On  9 Apr 98 at 12:02, Dan Epure wrote:

> I recently heard about the existence of MicroCode, a little program
> (about 2 Kb) that reside inside CPU.

  The original definition of "microprocessor", dating from the 1970s,
was that it was a CPU that used an internal machine language to
implement the external language in which applications are written.
There were a few experimental systems that were able to host several
different popular instruction sets by reprogramming the microcode,
but that never really caught on.

> I also heard that MicroCode can be modified by software (on Pentium or higher
> CPU).

  There are persistent rumours that Intel has included this facility.
But I don't know of any details being available -- imagine the havoc
that a virus could cause with something like this.
  I don't think there's a useful difference between "Intel can't tell
anybody about this" and "Intel never did it".  Microcode is a useful
thing for CPU designers to study, but not something anybody else
needs to worry about.

David G