Hardware Clinic - Upgrading the Mother of All Boards
by Mark Minasi
I'm buying a new motherboard because I'm tired of waiting for my 10-MHz 286 clone to do Windows redraws. I'm lucky because (years ago) I bought a clone in a regular generic AT-type case.
That means that the motherboard is standard size, so I can make the whole system a 386 screamer just by replacing the motherboard. Come on along and see how you can jazz up your system, too.
Oops--I forgot to tell you just what a motherboard is. If you take the cover off you PC, you'll see, off to the left, a bunch of upright circuit boards. Now take a look at what they're standing on--another circuit board, a big one lying flat on the bottom of your PC's case. The board lying down is the motherboard.
The motherboard is the Big Cheese in your PC. It's the board that most likely contains your main CPU, your math coprocessor, some memory, your BIOS, and other items.
Because it's so important, changing the motherboard changes your PC's entire personality, as well as its speed and, in some ways, its flexibility. Motherboard surgery isn't for everyone.
If you've got a strange-shaped case, like one of those "slimline" or "small footprint" PCs, your manufacturer achieved that small footprint by using a nonstandard-size motherboard, which pretty much lets you out of this discussion. (Stick around anyway; you'll get some tips on buying your next computer.)
I want a lot of features in a motherboard--some necessary, some merely nice. My necessary list includes room for at least 16MB of RAM on the motherboard; the ability to disable shadow RAM; a BIOS with user-defined hard drive type; BIOS contained in two ROM chips, not one; BIOS from AMI, Award, or Phoenix; eight expansion slots; and a 16-MHz 386SX or 20-MHz 386DX processor.
The unnecessary-but-highly-desirable list includes adjustable bus speeds, a faster CPU with cache, and a motherboard that's XT size, not AT size.
Thanks for the Memories
I dream of a day when I won't want more memory.
Sounds crazy when I say to you, "You must buy motherboards that accommodate at least 16MB of RAM," but it's true. Blame it on Windows 3.0. Blame it on 386Max and QEMM. The fact is that four megabytes is a bare minimum required to get anything done with Windows. Eight megs is much comfier. And more software's coming down the pike that will give you even more reasons to want more RAM.
So I figure that in a year or two, everyone will have eight megs and will want more. At about $45 per megabyte for RAM chips and SIMMs, it's not unreasonable.
But, of course, there are a few catches. In the XT and AT days, you just expanded memory by buying a memory expansion card, putting memory chips on it, and putting card into one of the PC's expansion slots. But you can't do that with faster PCs. No matter how fast your PC is--20, 25, 33 MHz--the expansion slots still only run at 8 MHz.
Why do the slots run so slowly? Because most expansion boards can't operate above 8 or 10 MHz. So whenever the system is accessing an expansion board, it slows down to 8 MHz. That sounds pretty awful, but it's not that bad. Most boards in expansion slots communicate with things that are fairly slow anyway, like floppy drives, printer ports, modems, and the like.
What really hurts is having to put a memory card in an expansion slot. Memory runs best when it runs at the full speed of the CPU, so it's a crime to make a 25- or 33-MHz machine slow down to 8 MHz when accessing memory.
By the way, a few motherboards give you the option to experiment with a faster bus. For example, I've got a 20-MHz 386 system that lets me set my bus speed to 6,8, or 10 MHz.
If all of your expansion boards are a bit faster than average, you can get away with running the bus at the practically illegal rate of 10 MHz, and speed up video and disk access in the process. More on this in a future column, but having the speed adjustment is a nice motherboard option.
Manufacturers have found two ways to avoid this problem. First, some manufacturers design a special high-speed slot for the motherboard that will only accommodate a particular card--a memory board.
If you buy a motherboard of this type, make sure you get the memory board at the same time you buy the motherboard, or you won't be able to put any memory on the system.
Other manufacturers put sockets for memory right on the motherboard, eliminating the need to deal with the expansion slots. If you buy one of these motherboards, ensure that there's enough room for at least 16MB.
Be warned, however, that most of the boards on the market only have room for 8MB, so pick carefully. Some motherboards, by the way, combine both methods--they have room for about 8MB on the motherboard itself and also have a highspeed memory slot for a board that will hold another 8MB.
You don't need the memory board until you're ready to exceed 8MB of total system memory, but buy the board immediately anyway. Why? Because the board may not be available when you need it in a year or two. These boards generally run $100-$200.
