreviated as
PCIe
. Most modern computers support both, so users can
attach new, fast devices to the PCIe bus and older, slower ones to the PCI bus.
While the PCI bus was just an upgrade to the older ISA bus with higher speeds
and more bits transferred in parallel, PCIe represents a radical change from the PCI
bus. In fact, it is not even a bus at all. It is point-to-point network using bit-serial
lines and packet switching, more like the Internet than like a traditional bus. It's
architecture is shown in Fig. 2-32.
