Protected Hardware Interrupt Handler Stuck? (DJGPP)

Question

I'm trying to set up a hardware interrupt handler in protected mode, using djgpp-2 for compiling in dosbox-0.74. Here's the smallest code possible (timer interrupt), I guess:

#include <dpmi.h>
#include <go32.h>
#include <stdio.h>

unsigned int counter = 0;

void handler(void) {
    ++counter;
}
void endHandler(void) {}

int main(void) {
    _go32_dpmi_seginfo oldInfo, newInfo;

    _go32_dpmi_lock_data(&counter, sizeof(counter));
    _go32_dpmi_lock_code(handler, endHandler - handler);

    _go32_dpmi_get_protected_mode_interrupt_vector(8, &oldInfo);

    newInfo.pm_offset = (int) handler;
    newInfo.pm_selector = _go32_my_cs();
    _go32_dpmi_allocate_iret_wrapper(&newInfo);

    _go32_dpmi_set_protected_mode_interrupt_vector(8, &newInfo);

    while (counter < 3) {
        printf("%u\n", counter);
    }

    _go32_dpmi_set_protected_mode_interrupt_vector(8, &oldInfo);
    _go32_dpmi_free_iret_wrapper(&newInfo);

    return 0;
}

Note that I'm not chaining my handler but replacing it. The counter won't increase beyond 1 (therefore never stopping the main loop) making me guess that the handler doesn't return correctly or is called only once. Chaining on the other hand works fine (remove the wrapper-lines and replace set_protected_mode with chain_protected_mode). Am I missing a line?

Answer 1

You need to chain the old interrupt handler, like in the example Jonathon Reinhart linked to in the documentation, as the old handler will tell the interrupt controller to stop asserting the interrupt. It will also have the added benefit of keeping the BIOS clock ticking, so it doesn't lose a few seconds each time you run the program. Otherwise when your interrupt handler returns the CPU will immediately call the handler again and your program will get stuck in an infinite loop.

Also there's no guarantee that GCC will place endHandler after handler. I'd recommend just simply locking both the page handler starts on and the next page in case it straddles a page:

_go32_dpmi_lock_code((void *) handler, 4096);

Note the cast is required here, as there's no automatic conversion from pointer to a function types to pointer to void.