Re: [RFC] P6 microarch (Pentium 2/3) DFA scheduler description

[Steven Bosscher <stevenb at suse dot de>]

On Thursday 26 February 2004 03:51, James Morrison wrote:
> > to ! ;; two uops, and simple read-modify instructions also take two µops.
>
>  uops?

Actually it is µops, but the mu symbol is usually typed as u for
simplicity.  This was probably copy-paste from some manual, but
I should use uops everywhere.


> > uop, ! ;; which is not exactly true because there are a few instructions
> > that ! ;; decode to 2 uops or microcode.  But this probably gives the
> > best
>
>  What does microcode mean here?

Microcode means that the sequence of uops generated for an
instruction is hard-coded on the chip, i.e. firmware.

> > ! ;; div and idiv are very similar, so we model them the same.
> > ! ;; QI,HI,and SI have issue latency 12, 21, and 37, respectively.
>
>  The code has magic numbers 19, 23, 39.  Is it obvious where the extra
> 7, 2, and 2 are coming from?

The difference of 7 is probably a typo, iirc the 12 should be 17.
The 19, 23, and 39 are the latencies for the _result_ of the
instruction, while the 17, 21, and 37 are the issue latencies,
i.e. a new idiv can only be issued 17/21/37 cycles after the
previous one.

> Thanks, this was fun to read.

Thanks for the corrections!

Gr.
Steven