SimCity2000 hangs on mouse actions

[stsp]

Describe the bug Scrolling the map with a scroll bar causes the game hang. dosemu crashes shortly after. Actually moving the mouse wheel is enough to crash everything.

To Reproduce Scroll the city map with the side-scroller, or just move the mouse wheel.

A regression? Very likely so.

[stsp]

Doesn't crash if you have stacks=0 in config.sys, or if you use something else than freedos or fdpp. And the problem is specific to dos4gw-1.95 which sc2000 uses. Hmm... Most certainly another freedos bug...

[stsp]

It appears dos4gw-1.95 somehow (???) maps the DOS irq stack (if one is configured), and uses it as a stack for prot mode IRQs! So its particularly sensitive to the IRQ stack usage strategy, which probably differs between freedos and ms-dos. So it seems this is not a freedos bug, just some incompatibility in an irq stack usage. But how can one map the IRQ stack of DOS? Is there any API to find its location and size?

[andrewbird]

I don't know much about DOS stacks, but this page https://sites.google.com/site/pcdosretro/dostables (search for Stacks block header) seems to have some info. Incidentally the guy that has that site pcdosretro used to work on PC-DOS 7 and until fairly recently was active on vcfed see https://www.vcfed.org/forum/member/50489-pcdosretro , maybe he can help?

[stsp]

Thanks, interesting link. If they are talking about the stack set with stacks= then indeed freedos doesn't maintain that header and the descriptors array. Anyway, that still gives us no info on how dos4gw maps these stacks. If I knew that, I could try to cheat him and give it some scratch area instead of real stacks. But since this is still unknown, I'll try another idea: if I "revector" the mouse interrupt, then it will not execute on the DOS irq stack. Which may solve the problem quite cheaply. Or maybe just disable those silly irq stacks by default?

[stsp]

Whoa, it all appears much simpler. :)

    SetSelector(DPMIclient[rmcb_client].realModeCallBack[num].rm_ss_selector,
                (SREG(ss)<<4), 0xffff, DPMI_CLIENT.is_32,
                MODIFY_LDT_CONTENTS_DATA, 0, 0, 0, 0);

... is what dpmi_realmode_callback() does. So dosemu2 itself maps the irq stack to dos4gw if the call-back is done from IRQ - something that never happened in dosemu1.

[stsp]

So there was no bug at all. Just that dos4gw can't work with 256b irq stacks, so lets create 512b ones.

[stsp]

That would sill leave sc2000 broken on freedos... I need to stop mapping irq stacks to DPMI instead, I guess.