Geeky question OTD

[figmo]

I upgraded my PowerBook last night from a 400MHz G3 to a 500MHz G4.

Virtual PC on my PowerBook used to think I was a Pentium II. Now it thinks I'm a "686." What's that? I was hoping to be a Pentium III or Pentium IV.

Comments

[keristor.livejournal.com]

Well, the slowest PC-class machine I have around here is an AMD K6-2 (P2 equivalent) 500MHz, and you're around half the speed of that. But I remember not long ago when any P2 type processor running at over 200MHz was regarded as 'fast'. Basically, don't even think about Windoze XP on the thing but it will be fine for running Linux (unless you want to do something like recompiling the kernel).

Note that apart from the speed the '686' classification implies the instructions available (P3 and P4 added instructions for specialised tasks, as did the AMD Athlon but a different set of extra ones), and is basically the 'core' set of all those in common to that type of processor, so it's saying that it hasn't found anything extra to identify it as one of the other processors.

[pbristow.livejournal.com]

Yep. So what this is telling us is that Virtual PC emulates the instruction set of a generic 686-class processor, with no extras. Presumably it's assessment of the speed will go up in proportion to the clock rate of your make, but you won't be able to jump class to a P3 or P4 without upgrading or reconfiguring Virtual PC itself.

N.B. The P4 actually has *fewer* special instructions than a P3: They took out the ones where the specialised circuitry just wouldn't cope with the higher clock speed they wanted to push the P4 up to! So a P3 at, say, 700MHz is actually more powerful (at ceratin tasks) than a P4 at the same speed; but the P4 can be run twice as fast.

All of which is irrelevant to Virtual PC of course: It just emulates whatever it's written to emulate, at whatever speed it can manage on the hardware provided.

[pbristow.livejournal.com]

Er... In the first para, "make" is a typo for "Mac". [BLUSH]

[figmo.livejournal.com]

Interesting. I just upgraded to Virtual PC 6 with the upgraded hardware and it still thinks I'm a 686 (Pentium II with MMX). Same megahertz. Go figure.