You can use something like that:

1		/* Switch the psp's */
2		dos.psp(pspseg);
3		DOS_PSP newpsp(dos.psp());
4		dos.dta(RealMake(newpsp.GetSegment(),0x80));
5		real_writew(RealSeg(sssp-2),RealOff(sssp-2),0xffff);
6		block.exec.initsssp = sssp-2;
7		block.exec.initcsip = csip;

in dos_execute.cpp, should correct a part of the problem.
The rest seems bad return stack layout.

> You can use something like that:

Thanks! However, AFAICS this modification affects the called program's stack only. The issue I was talking about is the value of SP of the caller.

EDIT:

this modification works:

1	if (flags==LOAD) {
2		SaveRegisters();  /* added */
3		DOS_PSP callpsp(dos.psp());
4		/* Save the SS:SP on the PSP of calling program */
5		callpsp.SetStack(RealMakeSeg(ss,reg_sp));
6		reg_sp = reg_sp+18; /* added */
7		/* Switch the psp's */
8		dos.psp(pspseg);

but looks a bit strange, since a IRQ in the VM might destroy the register copy saved onto the stack.

the +18 is probably to counter the the -20 at the start of dos.cpp ?

Qbix wrote:

the +18 is probably to counter the the -20 at the start of dos.cpp ?

no, it's just to revert the change done to this register inside SaveRegisters().

but looks a bit strange, since a IRQ in the VM might destroy the register copy saved onto the stack.

Maybe trace the stack of msdos, if they skip the register saving completely.
In that case you'd remove the SaveRegs calls in int21/4b functions, and the
RestoreReg ones in int21/4c

wd wrote:

Maybe trace the stack of msdos, if they skip the register saving completely. In that case you'd remove the SaveRegs calls in int […]
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but looks a bit strange, since a IRQ in the VM might destroy the register copy saved onto the stack.

Maybe trace the stack of msdos, if they skip the register saving completely.
In that case you'd remove the SaveRegs calls in int21/4b functions, and the
RestoreReg ones in int21/4c

The register saving is - of course - done by MS-DOS. The important thing is that SS:SP isn't changed, that is, the calling application can push/pop registers it wants to preserve onto the stack across the call.