Me and My Shadow
Many 386 systems have a feature called shadow RAM. It's supposed to speed up system response. Actually, shadow RAM has little real-world value and can cause trouble when running Windows 3.0 and other programs. I don't mind having the feature with the system, but I sure want to disable it. Make sure your system gives the option to disable shadow RAM.
You see--shadow RAM speeds up any attempt to read the BIOS, a basic, low-level piece of software that controls your keyboard, disk, screen, and printer.
The argument goes that any input/output operation will be sped up by shadow RAM. This argument is specious because it overlooks an important fact: Most software bypasses the BIOS and controls the the PC hardware directly in order to achieve maximum speed.
If software used the BIOS, shadow RAM wouldn't be a bad idea--but most software doesn't. It looks good on benchmarks (which politely access the hardware via the BIOS), but Windows, 1-2-3, and WordPerfect (to name a few) will be unaffected by shadow RAM, so don't feel bad about disabling it.
Buying the Best BIOS
I just mentioned the BIOS and that it's a piece of software. It's an unusual piece of software, however, in that it's encased in hardware. Where most software is loaded from a floppy or hard disk into the computer, the BIOS comes in a chip called a ROM (Read Only Memory).
When shopping for ROMs, it turns out you've got to be concerned both with the software in the chip and with the way the chip is packaged.
First, ask who's writing the software. The BIOS software must be very, very comptible with an IBM BIOS, or your system won't be 100-percent PC compatible. Developing compatibility takes lots of practice, so buy a BIOS from a vendor with some experience.
I'd recommend AMI (American Megatrends, Incorporated), Award Software, or Phoenix Software brands. That doesn't mean the other guys are trash, understand--they just need some more time. Stay with AMI, Phoenix, and Award, and you'll be OK.
And there's no reason why you can't get a BIOS from the vendor of your choice--all three make BIOSs for just about every 386 system under the sun.
The BIOS's original job was the low-level hardware functions I described, earlier, but nowadays there's more to look for. First and probably most important is a user-defined drive type. Since the advent of the IBM AT in 1984, 286/386/486 BIOSs have contained a table of hard disk drive types--descriptions of common hard disks.
As ROM space is limited, most ROMs only contain 47 drive descriptions, which, of course, aren't enough--there are new drives appearing every day.
If the drive you're trying to install doesn't match anything on the table, either you won't be able to install the drive or you'll have to settle for a description of a smaller drive, leading the PC to waste some space.
In 1989, AMI and Award introduced a useful new feature, the user-defined drive type. It allows you to describe a drive directly, rather than trying to match your drive to the cloest prestored type. Since then, most BIOS vendors have included user-defined drive types, so make sure your BIOS includes this feature.
Other desirable BIOS features are a built-in system setup (which eliminates the need to hunt around for the SETUP disk every time you make a change to the system), the ability to set memory wait states, the ability to enable and disable blocks of memory in the BIOS (rather than having to physically remove memory in order to isolate and test it), and the ability to set keyboard speeds via the BIOS (freeing you from having to use third-party utilities to make your keyboard more responsive).
The last BIOS feature has to do with its packaging. Most BIOSs are shipped as a pair of chips called the even ROM and the odd ROM. More recently, however, I've seen motherboards that use a BIOS packaged as a single ROM.
I would counsel you against these motherboards simply because it will be more difficult to find ROM upgrades in the future. There are several ROM vendors in the U.S., and none that I've talked to were prepared to offer a single ROM BIOS--they all use the more normal dual-chip BIOS.
So for the sake of easy upgrades later, stick to motherboards that use a pair of ROMs to house the BIOS. (Why would you want to upgrade your BIOS? To solve new compatibility problems or to support new hardware, such as the upcoming 2.88MB floppy drives.)
Odds and Ends
Most 386 motherboards used to be large, about the same size as an old AT motherboard. More advanced chip design has reduced the number of chips on the motherboard, reducing power consumption and leading to smaller, XT-size motherboards. I'd recommend the smaller XT motherboards because you can fit them into smaller cases, which take up less space on your desk.
But don't buy a motherboard that achieves smaller size by reducing the number of expansion slots--you want an eight-slot motherboard. The more slots, the more long-term expandability you'll have.
I know you're waiting for me to make a recommendation about a brand. I don't have space to discuss all the motherboards I like, so I'll just say that there are actually lots of terrific no-name motherboards. New vendors appear and disappear almost weekly. Check out your local clonemeister's offerings: A decent 33-MHz motherboard with cache will set you back about $900.
No matter which 386 motherboard you buy to replace your current 286 PC's motherboard, you'll be quite pleased by the results. But as you can see, attention to a few extra details will make using your newly improved system even more satisfying.