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SubjectRe: Linux Kernel Microcode Question


> On Mon, 22 Mar 2004, Timothy Miller wrote:
> > I don't see anything wrong with what he said. As I understand it,
> > Pentium 4 CPUs don't use microcode for much of anything. If an
> > instruction which was done entirely in dedicated hardware was buggy,
> > and
> > it's replaced by microcode, then it will most certainly be slower.
> >
> > You seem to have missed where David used terms like "theoretically
> > possible" and "an operation".

> No, that is not what he said and that (what you say) is certainly
> wrong,
> namely this bit:
>
> If an instruction which was done entirely in dedicated hardware was
> buggy, and it's replaced by microcode, then it will most certainly be
> slower.
>
> All instructions are done by means of microcode of some sort, i.e. the
> instructions are "compiled" as they are executed into a more primitive
> instruction set (called "microcode" or "u-code"). If a buggy
> instruction
> (or rather the sequence of microcode which corresponds to it) is
> replaced
> by a fixed version (i.e. by some other sequence of microcode) then
> there
> is no reason to say that the result will "most certainly be slower".
> For
> some bugs the fix runs faster than the broken code, for others it may
> be
> slower --- there is no way to tell apriori that it will always be
> slower.
>
> Do you understand now?

You are using the word "instruction" to mean something different than
what I am using it to mean. I am using "instruction" to mean "the
smallest cohesive unit of operation". I do NOT mean "instruction" as
in "an operation coded by the programmer".

I am talking about the case where an "operation" performed in buggy
hardware is replaced by an "operation" performed in fixed microcode.

By the way, the recent Intel patent lawsuit had this exact same issue.
The word "instruction" can refer to *any* cohesive unit that performs
some logical function.

DS

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