XTs and large hard disks

Eric J Korpela korpela at gmail.com
Mon Sep 5 00:50:57 CDT 2005


On 8/29/05, Wolfe, Julian <ISC277 at clcillinois.edu> wrote:
> 
> 
> So my questions are:
> 1. Does anyone know a good solution for putting a large drive in an XT,
> while still leaving it an XT?



There are a couple issues, the first being interface, the second being BIOS, 
and the third being DOS.

1. I'm not familiar with the IDE interface you mentioned. If it's an 8-bit 
IDE interface you're likely to have problems with 16 bit drives. The drive 
geometry information will be garbled at best... SCSI should work OK provided 
the drives support whatever the controller provides (which I would assume 
would be SCSI-1 (single ended, 5 MHz)) You should check whether TRM_PWR is 
supplied by your controller and try an active terminator, rather than any 
autotermination. I've found that some drives don't autoterminate SCSI-1 
properly.

2. Your XT IDE controller is likely to be limited to 504MiB drives without a 
BIOS upgrade. Your XT IDE controller is unlikely to be doing any translation 
of block addresses required for LBA. It's probably doing 
cylinder-head-sector addressing. If the BIOS was written for DOS 4.X or 
earlier it's probably calculating size as follows....
cylinders=true_cylinders mod 1024
heads=true_heads mod 16
sectors=true_sectors mod 64

Although a SCSI controller shouldn't have these constraints, you never know 
till you look.

3. Depending upon what DOS you use, you may have a limit of 128MiB (Dos 4.X), 
32MiB (Dos 3.X) or 16MiB (Dos 2.X and earlier).

Your best bet for debugging is to upgrade to Dos 5.X+ and get smaller drives 
and use the largest one that works.

2. Should I leave the V20 in? I've heard it has compatibility issues
> with some programs, but I'm not solid on my information.


I've never had a compatibility issue using a V30 in an 8086 machines, except 
with very early copy protected games... Fortunately I could select speed and 
wait states from the keyboard and found that 4.77 MHz with 3 wait states 
generally came close enough to an XT to fool the game.

Eric



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