🚀 go-pugleaf

In article <9008161307.AA11621@esegue.segue.boston.ma.us> johnl@esegue.segue.boston.ma.us (John R. Levine) writes:
>
>can't try it myself.)  If flag bit 18 really doesn't work, it might work to
>try wiggling bit 18 in CR0 which is unimplemented on the 386.
>

You can't access CR0 in CPL3.  So setting any bit in CR0 in V86 mode will
be trapped.  I have tried setting bit18 in EFLAGS, and found it not to
be latched in real mode.  I found this astonishing, and therefore posted
my results.  Since that posting I did further testing, and found my first
conclusion to be wrong.

-----------------------------------
Continuation of a posting that I didn't get a chance to finish:
-----------------------------------

As I said in another posting, trapping invalid opcodes, and setting
undefined bits in EFLAGS both have their pitfalls, and both are seemingly
valid techniques for determining CPU type.  However, setting undefined
bits in EFLAGS relies on (apparently) undocumented default settings for
these bits in EFLAGS.  

Deep down inside, I still feel more comfortable trapping invalid opcodes
because this method relies on documented differences in the CPUs.  Another
consideration is upward compatibility, and expandability.  What about the
'586?  Will the '586 behave in the same manner -- that isn't documented?
Since Intel published the CPU detection code themselves, that is a good
chance that the '586 will behave in the same manner.  But this 
conclusion assumes that the guy that wrote the code has good connections
in the CPU department.  If have found that Intel isn't like this at ALL.
One hand doens't know, nor have access to, what the other hand is doing.
So, just because Intel published the code, and the code relies on 
undocumented features of the chip, doesn't guarantee the code will work
on the '586.

Chosing which algorithm to use now becomes a consideration of the '586.
Which probability is greater:  the '586 having another new non-intrusive
opcode, or the '586 having another new bit in EFLAGS?



-- 
"Worship the Lord your God, and serve him only."  Mat. 4:10
Robert Collins                 UUCP:  ...!sun!altos86!rcollins
HOME:  (408) 225-8002
WORK:  (408) 432-6200 x4356