Hardware Reference
In-Depth Information
forms, the older PCI bus, and the new and much faster PCI Express (PCIe) bus.
The Universal Serial Bus is an increasingly popular I/O bus for low-speed periph-
erals such as mice and keyboards. A second and third version of the USB bus run
at much higher speeds. In the following sections, we will look at each of these
buses in turn to see how they work.
On the original IBM PC, most applications were text based. Gradually, with
the introduction of Windows, graphical user interfaces came into use. None of
these applications put much strain on early system buses such as the ISA bus.
However, as time went on and many applications, especially multimedia games,
began to use computers to display full-screen, full-motion video, the situation
changed radically.
Let us make a simple calculation. Consider a 1024
768 color video with 3
bytes/pixel. One frame contains 2.25 MB of data. For smooth motion, at least 30
screens/sec are needed, for a data rate of 67.5 MB/sec. In fact, it is worse than this,
since to display a video from a hard disk, CD-ROM, or DVD, the data must pass
from the disk drive over the bus to the memory. Then for the display, the data must
travel over the bus again to the graphics adapter. Thus, we need a bus bandwidth
of 135 MB/sec for the video alone, not counting the bandwidth that the CPU and
other devices need.
The PCI bus' predecessor, the ISA bus, ran at a maximum rate of 8.33 MHz
and could transfer 2 bytes per cycle, for a maximum bandwidth of 16.7 MB/sec.
The enhanced ISA bus, called the EISA bus, could move 4 bytes per cycle, to
achieve 33.3 MB/sec. Clearly, neither of these approached what is needed for full-
screen video.
With modern full HD video the situation is even worse.
×
It
requires
1920
1080 frames at 30 frames/sec for a data rate of 155 MB/sec (or 310 MB/sec
if the data have to traverse the bus twice). Clearly, the EISA bus does not even
come close to handling this.
In 1990, Intel saw this coming and designed a new bus with a far higher band-
width than the EISA bus. It was called the
PCI bus
(
Peripheral Component
Interconnect bus
). To encourage its use, Intel patented the PCI bus and then put
all the patents into the public domain, so any company could build peripherals for
it without having to pay royalties. Intel also formed an industry consortium, the
PCI Special Interest Group, to manage the future of the PCI bus. As a result, the
PCI bus became extremely popular. Virtually every Intel-based computer since the
Pentium has a PCI bus, and many other computers do, too. The PCI bus is covered
in gory detail in Shanley and Anderson (1999) and Solari and Willse (2004).
The original PCI bus transferred 32 bits per cycle and ran at 33 MHz (30-nsec
cycle time) for a total bandwidth of 133 MB/sec. In 1993, PCI 2.0 was introduced,
×
