Elizabeth Boston <[log in to unmask]> wrote:
>         I am currently studying NRI's PC Servicing course, and have a
> question I'd like to ask just for clarity...
>
> I know that there are three major buses in a computer,
> address, data, and control...    I spent last evening reading about
> the PC/XT, PC/At, VL and PCI buses. But nowhere (yet) do I see
> where the two fit together. Does the PCI bus for example contain
> address, data and control buses? (this is what I am assuming, but
> I hate to assume)  Or have I totally missed something somewhere?

You don't want to confuse the term "bus" used in different meanings. At a very
low level, you are correct that a computer has different buses, usually address,
data, and control.

On a higher level, the term PCI, VL, and AT (more commonly known as ISA) refer
to standards that encompass several concepts, including physical layout (card
edge placement and card dimensions), electrical specifications (voltages,
timing, etc). The purpose of these standards is to make sure that you can get a
PCI based card, and have it work in any computer that supports PCI.

The oldest bus in the PC was the 8 bit ISA found in the original PC/XT. It was
expanded to 16 bits in the AT. The ISA bus has several limitations including how
it handles IRQ's, limited address space, and speed.

The next bus to be introduced was the VL (Vesa Local) bus. This was primarily
intended to solve the speed/bandwidth issue for video cards, but also became
popular for disk controllers. This bus also has its limitations, only one of
which is the fact that the organization that pioneered it later decided that
they wanted to enforce their patents and put severe limitations on its
commercial use. The VL bus is all but dead. No new systems in the last 2 or 3
years uses it anymore.

The latest is the PCI bus which has higher bandwidth and is more flexible,
especially in IRQ use. These are common on all current systems. Even Sun now
embraces PCI for their newer workstations. There are variations on PCI coming
out. The most common standard is 33Mhz, which means it clocks at 1/2 the
processor core speed (or 1/3 for PC100 systems). A newer version is starting to
debut that runs at 66Mhz.

AGP (Accelerated Graphics Port) is also fairly common now, as it provides a
flexible way to allow a video card to be "closer" to the processor and memory,
one advantage is to allow system memory to be used for video purposes.


--
Russ Poffenberger               Engineering Specialist
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