[SOLVED] Where does /proc/cpuinfo model name data originate?

astrogeek · 12-09-2013, 05:55 PM

I am performing another resurrection of an old box and just discovered that the CPU is a faster version than I had expected - pleasant surprise. But now I am confused as to what I am seeing when I cat /proc/cpuinfo. I always thought that was reported by the cpu itself - apparently not.

The motherboard is an SV266A and the CPU is AMD Athlon. When I received the corpse it had 256mb RAM and cat /proc/cpuinfo reported:

Code:

processor       : 0
vendor_id       : AuthenticAMD
cpu family      : 6
model           : 8
model name      : AMD Athlon(tm) 
stepping        : 0
cpu MHz         : 1248.759
...

It seemed to work fine with a good hard drive and network card so I thought it worth adding some RAM and bringing it back to life. The RAM arrived today and I had found a datasheet on the motherboard.

While setting it up I pulled the CPU heatsink and looked up the part number - and found that it is actually an 'Athlon XP2000+' CPU that operates at a higher clock rate. So I configured the motherboard and RAM timings for higher speed and it works great.

But now cat /proc/cpuinfo reports:

Code:

processor       : 0
vendor_id       : AuthenticAMD
cpu family      : 6
model           : 8
model name      : AMD Athlon(tm) XP 2000+
stepping        : 0
cpu MHz         : 1662.003
...

So obviously the model name does not come from the CPU itself, but must be looked up by the kernel based at least in part on the clock rate. In other words, reported CPU model is inferred, not read directly from the CPU.

That being the case, perhaps I should look under the heatsink on some of my other zombies!

I know this is ancient hardware for most of you, but would be curious to know if anyone else has seen this, or if I have stumbled on something known to everyone else.

NOTE: It is running Slackware -current headless.

TobiSGD · 12-09-2013, 06:31 PM

It depends on the CPU. The Athlon XP CPUs never really had the knowledge about their type, so from a software point of view the only way to determine which Athlon XP was actually in the machine was a lookup table.
This is different with modern CPUs that switch their clockspeed dependent on load, they know their type and nominal frequencies.

genss · 12-09-2013, 06:59 PM

the name does come from the cpu itself
well... yes, but not human readable

"modern" cpus (past 486) have an extra opcode called cpuid that fills the registers with info on the cpu
first register then holds the stepping, model, family and such
while other hold info about the MMU, flags for extra capabilities like fpu, sse, nx etc. etc.
any program can execute it

there is a fairly detailed example on wikipedia

i guess the commercial name changed 'cuz AMD made more then one series with exactly the same core (they maybe changed a few things not related to the architecture)
so the kernel guessed based on the base frequency

when that series went out the "war" with intel was flamin' if i remember right

edit: TobiSGD said it good, i just misinterpreted "type" for the general info
this still stands thou, just less on topic

astrogeek · 12-10-2013, 01:00 AM

Thanks TobiSGD and genss - exactly what I was looking for , very helpful!

I had a mental note that CPUs included such info since Pentium days, but did not really know the specifics. When I saw the reported model name change based on clock freq instead of the other way around I found it a little confusing.

Marking solved.