CPUID varies with multiplier setting in AMD K5

[Neon]

I bought a AMD-K5-PR200ABX processor last week. While testing, I found that CPUID changes depending on the multiplier setting. Previously, I had assumed that CPUID is "hard-coded" in the processor such that the same result is returned upon query, independent of the processor speed. This seems to be false, according to these results:

[Neon_WA]

Well done at finding this info out

[Neon]

I know that certain default CPUID data can be overwritten by the system BIOS at boot time, but I did not think that the basic family, model, stepping data would get altered. I am not smart enough to know whether this is:
a) just a quirk of my particular motherboard BIOS, or
b) a general property of the K5 processors.

It would be good to hear about others' experiences with these processors.

[Elar]

From K5 datasheet, PR200 not shown, but fits in as model 3 with 2x multiplier and BF pins at "11"

[6a6ar09a]

Some more info about PR200ABX

http://www.cpu-collection.de/?l0=co&l1=AMD&l2=K5#AMD-K5-PR200ABX

[Xaar]

Hello!

That's a good Information!

[Neon]

OK, good! Your results for PR150ABR agree with my results.

The PR150 is a strange animal, and apparently AMD did not publicly document its specification. There is conflicting information on websites. Some sites claim that PR150 settings are 2.5x 60MHz = 150MHz, with internal frequency 105MHz. This is logical because it would complete a set of 3 analogous pairs of products that use 60MHz/66MHz bus speeds: PR90/PR100, PR120/PR133, and PR150/PR166.

However, as you noted, the PR150 (all that I have seen) are marked 66MHz bus. So ???

I speculate:
a) From a manufacturing point of view, PR150 is identical to PR166, and it performs the same;
b) From a product point of view, customers who have motherboard with maximum bus speed 60MHz will want PR150 because it is the fastest AMD upgrade possible.

[Neon]

I just completed some additional testing with K5-PR166ABR and K5-PR133ABR.

For the PR166ABR, the results are exactly the same as above for the PR200ABX. This includes motherboard setting 3.0x 66 = 200 (internal frequency 133MHz, a 16MHz overclock of nominal 117MHz). At this setting, PR166 has CPUID 534.

For the PR133ABR, at motherboard setting 2.5x 66 = 166 (internal frequency 117, a 17MHz overclock of nominal 100MHz, woo-hoo!). At this setting, PR133 has CPUID 524.

In summary, for K5-PR200ABX, PR166ABR, PR150ABR, and PR133ABR processors, the CPUID value returned depends only on the motherboard multiplier setting:
2.0x --> CPUID 514
2.5x --> CPUID 524
3.0x --> CPUID 534

[Xaar]

[gshv]

The document below has CPUID, clock speed and multiplier for PR150 processor:
http://www.amd.com/us-en/assets/content_type/white_papers_and_tech_docs/21062.pdf

I don't know if any other family that has the same "feature" - different CPUID based on clock multiplier. It could be unique to K5 family.

Gennadiy

[Neon]

The datasheets explain a lot - I should probably learn to read before posting my "discoveries". If I understand correctly, the motherboard multiplier determines the signals to model 1, 2, or 3 CPU pins BF0 and BF1. This determines the processor internal multiplier, and that determines the CPUID model value.