Going realmode in kernel drivers?

[Peter B]

i386/FreeBSD-4.x/lkm.

How does one get into 'realmode' inside a kernel driver?

The reason for the need is a tight timeing loop that measures the lenght of
pulses. And disableing interrupts is just not enough.
Target cpu's are AMD K5 + AMD XP.

Asfair when reading cycles per opcode. The number of cycles required increase
about three times when useing protected mode or similar.

Code excerpt:
    u_int32_t register      cnt1;
    u_int32_t register      cnt_max=0xFF;
    u_int32_t register      *store_ptr;
    u_int32_t register      *store_end;
    u_int8_t  register      last_val=0;

    store_ptr = ..;
    store_end = ..+ SIZ;

    disable_intr();
    for(;;)
        {
        for(cnt1=0;  cnt1<cnt_max && ((inb(0x379)&0x20) != last_val);  cnt1++)
            ;
        *(store_ptr++) = cnt1;
        if(  store_ptr>=store_end  )
            break;
        last_val ^= 0x20;
        }
    enable_intr();

(Will start on a new count every signal flank).

   /P