Interrupt, without apology!

by Alan Zisman (c) 1998. First published in Canadian Computer Wholesaler, February 1998

Oh where, oh where have your Interrupts gone?

I don?t mean the kids who butt in whenever you try to have a conversation with your spouse. Instead, take a look at the interrupts in your computer system.

Sort of like your kids (or mine), whenever any of the devices attached to the computer need to get some attention from the CPU, they have to interrupt it. Unlike our kids, they can only do that if they have an Interrupt Request (IRQ) number? sort of like a ticket giving them the ability to butt in on the CPU?s time.

Part of the appeal of the PC-style computer is its upgradeablity?sealed box, non-upgradeable versions have been released from time to time, (and are under discussion yet again as one way to lower corporate computing?s Total Cost of Operation (TCO)), but have never attained mass market acceptance.

But in order to be able to easily add hardware to a PC, there need to be free IRQs? otherwise, the new devices can yell for attention all they want, but the CPU will never hear them. In other words, they won?t work.

The original IBM PC and XT offered a mere 8 interrupts, most of which were already spoken for, by the keyboard, the floppy disk drive, and so forth. In 1984, IBM doubled the number to 16 with its AT model. And that?s were it?s stayed ever since?14 years, during which processor speeds have gone from 6 MHz to 333 MHz, and hard drives have grown from the AT?s original 10 Megs to nearly 10 Gigs (a thousand-fold increase).

It used to be that a typical new PC had somewhere between 3 to 5 free interrupts, but those numbers have been slowly whittled down as systems come more fully equipped. You may be shipping (or buying) systems with empty slots (particularly as more and more functions come integrated right onto the motherboards), but unless you have free IRQs, these slots are unusable. And some of today?s fancy adapters require two interrupts each!

Let?s take a look at one of my PCs? a relatively modest 1996-model Pentium 166. Here?s what?s reported by the Win95 Device Manager, sorted by IRQ number:

IRQ 0 System Timer
IRQ 1 Keyboard
IRQ 2 Programmable Interrupt Controller/Floppy disk controller
IRQ 3 Com 2
IRQ 4 Com 1
IRQ 5 Sound card
IRQ 7 LPT 1
IRQ 8 Real Time Clock
IRQ 11 network card/PCI steering
IRQ 13 Numeric Processor
IRQ 14 Hard disk controller 1
IRQ 15  Hard disk controller 2

So I?ve (maybe) got a few IRQs (6, 9 ,10, and 12) free. Microsoft?s MSD DOS utility claims the floppy disk controller is really using IRQ 6. IRQ 9 is iffy, being the so-called bridge between the XT?s original eight and the AT additions. A PS/2 mouse port would have taken IRQ 12. Add a SCSI card, and I?d lose another. Add that second network card I?m thinking of, and suddenly, we?re about full up.

Ironically, while the Macintosh started life as the proverbial sealed box, lacking expansion ports, the adoption of built-in SCSI in 1986?s Mac-Plus has proven a wise choice. SCSI allows relatively easy addition of up to six devices, and when implemented on a PC, these devices all share a single interrupt. But aside from a few areas such as high-end network servers, SCSI has never really caught on as a PC solution, where the added cost and complexity of SCSI hampered its adoption.

There have been other recent proposals to get around IRQ rationing. Like Universal Serial Bus, allowing connection of up to 256 devices (!) sharing a single external USB bus. Presumably, we could be seeing keyboards, mice, sound cards, scanners, and more, all using a single interrupt.

But USB still hasn?t caught on, though most modern motherboards are shipping with at least the possibility of adding a USB port. We?ll see if the upcoming release of Windows 98, with built-in USB support, provides this technology its long-needed jump-start.

The villain in this scenario is the AT?s now antiquated ISA bus. Even when you?re not using any ISA cards, on today?s motherboards the keyboard, the  parallel and serial ports, hard drive and floppy controllers are all connected to the ISA bus, along with other functions built into the motherboard. Ironically, lower-priced machines typically are shipping with more free resources than their pricier, more feature-packed competition.

Last year, when Microsoft released its PC-98 specifications, it boldly called for the demise of the ISA bus. Not on January 1st 1998, but over the upcoming year, by computer manufacturers wanting the right to include a ?Designed for Microsoft Windows? logo.

But even mighty Microsoft has lacked the power to impose its will on the chaotic PC industry. There?s not a chance that the legacy ISA bus will be dead in 1998.

Now it?s a year later?the seventh annual WinHEC (Hardware Engineering Conference) was held March 24-27, with Microsoft again attempting to set standards, this time presenting a trial version of recommendations for PC99, realistically aimed for implementation in Year 2000 models. In the meantime, implementation of the PC98 proposals for finally seeing an end to the ISA bus and the shortage of IRQs have been postponed, yet again.

PC99 proposals look at a minimum 300 MHz P-II with at 32 Megs of RAM (64 Megs recommended). CD-ROMs will be replaced with DVD, and (yet again), no ISA bus. On-Now power management is expected to be implemented, permitting instant startup.

In the meantime, take a good look at the systems you?re buying, selling, or distributing. Use the Win95 Device Manager (or even the old DOS MSD utility), and check how many IRQs are in use, how many are free for new devices. Be prepared?it?s a question that your more aware customers may very well be asking you.

But who among us has the courage to be the first to release a PC without a 1984 legacy ISA bus?