Blackthorn00 wrote on 2025-12-23, 10:11:

I speculate it's the first on 286 and the second on 386

EMM386 doesn't work on a 286.

If you want software EMS on a 286, you need some third-party utility, eg. EMM286 - which of course does the memory copying.

I don't know how exactly EMM386 works - I think the only way to learn is by looking at the leaked source of MS-DOS 6.0.

Grzyb wrote on 2025-12-23, 11:28:

I don't know how exactly EMM386 works - I think the only way to learn is by looking at the leaked source of MS-DOS 6.0.

Or look at MEMM - the predecessor of EMM386 - of MS-DOS 4.0: https://github.com/microsoft/MS-DOS/tree/main/v4.0/src/MEMM

Blackthorn00 wrote on 2025-12-23, 10:11:

So which one is the truth? (I speculate it's the first on 286 and the second on 386).

2)

Other posts suggest that since EMM386 puts the processor in v86 mode, then it enables paging. As a result, swapping those 16k pages is just a use of virtual memory (mapping logical pages to phisical ones) which doesn't require a copy and thus is very fast.

EMM386 switches your 386+ processor into protected mode and enables the paging unit.
It installs a kind of hypervisor and lets DOS run in a virtual 86 machine.
Memory switching as required by moving EMS windows will be fast using the paging unit.

As EMS memory is allocated, available XMS memory will be decreased.

However, enabling protected mode and paging will cost a bit of performance. And not every DOS program likes to be run under a hypervisor.
A 386sx will suffer most from an active EMM386.

There also were Compaq's CEMM and the EMS providing VM monitor as part of Windows/386.
Both are predecessors to EMM386 in some ways.
(Btw, the MemMaker utility used for EMM386 was based on the Customize utility of Helix NETROOM 3. Video: https://www.youtube.com/watch?v=NF83dbkuunA)

Then there was IBM's EMS counterpart called XMA.
It initially had compatibility issues with regular EMS hardware, if I understand correctly.
The number of handles or something along these lines was different, so users of the time thought IBM's software was buggy.

Speaking of EMM286, there are some other LIMulators that are more compatible.
So far, EMM286 only worked with AutoSketch 3.0 (DOS) for me.

By contrast, commercial LIMulators such as Above DISC or Turbo EMS also especially supported Windows 2.x.

Videos:
https://www.youtube.com/watch?v=539tnsX-Zso
https://www.youtube.com/watch?v=DSTzAeWWDYo&t=660s

EMS Magic also used to be commercial, but was mainly meant for VDMs on Windows NT or OS/2.
Still, it's also usable on real DOS and a good alternative to EMM286.

A very rare solution for EMS are external MMUs, such as the ALL CHARGECARD.
They existed as processor interposer cards (for 80286 systems), for example.
Video: https://www.youtube.com/watch?v=xgMIbo6QoM4
PC-MOS/386 supported some of them, if memory serves.

Edited.

EMS cards used mapping hardware
EMM386 and alternatives like Jemm386 use the 386 V86 mode for memory mapping, same way it can map UMB blocks
Inbetween, there were disk based emulators, absolutely copy based, well load and save based and also XMS copy based that could work on a 286.
And then there was chipset based EMS on some 286 systems, which used hardware mapping