Information Technology Reference
In-Depth Information
4.9
Notes from the author
There is an amusing statement that was made in 1981, in the topic 30 Hour BASIC Standard,
1981:
Microcomputers are the tool of the 80's. BASIC is the language that all of them use. So the sooner you
learn BASIC, the sooner you will understand the microcomputer revolution
Well, as it has proven, a good knowledge of BASIC will not really help your understanding
of microcomputers, but if there is one bus that you need to understand in the PC, it is the PCI
bus. This is because it is the main interface bus within the PC. Most external devices eventu-
ally connect to the PCI through bridge devices. There were several short-term fixes for local
bus technology, but the PCI was the first attempt at a properly designed system bus. It allows
the PC to be segmented into differing data transfer rates. PCI provides a buffer between the
main system core, such as the processor and its memory, and the slower peripherals, such as
the hard-disk, serial ports, and so on.
With interrupts, the PCI has the great advantage over ISA in that it allows interrupts to
be properly assigned at system start-up. The BIOS or the operating system can communicate
with the PCI-connected bus with the configuration address area. From this, the system can
determine the type of device it is, whether it be a graphics card or a network card. The sys-
tem can then properly configure the device and grant it the required resources. The days of
users having to assign interrupts (using card jumpers, in some cases) and I/O addresses are
reducing (thankfully!).
The great leap forward in PC systems happened with local bus technology. The demand
came from graphics cards as Windows 3.0 was being adopted. The ISA bus was far too slow,
as it only supported 8MHz transfers. Graphic card manufacturers got together and devel-
oped the VESA-backed VL-local bus standard. It showed how fast transfer devices could be
connected to a local bus, while other slower devices had to access the processor through a
bridge, which allowed a different clock speed, and a different data and address bus. Most
PCs are now based around this local bus idea, and they can be split into there main areas:
Local processor bus. Direct connection of the processor to its local cache memory (ei-
ther Level-1 or Level-2 cache.
Local bus. Connection onto the PCI bus. This connects to the local processor bus via a
bridge.
External bus. ISA, IDE, RS-232, and so on. This connects to the local bus via a bridge.
There is great potential in the PCI bus. At present, most systems use 32-bit data transfers,
but there is scope for 64-bit data transfers. Also, the 33 MHz clock can be increased to
66MHz with double edge clocking. A new enhanced PCI-based system called the AGP (Ad-
vanced Graphics Port) has been developed which allows for data transfers of over 500 MB/s.
I'm slightly annoyed with the success of the PCI bus, as I've got an ISA-based sound
card, an ISA-based Ethernet card and an ISA-based video camera, and I've only got two ISA
slots. So, every so often, I have to swap the sound card for the video camera, and vice-versa.
At present, I've got four empty PCI slots, and I think one of them is waiting for a PCI-based
Ethernet card. Then I'll be able to have a proper video conference, with sound and video.
But, never mind, I've just got myself a lovely new Dell notebook, and a USB-based video
camera, and a single PCMCIA card for my modem and network connections, so I may never
need my desktop computer again (here's hoping).
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