View Full Version : Dynamic Disk overlay on 286

June 30th, 2006, 11:22 PM
I'm currently messing around with my old 286 which has a 1280MB Quantum Fireball built into it. Of course this is a bit large for BIOS which only supports 528MB. Surprisingly, the disk works fine most of the time. But since there occur errors once the disk is about half full I'd like to set up a more reliable solution.

I've gotten hold of an old Disk Manager software from Ontrack but I currently can't test it because I'm not at home. Do you think it works with a 286? It also consumes conventional memory which is of course bad since I can't use Emm386 with a 286.

Does anyone here know of another solution? Would it be safe to use the drive if I partitioned it using smaller partitions?

July 1st, 2006, 06:21 AM
I have very limited experience with disk overlay software, but I think you'll be fine.

I put a fairly large drive on an original 6Mhz IBM PC AT and had to use something like OnTrack Disk Manager to make it work. The machine had the original Western Digital based controller and a full height 5.25" hard disk, which was unusable. (Bad bearing .. it sounded awful.)

I replaced the MFM controller with a small IDE 'paddle board' that didn't have BIOS. (It was all I had on hand at the time.) The BIOS built into the AT could drive the IDE interface, but still with only the drive types built into the BIOS.

The disk overlay software takes up almost no memory. It just has to take up enough code and space to provide a new drive parameter table entry. The entries are a grand total of 16 bytes, and the code (which is just needed at boot time) is probably less than 512 bytes.

Once you take this route you can still boot from a floppy, but it has to go to the hard disk first, fixup INT41h, and then boot from the floppy. That lets whatever you boot see the hard disk. If you boot with a diskette in the drive first, the hard disk will be physically present, but the DOS on the diskette won't be able to access it by drive letter.

July 1st, 2006, 06:54 AM
Thanks for the info. I've had time today to try the older Ontrack software and it works perfectly. The wrote in the readme it would take 5K but it seems to be less. With 4MB (!) in my 286 I'm not that short on memory (though there's of course the conventional barrier and no UMBs).

I just wonder if there are some games that use some kind of low level disk access which could mess things up. Do I need to do something to Win 3.1 or will it run out of the box?

Concerning the boot disk, I've yet to investigate this. I think my BIOS behaves a bit strangely since prior to installing the DDO I could access the full disk capacity. It's just that read-write errors occured after the disk was about half full. So it might be that I can still see the disk even if I boot with a non DDO-enabled boot disk.

July 1st, 2006, 06:59 AM
Have you run some sort of disk test program to determine what the errors are and where they are? If it is just a bad spot on the disk that is easy to take care of. Disk tools will mark those sectors as bad and not use them. If it only starts to give errors after it gets half full and it is not a particular set of sectors that would be quite strange.

As for Win 3.1, it will probably work. I wouldn't install any enhanced disk drivers though - that might hose it.

July 1st, 2006, 02:29 PM
Well these errors all occured before installing the DDO. As the BIOS cannot address more than 528MB, I guess some errors are to expected. I will yet have to see if any errors occur now after installing the DDO; I hope not.

July 1st, 2006, 02:32 PM
My GEM 286 has a Disk Overlay for the 540MB drive it has installed in it, you should have no problems whatsoever. Nice to know since before too long I plan to possibly slap a 2.1 GB drive in there.

July 1st, 2006, 02:50 PM
You won't get read write errors just because all of the drive is not addressable by the BIOS. You get read write errors because there is a legitimate problem on a sector, or you try to reference a sector that doesn't exist.

If you were referencing sectors that don't exist because of a geometry mismatch, you'd have hundreds or thousands of consistent errors. This is unlikely, as almost any newer drive easily has more heads, cylinders, and sectors per track than any drive in the built-in BIOS table.

Best thing to do is to install the drive overlay software, and then run something like Norton Utilities over the disk to find and mark bad sectors.

July 1st, 2006, 06:13 PM
Regarding the data errors past 'half full'. That might be caused by the drive's write-precompensation occuring too early or too late. See my June 15th entry in the 'My IBM 5160 - questions' thread.

Norton's Utilities does a lot of stuff at a low level and thus may not 'see' the drive overlay. If that's the case, I recall that you could configure Norton's to look at the drive in either physical or logical mode. Logical mode (via DOS I presume) might get around that.

July 2nd, 2006, 01:27 AM
Usually if sectors are physically defect, you have a noticeable delay and audible retries. Neither of these things were there. It simply instantaneously displayed read/write errors. Still, the errors occured consistently at the same places.

I should clarify the "about half full thing". It's just that I copied the files on the disk with another computer which is perfectly able to address the whole disk natively. At first that was no problem but when I copied more on the disk later on (with the newer computer) errors occured (the disk was then approximately half full).

Perhaps the best thing would be to physically check it in the newer computer. I guess Norton or PC Tools could mess things up with the overlay installed. I'll look into it next week when I'm home again.

July 2nd, 2006, 02:26 AM
Regarding the data errors past 'half full'. That might be caused by the drive's write-precompensation occuring too early or too late.
Sorry. I had a blonde moment! A 1280MB Quantum Fireball is an IDE drive (i.e. write-precomp controlled by the drive itself).

Terry Yager
July 2nd, 2006, 09:09 AM
Sorry. I had a blonde moment! A 1280MB Quantum Fireball is an IDE drive (i.e. write-precomp controlled by the drive itself).

I musta took a blonde pill this morning, cauze I was thinking the same thing myself about the wpc...