That was one option I thought of at first (mentioned in the first section), but the info I found indicated that the /370 models used the same firmware as the "plain" 5170s - if there were any BIOS extensions, they were probably somewhere on the add-on cards. The AT/370 also had 512K of on board RAM, while this BIOS seems to indicate 640K.

Plenty of people remember, and used, them. Just not people who tend to hang out here. I knew several IBM VM dev types who had them as light dev/remote mainframe access machines, usually at home. They were popular enough there was a followon product: the PC/390 which was the same idea, more advanced processor, based on a PS/2 microchannel platform (and, AFAIK, OS/2).

You want really obscure? Unisys had the same idea with the "Micro-A", which was a PC running OS/2 with a coprocessor card with a single chip implementation of an A-series mainframe. I know of 2, possibly 3, still around.

Details: The IBM AT/370 used standard bios on the motherboard, and the two 68k custom cards had their own bioses. The 68ks were very heavily modified by one of the motorola engineers.

Its the second version of the AT Bios that was disgusting was verion 2, that ran on 6mhz 286s and prevented you from swapping the crystal for a 16Mhz/8Mhz speed up. The first version had bugs, and the third version was for the 8Mhz machines. ( still a few bugs ).

This is the AT/370:

https://en.wikipedia.org/wiki/PC-based_IBM_mainframe-compati...

https://www.cpushack.com/2013/03/22/cpu-of-the-day-ibm-micro...

https://anycpu.org/forum/viewtopic.php?f=22&t=350

There was one additional model of the IBM AT: THE IBM XT/286: An AT class mother board in an XT sized case.

https://www.dosdays.co.uk/computers/IBM%20PC-XT-286%20(5162)...

Oops. Anyway, I remember attending a talk by one of the IBM engineers back when they first released the XT/370. He said that they looked at all possible ways to integrate their production line as a kind of secondary track off of one of the main production lines for the PC/XT, but the most economical option ended up being a separate facility that would receive normal pallets of regularly boxed, end-user XTs from the main factory, unbox them, make the mods, and pack them back into XT/370-labeled boxes for shipping.

Article discusses and dismisses that possibility

I remember that. I think it ran VM/SP or whatever it must have been called.

I recall the 370 part was on a card.