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Clearing P-RAM and resetting the PMU are different
Authored by: danieleran on Aug 30, '05 04:52:12PM

Macs' 'Parameter-RAM' and 'NonVolatle-RAM' are like a preferences files in firmware. Clearing the P-RAM is useful only when there is something corrupt written to this battery backed RAM. Holding down Apple+Option+P+R on boot will reset the P-RAM, and sometimes fix strange behavior such as failure to find the startup volume. P-RAM was used more extensively under Classic OS 9, and therefore was a primary troubleshooting step.

The Power Management Unit, or PMU, is the self contained computer on Macs that (among another things) allows you to schedule it to turn itself on or off. If this gets messed up, you can similarly reset it. On PowerMacs prior to the G5, you had to open your Mac up and carefully touch the PMU reset button on the logic board. If you touched it twice, or for too long, you risked crashing the PMU computer and draining your battery, which would then cause P-RAM to fail to hold values (including the time and date for the clock).

On newer Macs, including the latest G5s and apparently the Mac mini, you can reset the SMU (System Management Unit, which replaces the former PMU) by simply unplugging the unit and letting it sit unpowered for a 15 seconds.

PMU:
http://docs.info.apple.com/article.html?artnum=2238

SMU:
http://docs.info.apple.com/article.html?artnum=300341



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Clearing P-RAM and resetting the PMU are different
Authored by: macshome on Aug 31, '05 03:40:11PM

Just to fill in some fun backstory from WWDC this year...

It's not cmd-opt-p-r. It's cmd-opt-r-p. The original designers were thinking "reset pram" not "pram".

I've always gone with the cmd-opt-p-r though, as I could tell users to remember the word "copper".

Josh

---
http://www.afp548.com
Breaking my server to save yours.



[ Reply to This | # ]
Clearing P-RAM and resetting the PMU are different
Authored by: macshome on Aug 31, '05 03:40:12PM

Just to fill in some fun backstory from WWDC this year...

It's not cmd-opt-p-r. It's cmd-opt-r-p. The original designers were thinking "reset pram" not "pram".

I've always gone with the cmd-opt-p-r though, as I could tell users to remember the word "copper".

Josh

---
http://www.afp548.com
Breaking my server to save yours.



[ Reply to This | # ]