50% CPU limit with ACPI multiprocessor

Hi,
Can someone please explain this one to me?
When I run an extremely CPU-intensive task, such as recalculating a vast Excel spreadsheet, the CPU graph in Task manager sits on 50% not 100%.
Does this mean that half of my CPU capacity is not being used, or is it just a quirk of the Task Manager display?
Also, some forums have suggested to me that I should see two CPU graphs in TM, but I only see one. Which is correct?
Info:
Device Manager says the ACPI multiprocessor driver is OK. NUMBER_OF_PROCESSORS env variable is correctly set to 2. I'm running Win XP Home on a Gigabyte 8KNXP MB with latest BIOS, P4 2.8GHz, 2GB RAM

 

It means the task only requires half of the processor power. If it required 100% the graph would show that.
XP Home doesn't support more than one processor. That may be the reason you only see one graph. With an HT enabled you should see 2 graphs. My XP Pro shows 2.

 

Thanks Bob,
May I follow up on your reply? First of all, my post was misleading: due to a temporary attack of stupidity, I failed to spot that my TM display does have two processor graphs (I was expecting to see them one under the other, not side by side).
Could you expand a bit on the XP Home issue? My machine appears to be running two CPUs if I believe the Device Manager properties dialogue, but I am a bit sceptical that the task only requires 50% of the CPU - when I run a pure CPU task, it sticks for ages on exactly 50%, not (say) 49% or 51%, and having 2Gb RAM my swap file usage is permanently zero.
Could it be that whilst XP Home doesn't complain about not being able to utilise the second ACPI virtual processor, it simply ignores it without telling me, and only uses half of the machine capacity?
Regards,
Mike.

 

That is a possibility. If it was a true dual processor box XP Home would install the multi-processor HAL, but, wouldn't utilize both processors.
Have you tried to run prime 95 to really tax the CPU? It should utilize 100% of both processors.

 

Another problem that you may be having is that some tasks just can't make effective use of two processors. If each step requires the result of the previous step to compute, you can't really do two steps at once that easily. Also, most programs aren't written to be multi-processor capable; I know that MaxPC has a difficult time finding stuff that will actually stress a two-processor machine with a single app (of course, if you run two processor-intensive tasks at the same time, you see the benefits of multiple processors). I'd trya stress utility like Bob mentioned, or running two big apps at once (say, a nasty PS filter on a huge image, at the same time as recalculating your monster spreadsheet)