Riding the Local Bus

by Alan Zisman (c) 1993. First published in Our Computer Player, 21 September, 1993

What's with this 'local bus' stuff, anyway? I don't need to take the
bus... I drive to work. (Har, har!)

Well, your computer can only run as fast as it's slowest parts (sort
of like that cliche about strong chains, right?)... and even if you've
just bought your dream 486 DX-2-66, you may find yourself with a super
fast CPU that spends a lot of time waiting, and waiting, on other
parts of the machine.

On many computer motherboards, that slowest part is the 'bus'...
that's the part of the computer where signals get on and off, and
get grouped together, for the ride to and from the CPU (sort of like
commuting to Coquitlam, get it?) Physically, if you look inside your
computer, you'll see a series of parallel slots to connect your
motherboard to your video, your drives, your printer, and more.

And most of our computers still use the so-called ISA, industry-
standard bus. The one designed by IBM for the 1984 roll out of the
AT. That computer ran at 6 mhz, and the bus was designed, with some
room to grow, to run at a maximum speed of 8 mhz. Plenty fast enough
for the peripherals of that era. And like the 286-based AT, the ISA
bus is 16-bits wide.

But today's computers are 32-bit machines. Running at speeds of 25,
50, even 66 mhz. For some operations, a faster bus doesn't matter.
You don't type any faster with a faster CPU. A serial port still
brings in data one-bit at a time, whether it's connected to a 32-bit
computer or not.

But hard drives want to transfer millions of bytes a second, but can'
t, because the bus jams up. And video's even worse. This became
apparent with the widespread popularity of Windows. Speed up the
video, and Windows becomes a much more usable environment. Without
speeding up the CPU or adding RAM (though these can help, too). An
800x600 screen, with a modest 256 colours, takes almost half a
million bytes to represent. Everytime you update that screen, you've
got to shove another half-million bytes through that bus. Now try to
imagine full-screen video at 30 frames per second? Even a modest
spreadsheet macro can tax the ISA bus. (Try and visualize all your
little bytes waiting patiently at the ISA-SkyTrain station at rush
hour, and you've got the picture).

IBM tried to replace the ISA bus with 32-bit MicroChannel, back in
1987, and their competitors produced the EISA bus a few years later,
to avoid paying royalties to IBM (and to provide compatiblity with
standard ISA cards). But neither has caught on too widely, and while
both provide 32-bit bus-widths, neither truly support the high clock
speeds of todays computers.

Several manufacturers independently came up with a solution... a '
local bus', that ran at the same speed as the CPU, with direct, 32-
bit connections to that CPU. Sort of a high-speed, express-lane,
commuter special.

These proprietary solutions had a problem, however... each
manufacturer had their own standard. Cards had to be designed just
for those machines, and wouldn't be widely available. Without
competition, prices would remain high.

In 1991, VESA, a group composed of video card companies, met and set
a standard for a local-bus... the so-called VESA-standard local bus.
This standard quickly gained widespread popularity, as it could be
easily added to current motherboard designs, and added little to the
cost of the motherboards, or to add-in cards. Now the majority of
486 computers are being manufactured with VESA local bus, or VL-bus. Let's look
at what it implies:

-- Typically, a 486 motherboard will include 2 or 3 local bus slots.
Because they're directly connected to the CPU, having more local bus
slots would strain the CPU's resources, and slow down the whole
computer. Besides, as we've seen, many bus processes, such as
running a modem through the serial port, wouldn't benefit.

-- You can't add a local bus to your current motherboard. Sorry. At
minimum, you'll need to replace your motherboard. And then...

-- These local bus slots still let you use your older ISA cards, but
you don't gain any benefits. To get improved performance, you need
cards designed to work in a local bus. (So if you're replacing your
motherboard and your video card anyway, maybe it's time for a whole new
computer?)

-- Video shows the most dramatic improvement. Hard drive performance
can also be boosted, though in many cases, you'll need a hard drive
that's capable of sending the extra data.

-- The VL-bus was designed to max out at about 40 mhz. This
is fine for 486-25s and -33s. And the DX2-66s are also okay, as they'
re double-speed CPUs, that are running in a 33 mhz motherboard.
Users of 486-DX50 chips may have some problems, however. Some local
bus video cards CAN be successfully run at 50 mhz, but others
cannot. Check before buying!

-- There are still a few non-standard local-buses around. If you buy
one, you'll find yourself a slave to a single manufacturer. The
words "local bus" aren't enough... make sure that you're buying a '
real' VESA compatible local bus.

With the exception of some 486-50 users, the VL-bus provides
a good match for the 486's performance. It doesn't seem, however,
like a good match for the next generation... whether that turns out
to be the Pentium, or the Power-PC, or some other contenter. These
chips are all designed as 64-bit chips, and will be running at
speeds of 100mhz or above.

Intel has proposed an alternative, more sophisticated local bus
design, the PCI-bus. This is not currently available, but seems like the best
choice for next year's Pentium powerhouses.

In the mean time, the VL-bus seems like the best choice for
users looking to get the most for their money, out of today's
technology.