Is this a missing feature in Dosbox?

Right, there's no real task switching for the 286-style descriptors implemented.

Which modifications should be made to make it run?

Not sure, but you'd certainly have to implement some pieces in CPU_SwitchTask,
and add the missing case entry for CPU_JMP (easy part). You might run into other
problems after that, or not.

[Thread moved to DOSBox Development Section]

Thanks for your fast answer!

I've been making changes to the CPU_SwitchTask method. Basicaly I have commented the 'E_Exit("286 task switch")' lines and applied the same operations used in the 386 part but using non extended registers (I mean AX instead EAX, for example) and TSS_16 descriptors. I'm not using paging mechanisms as I'm trying to emulate a pure 286.

Up to now, the code is executing properly 😀

At least it runs protected tasks but I have to check if everything works fine.

Thanks for your support!

I've been making changes to the CPU_SwitchTask method. Basicaly I have commented the 'E_Exit("286 task switch")' lines and applied the same operations used in the 386 part but using non extended registers (I mean AX instead EAX, for example) and TSS_16 descriptors.

Yes that may be enough, i've checked the io protections and they already have
the correct 286/386 tss check there, but things like that may pop up elsewhere
and are hard to figure out.

Hi! I've modified the CPU_CALL method on the cpu.cpp file adding the DESC_286_TSS_A case on the DESC_386_TSS_A branch. This works fine for the type 0x01 descriptor.

Now I've found another problem: the DESC_TASK_GATE (0x05) descriptor. I have found a 0x9A opcode that uses this kind of descriptor and there is no "case" for it, so it throws an error ("CALL:Descriptor type DESC_TASK_GATE unsupported")

The 0x9A opcode is a CALL to direct intersegment via task gate, as specified on the 80286 manual.

Is this code implemented elsewhere? How should I treat this case to make it work?

Thanks in advance.

Is this code implemented elsewhere? How should I treat this case to make it work?

No it is not implemented, not sure how exactly CALLing a task gate would work
but you can start with the code for DESC_TASK_GATE in CPU_Interrupt() and
check differences in the intel/amd manuals and bochs sources.

Great, that's what I was doing. I have copied the code from CPU_Interrupt renaming the Descriptor from "gate" to "call" and it seems to be working up to now.

Thanks!