Can't use interrupts after returning to realmode under Bochs

[yakujabi]

I'm trying to set a video mode by returning to real-mode and issuing an interrupt.

It works well under all VMs I've tried, but Bochs just hangs in the interrupt handler without returning. I also get the following line spammed in the logfile:

io write to address 00000000, len=2

The steps I've taken to return to real-mode are:

0) cli
1) copy some 16-bit code to a low address
2) far-jump to that code on a 16-bit, 64k-limit code segment
3) clear first bit of cr0
4) far-jump to the next instruction
5) set proper real-mode segment register values
5) reset IDT & GDT to the real-mode defaults
6) set proper real-mode segment register values again (don't know if it's needed)
7) issue an interrupt

I'll appreciate it if anyone points out any mistakes I've made or any Bochs-gotchas that could be causing this behaviour. Thanks

[Masterkiller]

Did you check memory location 0000h:0000h? It should containt Interrupt Vector Table. IDT is not valid in real-mode, you could even keep it. Address 0x00000 shoud contain 32-bit value seg:offset for INT 0 and so on up to 0x3FF.

[Brendan]

Hi,

I think you might end up with a 32-bit stack (where "push byte 0x00" decreases SP by 4). Try this to see:

0) cli
1) copy some 16-bit code to a low address
2) far-jump to that code on a 16-bit, 64k-limit code segment
3) load 16-bit data segment registers (especially SS)
4) clear first bit of cr0
5) far-jump to the next instruction
6) set proper real-mode segment register values
7) reset IDT to the real-mode default (don't worry about the GDT)
8 ) issue an interrupt


Cheers,

Brendan

[Dex]

You could also compare your code to my vesa demos, as the does what you want to do.
http://www.dex4u.com/demos/VesaDemo.zip

[yakujabi]

Hi,
I think you might end up with a 32-bit stack
Brendan

Thanks Brendan! The problem was indeed stack-related,it's fixed now, and the interrupt returns just fine

...except that it doesn't much more than that
Doesn't crash, doesn't print anything to the logfile, and worst of all, doesn't change video-modes

Any ideas on this one?

Thanks

[yakujabi]

Well, it turned out that I had a 16-bit stack that uses ESP, so now everything works well.

Thanks Brendan!