Does the screen reach 64KB and stop? How much time have you waited? It should boot into Basic if you waitlong enough.

In any case it could be bad RAM.

I have waited long enough. However, there's another thing I've tried.
Despite the fact this is "64-256KB SYSTEM BOARD", the board came with 640KB populated, but without CPU. I assumed someone just found a board with empty memory sockets and stuffed some random memory into it but didn't have CPU. As such, I have removed the 256Kx1 chips and placed 64Kx1 chips into banks 0 and 1, and this leads to the memory test stuck at 64KB.
I've fitted the bank 0 with 256Kx1 chips and now the POST continues!

I'm puzzled - it looks like the board was modded for 640KB somehow, although I see no wires. The only suspicious thing is two socketed DIL16 chips (U44, U84) with paper sticker on them, saying "1" and "2". According to schematics, U84 is a multiplex for RAM address line MA8 and U44 has something to do with RAM address selection. My assumption is that although this board is marked 64-256, the only difference to 256-640 board is those two chips.

In fact, the whole design might even support UMB 😀 I need to do some research on that. After all, I like this board more and more.

I'm glad you made some progress here!! 😀 640 KB wasn't the absolute limit, actually, by the way.
There used to be hardware mods for the PC/XT to overcome that limit (by modifing U44).
PC/XT boards can address up to ~900KB of main memory if there's nothing in "the way".
Id largely depends on which graphics hardware and Option ROMs are in the adapter's segment.
With CGA and/or Hercules, you can have 704KB without compatibility issues.
Roughly 736KB are possible with a CGA card in the system.

Re: ISA RAM card, my easter project
Re: Is it possible to go over the 640KB barrier on an 8088 using some software ?

Re: PCEm. Another PC emulator.
Re: PCEm. Another PC emulator.

Yep, I found some info on that U44 as well, and I will continue research as time permits.

In fact, I will be using VGA, thus the amount of UMB will be severely limited, and conventional will be limited to 640KB. On the other hand, I hope to be able to squeeze all the important ROMs into free space in BASIC ROM so at least C800-EFFF should be free for UMB.

That's cool, good luck! If you can, tell us about your results.
Spekaing of UMBs, there where some useful programs at vcfed.org.
They allowed to use that memory on PC/XTs running DOS 3.x or higher.
In the past, QRAM and other tools allowed the uploading of the DOS kernal, too.

Actually, packing all the BIOSes into F segment was dead simple - I just had to reconfigure the XT IDE BIOS to support regular 16bit IO card in 8bit mode, and then rebuild the whole "Super XT BIOS". I can see both option ROMs (floppy and XT IDE) starting, and I am booting off the CF card.

However, I'm having problems with keyboard, so next side project will be AT/XT keyboard interface 😀

As per my suggestion, Plasma made it possible for the Super XT BIOS to look for option ROMs up to FE00 segment. The reason for my request is that many of XT clone boards have a single or several empty sockets for ROM CASSETE BASIC. This way it becomes easy to use the empty socket for option ROMS.

Maybe if you don't care about cassete BASIC you could do this but it seems you have everything sorted by now.

Actually, the BIOS itself is 8KB, Basic is 32KB so there's still 24KB free in F segment. Both XT-IDE and Sergej's floppy ROMs have 8KB, which means I still have another 8KB free. LAN boot? 😀

And yeah, scanning up to FE00 was a great idea!