[syslinux] reboot from com32

[Miguel]

>> Q: Is there a way to do a reboot (either warm or cold) from
>> a com32 program?
[snip]
> Not really.
>
> The easiest way is to use __farcall() to call to 0xf000:0xfff0:
>
> __noreturn syslinux_reboot(int warm)
> {
>   static com32sys_t dummy_regs;
>   uint16_t * const reboot_flag = (uint16_t *)0x472;
>
>   *reboot_flag = warm ? 0x1234 : 0;
>   __farcall(0xf000, 0xfff0, &dummy_regs, NULL);
> }
>
> I just added syslinux_reboot() to the git tree.

Looks great to me.


*** 5 minutes later ***

Hmm ... it isn't working ... and I'm puzzled.

I pasted this exact code into my com32 source file. When I compiled I got
a warning from gcc saying:

foo.c: In function ‘syslinux_reboot’:
foo.c:127: warning: ‘noreturn’ function does return

I shut this up by turning the __noreturn into a void.

At runtime, it returns. I put in printf statements before and after the
reboot:

void reboot()
{
  printf("getting ready to call syslinux_reboot(1)\n");
  syslinux_reboot(1);
  printf("hmm ... why am I here?\n");
}

Both print messages come out. And I get back to the pxelinux boot: prompt

*** 5 minutes later ***

I made 2 changes
 * made it say __noreturn instead of 'void'
 * zeroed out the dummy_registers

Behavior is slightly different ... execution seems to be continuing at
some other place in my com32 program.

*** 15 minutes later ***

I'm stumped.


Miguel