iMac G5 processor attenuation

Discussion in 'PowerPC Macs' started by Chasealicious, Mar 9, 2006.

  1. Chasealicious macrumors member


    May 6, 2005
    Fayetteville, AR
    I set my iMac up last night to burn an hour and a half long DVD with iDVD, so I knew it was going to take at least two or three hours to encode.

    Even working off of an external hard disk, the CPU temperature immediately shot up to a consistent 195 degrees. Having had a Rev. A iMac that had both a fried hard drive and logic board, I'm pretty sensitive to the operating temperature of this machine.

    Not wanting to leave the machine running at that temperature for such a long time, I went to system preferences and set the processor performance to "reduced." Immediately the temperature dropped to a steady 165-170 degrees.

    My question is, what technically happens when you reduce the performance? Does the chip downclock itself? I'm just curious as to how exactly the process works. Wondering if anyone might know.
  2. Unorthodox macrumors 65816


    Mar 3, 2006
    Not at the beach...
    Yeah it just throttles down the voltage.
    I also have a rev. A iMac. I'v never fried the CPU or MB. But sometimes, on a hot day, under full load, it will automatically power down.
  3. mmmcheese macrumors 6502a


    Feb 17, 2006
    I don't know how the G5 handles it, but there are a couple of ways to do it. Some chips support this natively and either the FSB, or the multiplier is changed speed up and slow down the processor. This is very typical of mobile processors such as the Pentium M and Core CPUs from Intel.

    Another approach (typically used in cheap PC laptops that use desktop processors), is to have the OS send sleep commands to the CPU at a certain interval (ever second operation for example). This causes the CPU to idle a certain amount (sometimes there's only 1 actual operation and 3 sleep operations...essentially reducing the processor speed to 1/4 of the original speed), causing less heat. This approach isn't always that good, but if the processor doesn't support speed scaling natively, you don't really have any other choice.
  4. Eidorian macrumors Penryn


    Mar 23, 2005
    So what happens when it throttles down the voltage? (I know about the voltage, but performance wise. :rolleyes: )

    I notice a big slow down from automatic to reduced. I just dusted out my G5's CPU duct. It was packed with dust. :eek:


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