Hi Uli and Abe,

indeed it seems like it is not trivial to find the right
excluded areas for UMB to have stability. Having no page
frame is okay - EMS 4 aware software can still use EMS,
only EMS 3 software will miss a frame.

Normally it is enough to have 550k conventional free.

To explain the different memory types:

low DOS memory - the usual 640 kB that you always use,
  where you want to have at least 500 something free.

HMA - provided by HIMEM - lets mainly DOS kernel load high

XMS - provided by HIMEM - lets various DOS programs enjoy
  extra megabytes (ramdisk, dos extenders, caches etc.)

UMB - provided by EMM386 - lets you load various drivers
  high, can also be provided by UMBPCI or other hardware
  drivers, can cause stability issues in conflict cases

EMS - provided by EMM386 or old special hardware - lets
  old DOS programs enjoy some extra memory for page swap
  and extra data storage, not often needed by modern apps

VCPI - provided by EMM386 - lets DOS extenders share the
  protected mode with EMM386, so if you do not load EMM386
  in the first place, you will not need VCPI either. The
  special GEMMIS feature is similar, but for Windows 3.

DPMI - provided by Windows and some DOS extenders - lets
  programs which use DOS extenders share protected mode,
  popular for modern games. Often, games come with their
  own DPMI driver to be able to run outside of Windows,
  using XMS or raw memory in that case.

Raw memory - if you use protected mode "by hand", you can
  of course use all those megabytes outside the first DOS
  megabyte-and-a-bit, too. But using XMS or DPMI often is
  more convenient.

What does this tell you for normal users? You should load
HIMEM for HMA and XMS. If you need space to load drivers
high, you should also use EMM386. The rest will be magic
DOS extender use of whatever suitable memory you have and
only in rare cases you would actually need EMS :-)

Note that HIMEMX and XMGR are like HIMEM and JEMM386 is
like EMM386, while JEMMEX is like both HIMEM and EMM386
combined into a single driver, with some pros and cons.

Regards, Eric

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