Vipersan wrote on 2023-03-29, 00:14:
Definately debateable Jo..
Definitely. 😁
But as I said, I don't know these PC models, so I can only guess. 🤷♂️
Vipersan wrote on 2023-03-29, 00:14:
The existing drive interface is odd indeed...and proprietory to Compaq.
For example this is circa 1987- ish..and maximum hard drive size would have been around 40mb.
Anything larger was inconcievable at that time....and certainly not in the hard drive list.
It's likely not the norm, but the Commodore Setup of the time did list a 112MB type.
Re: Emulation on MS Windows 3.1x ?
My old 286 using a Schneider BIOS (Phoenix derivative), didn't by contrast.. So it depends.
Vipersan wrote on 2023-03-29, 00:14:
Hard drives at this time were set by Type ...and geometry ..rather than auto detect.
You would have to tell it what the drive was.
I know, I literally grew up with a 286 PC.. 🙂
My father and me installed an 80 MB Conner drive in it, but there was no Type 47 (user definable).
So my father did choose a 40 MB model from list.
The drive didn't complain, since the drive geometry wasn't exceeding overall drive capacity.
That being said, AT-Bus or ESDI drives of the late 80s do implement some form of geometry translation.
They can use L-CHS (logical CHS) as opposed to P-CHS (physical CHS).
For older HDDs (P-CHS), not exceeding drive capacity wasn't the only factor to use fake geometry.
To my understanding, not exceeding the actual numbers of physical heads and sectors was also required.
Last but not least, some 386 BIOSes frpm '92/' 93 can auto-detect CHS values.
They have an option for this in CMOS Setup.
XTIDE Universal BIOS has the same ability, I think.
It will try to figure out drive geometry in auto-detection mode.
Also works for old, pre-LBA drives, thus.
PCem/86Box rely on XTIDE Universal BIOS, too, for their PC/XT emulation.
As far as I know, auto-detection itself was no problem for IDE/AT-Bus HDDs of the late 80s/early 90s.
It rather was the PC BIOS that was so limited.
Here's an interesting site with information about early HDDs.
Drive firmware from the 1980s onwards wasn't simple, at all.
It had "sector translation" already.
http://redhill.net.au/o/glos2.php#sectrans
https://www.redhill.net.au/d/2.php
Vipersan wrote on 2023-03-29, 00:14:
In actual fact I have seen such hard drives described as AT/XT.
[..]
A very early version of XT...and at the very end of AT.
I believe you. Those early HDDs supported both 8-Bit and 16-Bit versions of IDE.
Like Compact Flash cards still do today.
Some of those Commodore PCs had XT-IDE on-board, AFAIK.
On the PC side, however, XT-IDE and IDE are differently implemented.
They occupy different memory addresses, different I/O ports.
They're not on the standard addresses the 90s era IDE channels are.
In general, XT BIOS itself differs a bit from AT BIOS in how it uses memory space to store its parameters in RAM.
Also, there's a difference in how IDE/XT-IDE communicate.
XT-IDE is using 8-Bit transfers, while IDE uses 16-Bit transfers.
On an XT class system, 16-Bit transfers aren't possible. Not directly, at least.
Multiplexing is required (converting to 2x 8-Bit transfers).
Some of the XTIDE cards may do this, yes, but it's not the old XT-IDE anymore, but normal IDE. The change was likely made to enhance compatibility with IDE hard disks or non-standard CF cards.
The old XT-IDE interface was intended to easily connect HDDs using 8-Bit I/O, because 16-Bit I/O wasn't available on PC/XT bus.
AT class systems with a 16-Bit ISA bus usually don't physically need XT-IDE anymore, since they can use regular IDE/ATA interface.
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