September 22, 1997
The Case of the Missing IRQs
By Bob O'Donnell
One of the great things about personal computers is that you can customize them in lots
of different ways. If you decide you want to attach a scanner to a machine or upgrade to a
faster network card, you can do it.
In fact, this customization capability is one of the main reasons NetPCs and other
network computer variations have yet to receive much support from the IT community (or
anyone, for that matter). IT managers want and need the ability to adapt existing machines
to various purposes. The only way they can do that is with upgradable hardware.
The problem is, determining what's upgradable isn't as easy as it sounds. In the past,
all you had to look for was a bunch of open expansion slots. Ironically, many of today's
new, well-equipped computers are proving to be difficult to upgrade, despite coming with
numerous open expansion slots. The reason? Not enough IRQs (Interrupt Requests).
IRQs, which are part of a system that allows communication between various components
of a computer, are often thought of as a relic from computing's past. In fact, most people
think of IRQ problems as something that Windows 95's Plug-and-Play technology was supposed
to take care of (and, for the most part, does). But all Plug-and-Play can do is help
ensure that system resources and devices are each assigned their own unique IRQ and don't
step on each other. It doesn't (and can't) add IRQ addresses to the system and that's the
real crux of the problem. Personal computers have been limited to 16 IRQs since the very
first IBM PC back in 1982, and despite all the advances in processors, memory and other
technology, that number has not increased for desktop computers. (Some multi-processor
enabled servers running Windows NT support more.) This IRQ problem isn't a new one, but as
more and more well-stocked new computers are hitting the market, it's becoming an
increasingly common one.
Of course, theoretically, there are solutions. The PCI bus, for example, is supposed to
enable up to four devices to share a single IRQ. The problem is, for a variety of reasons,
including required support for legacy devices and lack of OS and driver support from
Microsoft and PCI card vendors, most PCI cards end up using their own IRQs. Computers that
support Microsoft and Intel's PC97 and PC98 specs are supposed to require PCI cards that
can share IRQs, but not many current systems support those specs.
In addition, Universal Serial Bus promises to host up to 127 devices off a single port,
which again only uses a single IRQ. Unfortunately, Universal Serial Bus (USB) is still
very much in its infancy and at least a year away from working as originally promised. The
result is that many new computer owners are stuck with some very surprising and
little-known limitations.
Let me give you an example to bring the point home. I'm currently using a Dell
Dimension XPS H266 loaner that could only accept a single upgrade card in its basic
configuration before it ran into IRQ problems. Admittedly, the system is well-stocked, but
it's not at all extravagant. It houses a CD and an internal Zip drive (and a second IDE
controller to run them), built-in 16-bit sound (on the motherboard), an internal US
Robotics X2 modem (on an ISA card), and a Matrox Millenium II PCI video card. It does not
have a network card. (If it did, it would have been unable to accept a single upgrade card
without running into problems!)
I added a Pacific CommWare TurboExpress Port 920 ISA fast serial card to the system,
but as soon as I tried to add a SCSI controller (originally an Adaptec 2920 PCI card, but
subsequently a 1520 ISA card), I ran into problems. There were no more IRQs available. I
finally resolved the issue by disabling my currently unused USB ports to free up its
precious IRQ. But if I ever want to run any USB peripherals, I'm out of luck.
This is not a Dell-specific problem. The same thing would be true of any other vendor's
system using the same components. This is an industry-wide problem that many unsuspecting
buyers are finding themselves running into on an uncomfortably regular basis. So if you're
buying new machines in the future, don't worry about the expansion slots; just remember to
mind your Ps and IRQs.
©
Copyright 1997, by InfoWorld Publishing Corp., a
subsidiary of IDG Communications, Inc. Reprinted from InfoWorld,
155 Bovet Road, San Mateo, CA 94402. Further reproduction is prohibited.
