[SeaBIOS] Check for valid MBR before adding drive as "bootable"

[Steve Goodrich]

Back in July I was working on a problem that involved determining if a drive was bootable (see emails from circa 13 July 2012, "Bootorder Failover (Patch)").  I now have a few extra cycles and I'd like to try to do this the right way.  What I'm trying to do is add the "drive_is_bootable()" call to the various boot_add_xxx methods, similar to the following in boot.c:

void
boot_add_hd(struct drive_s *drive_g, const char *desc, int prio)
{
    if (!CONFIG_CHECK_BOOTABLE || drive_is_bootable(drive_g))    // <<<<< new code
        bootentry_add(IPL_TYPE_HARDDISK, defPrio(prio, DefaultHDPrio)
                  , (u32)drive_g, desc);
}

The drive_is_bootable() function would read the MBR from the drive, check a few things to ensure that the drive is bootable, and return 0 (fail) or non-zero (success).  Per the email thread referenced above, I've got my own copy of process_op() from which I've removed the ASSERT16().  Unfortunately, there are ASSERT16()'s deeper in the call tree -- specifically in the code used by process_floppy_op(), process_ata_op(), and process_cdemu_op().

I think the proper approach would be to put the processor in 16-bit mode and use the original process_op() function (returning to 32-bit mode afterward), but I'm not sure how to do that here.  Can anyone offer some guidance here?

Thanks,
    -- Steve G.