Reviving an IBM AT: Part 2 - BIOS and Hard Drive Setup

The AT unclothedIn Part 1 of this blog entry, I wrote about diagnosing and fixing the cause of a non-start from the AT's power supply. With the machine now booting up from floppy, it was time to see if those old two hard drives had survived their years of inactivity.

BIOS Setup

I'm not sure if the IBM AT pioneered this or not, but it was the first computer to popularise the concept of battery-sustained BIOS (Basic Input/Output settings) held in a CMOS IC. Most readers will know what these are but for those that don't, these settings were necessary to let the computer know the time and date, and what peripherals were attached such as number and type of floppy and hard drives, type of display card etc. Computers before the AT, such as the original IBM PC, had these configurations on a number of dip switches which needed to be set. With CMOS BIOS, this could all be done via software with a setup program.

From the early 1990's, setup programs were smart enough to auto-detect some of the trickier settings, like the numerous hard drive parameters of the many hard drive makes and models. However my AT well predated that time, so I had to make sure I had the information at hand.

First I needed the setup software. A member of the Vintage Computer Forums (VCF) helpfully pointed me to a generic version of an AT setup program. Armed with this on a bootable floppy, I was ready to go. My two hard disks had nothing but the model numbers on them but thanks to Google, I was soon provided with the parameter information I needed. I discovered that my large full height drive was a "type 2" while the 1/2 height hard drive was a "type 3".

AT Bios Data

Figure 1. Generic AT Setup screen showing BIOS parameters

This data was duly entered (I was using a VGA not CGA card during this setup phase), the boot floppy removed and and the machine rebooted.

Hard Disk low-level Format

In the famous words of Scott Adams (the adventure game writer, not the other Scott Adams) "Strange, nothing happened?". The main hard drive ground away for a while then came up with a boot error. Hmmm.

I booted with a floppy again, and checked if the machine could actually see the hard drive. Yes it could...actually it could see both of them. They both seem to have a copy of Dos 6.2 installed, but that was about all.

I figured there was probably a few bad sectors there, perhaps even where the boot file was written, thus preventing the boot from happening. Running a scandisk from the a: drive revealed eight bad sectors on the C: drive and one on the D: drive. This was not unusual for disks this age. As there was no software I wanted on the drives I decided the best medicine for them would be a low-level format. This re-writes the tracks, and is often advisable with MFM drives of this vintage as the heads can drift somewhat over time.

Next problem. Where to get a low-level disk formatter? Some googling around told me I should have one, right in the ROM in the floppy controller and all I need to do was to load MS-DOS's DEBUG program and type -g C800:5.

Did this work? No.

More googling revealed that while most AT-type machines have this small ROM-based utility, MY particular controller card did not! Hurrumph! Anyway, the day was saved by the guys following my progress on the VCF, and I was soon in possession of a great old school hard disk setup utility called "Disk Manager". This did the job wonderfully well, and I was able to low-level format both drives locking out all bad sectors. I then formatted both with MS-DOS 3.3, making the C: drive bootable. Cool!

Disk Manager

Figure 2. Disk Manager (as running in Virtual PC)

Incidentally, I'd forgotten how noisy these old hard drives could be. Powering up and down is a cacophony of clunks, grinds and beeps. Quite alarming really but I'm been told it's quite normal!

Original CMOS batteryBIOS Battery

So, I now had a machine that powered up, and booted off the C Drive. Only one issue was left. The CMOS battery.

The BIOS data is kept refreshed by a small external 6v Lithium battery screwed onto the inside back of the case (photo opposite). I was astonished to find this actually held the data successfully for 24 hours!? It looked original though, and I didn't expected to hold a charge much longer than that.

A fellow AT owner following my progress informed me that 3.5 external lithium pin-compatible batteries could be bought new at the local electronics store. Although lower than the original, the voltage was enough to do the job, so the old battery was replaced with a fresh one.

Clean Machine

The final act was to give the machine a good clean. Some cleaning liquids and elbow grease and the job was done. A working IBM AT, almost fully restored to it's glory days.


Photo 3. My fully-working IBM AT

I say ALMOST fully restored as it's not quite there yet. It doesn't have a genuine keyboard and, as someone pointed out, it should be decked out with PC-DOS (presumably 3.0) rather than MS-DOS for true authenticity. If I can find a copy somewhere, I'll do it. I'll also load it up with some classic AT software in the coming months.

In the mean time, it joins my other models as a valued member of the collection.

Many thanks to the guys of the VCF especially Paul, Chuck(G), modem7, Chromedome45. Your tips, advice and resources kept the project rolling along to a happy conclusion.


20th August, 2009


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