For example Wolfeinstein3D runs only in virtual mode cause I occupy too much low memory at boot in real one, but i didn't expect that it would be so slow. It's probably as slow as my old 386SX-20 in real mode as I remember...

I seem to recall playing with UMBPCI or something similar on a 386 - the key was that the BIOS had options to enable shadow on various segments, and then enabling it with a utility - ah, maybe it was HIRAM

Questions about old hardware and its configuration should be asked in Marvin. This forum is specifically about DOS games.

Moved.

You can find the answer on the Protected mode page of Wikipedia, which states that:

Virtual 8086 mode, however, is not completely backwards compatible with all programs. Programs that require segment manipulation, privileged instructions, direct hardware access, or use self-modifying code will generate an exception that must be served by the operating system.[30] In addition, applications running in virtual 8086 mode generate a trap with the use of instructions that involve input/output (I/O), which can negatively impact performance.

Some additional explanation in the 24 Nov 1992 issue of PC Mag (Google Books).

If your 386DX-40 slows down to the speed of a 386SX-20 there is certainly something wrong. Usually the performance hit by loading e.g. QEMM386 is not that high. It also depends strongly on the application you use (e.g. Real Mode program vs. programs using DOS Extenders).
However if you give some bench values from common applications I can run it on one of my 386 as well for comparison.

Matth79 wrote:

I seem to recall playing with UMBPCI or something similar on a 386 - the key was that the BIOS had options to enable shadow on various segments, and then enabling it with a utility - ah, maybe it was HIRAM

Quite a few of these shadow memory un-lockers were released that created UMBs without using the 386+ MMU. I have "The Last Byte Memory Manager" on my 486 for that purpose. Supports a wide variety of pre-PCI chipsets too.

elianda wrote:

If your 386DX-40 slows down to the speed of a 386SX-20 there is certainly something wrong. Usually the performance hit by loading e.g. QEMM386 is not that high. It also depends strongly on the application you use (e.g. Real Mode program vs. programs using DOS Extenders).
However if you give some bench values from common applications I can run it on one of my 386 as well for comparison.

Well, the only thing that often seems to be affected is video memory where sometimes I get 1300kbyte/s (both in speedsys and PC-Config) and sometimes 500kbyte/s... on the cpu side I get 9.2 on speedsys and 16% of a Pentium on PC Config.

NJRoadfan wrote:

Matth79 wrote:

I seem to recall playing with UMBPCI or something similar on a 386 - the key was that the BIOS had options to enable shadow on various segments, and then enabling it with a utility - ah, maybe it was HIRAM

Quite a few of these shadow memory un-lockers were released that created UMBs without using the 386+ MMU. I have "The Last Byte Memory Manager" on my 486 for that purpose. Supports a wide variety of pre-PCI chipsets too.

While this is often quite convenient it comes with a trade-off. UMBs from shadow memory (instead of XMS) are usually not capable of ISA-DMA. This is explained e.g. in the documentation of UMBPCI: http://www.uwe-sieber.de/umbpci_e.html
There is even a tool called DMACHK.COM to test for ISA-DMA capable memory segments.
A strong indication is that reads from floppy fail as data gets corrupted.

Pretty much the only application I've had a problem with ISA-DMA is disk caching drivers like smartdrv. The solution was either disable floppy disk caching or use a "split loading" technique documented in the memory manager's readme to ensure those floppy transfers DMA into conventional memory.