http://cvscompile.aep-emu.de/cl/dosboxChangeLog.txt

2006-03-12 21:31 c2woody

* src/ints/ems.cpp: limit VCPI support to JEMM games

real emm386 with ems and vcpi requires the processor to be in v86 mode all the time.
We don't like that.

So we have a hardware ems emulation and vcpi for those few games that really need it.

okay, that makes sense, but the register check means that VCPI can't be detected.
Then if I disable EMS from dosbox.conf and try to load JEMM, DOSBox crashes with a stack overflow...

that register check makes sure that only the jemm games detect vcpi
no one else as they then assume the processor is in v86 mode and will sometimes exit if the processor isn't.

why disable ems if you run a jemm game ?

I disabled EMS to see if JEMM would load. I thought that JEMM games would require JEMM. Anyway, the stack overflow is a bug.
So you have a problem of apps exiting because they find VCPI and not v86, and I have a problem of apps exiting because they don't find VCPI at all. :-)

i don't remember the wing commander series crashing when we didn't have the vcpi emulation.
Which game did you try ?

In the sourcefile ems.cpp you can enable vcpi if you want for your custom apps

You mix up the definition of what JEMM is, the games relevant system is
the emm-extension found in games like privateer and strike commander,
not the open-sourced emm386 replacement of japeth derived from the
freedos emm386 implementation.
The vcpi implementation in dosbox is far from complete but necessary for
ipx support under jemm games. So it's made available ONLY for those games
and nothing more. Especially as there's no need/use for a vcpi interface
under a hardware ems implementation like dosbox supports.

and I have a problem of apps exiting because they don't find VCPI at all

Well it's one demo, right? So not that much lost, especially as all halfwhat
intelligent extenders have a dpmi/vcpi/raw fallback strategy. I'm not sure
why they'd want to always use vcpi, but didn't check their code yet.

okay, now I understand. JEMM and JEMM. Easy to confuse.
The demo works only in VCPI mode because of a bug in the demo.
I'll see about making a patch for it.

Hm according to the docs it might even be intentional that it only works with
vcpi enabled, but that text could have been added later.

The code exists to run in raw and XMS mode, too, but it doesn't work properly. The documentation says that it requires EMS because they probably couldn't work out what the problem was.