[syslinux] reboot from com32

[H. Peter Anvin]

Miguel wrote:
> Q: Is there a way to do a reboot (either warm or cold) from a com32 program?
> 
> I have a com32 program that sometimes (based upon conditions) needs to
> reboot the system.
> 
> I would prefer to do a warm reboot (40:72 = 0x1234), but at this point
> cold would be OK too.
> 
> grepping through syslinux-3.51 I only found one reference, in
> iso/pxelinux.asm at the tail end of 'kaboom' ... but presumably that only
> happens after things go wrong.
> 
> It isn't clear to me how to do this since the com32 program is running in
> 32-bit mode.
> 
> Q: Can I just make a pointer to a function, set the value to 0x000FFFF0
> and make a call into the BIOS ROM, even though I am running in 32-bit mode?
> 

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.

	-hpa