"Constant Brouerius van Nidek"
<[EMAIL PROTECTED]> wrote:
> Nice to have survpc back online.
> When I use some programs on my 486 I get the info
> that emm386 has discovered... and the system may
> be unstable. The error discovered is every time
> and again " Exception 6 (invalid Opcode)"
<snip>
> What is Exception 6 and what is an opcode?
Most commonly used in discussions of the CPU
and/or assembly language, the term "opcode"
(short for "operation code") is synonymous with
"instruction" (a machine language or assembly
language "verb" which instructs the CPU to do
something).
For example, in the instruction "MOV AX,BX"
the "MOV" is the opcode, while "AX" and "BX"
are operands.
There are only certain binary values which can
be executed as instructions (opcodes) by the
CPU. If the CPU encounters a "bad instruction"
value in the stream of instructions it's
executing, it has to have a way of dealing
with that.
This is generally handled by having a special
address list to which the CPU "vectors" (transfers
program control) in the event of any "do something
undefined" kind of event -- and this kind of event
is classified as an "exception."
Exceptions also include such things as accessing
a piece of memory belonging to another process
(a "protection" fault) and dividing by zero (kind
of a "do something impossible" instruction).
To allow the computer to continue execution past
something like this would be meaningless -- and
you can't have your computer running around
"gibbering to itself" -- the results would be, as
engineers like to say, "undefined."
Imagine "... I have no idea where I'm going, but
I'm making really good time ..."
So, the exception vector usually just prints out
some terse "we're all going to die now" message and
executes a "HALT" instruction (guess what that does).
I hope this doesn't make it more confusing that it
needs to be, but, there you go.
~~Garry
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