Hardware Reference
In-Depth Information
some companies like to repartition the PC, with the CPU and memory in a tiny
sealed box and the hard disk inside the monitor. With PCI cards, doing this is im-
possible.
Several solutions have been proposed, but the one that won a place in all mod-
ern PCs today (in no small part because Intel was behind it) is called
PCI Express
.
It has little to do with the PCI bus and in fact is not a bus at all, but the marketing
folks did not like letting go of the well-known PCI name. PCs containing it are
now the standard. Let us now see how it works.
The PCI Express Architecture
The heart of the PCI Express solution (often abbreviated PCIe) is to get rid of
the parallel bus with its many masters and slaves and go to a design based on high-
speed point-to-point serial connections. This solution represents a radical break
with the ISA/EISA/PCI bus tradition, borrowing many ideas from the world of
local area networking, especially switched Ethernet. The basic idea comes down
to this: deep inside, a PC is a collection of CPU, memory, and I/O controller chips
that need to be interconnected. What PCI Express does is provide a general-pur-
pose switch for connecting chips using serial links. A typical configuration is il-
lustrated in Fig. 3-56.
Level 2
cache
Bridge
chip
CPU
Memory
Switch
Paired serial links
Graphics
Disk
Network
USB 2
Other
Figure 3-56.
A typical PCI Express system.
As illustrated in Fig. 3-56, the CPU, memory, and cache are connected to the
bridge chip in the traditional way. What is new here is a switch connected to the
