http://www.adobe.com/support/techdocs/a096.htm

Support Knowledgebase

Document 315344

Product
All
Platform
Macintosh
Last Edited
01/17/2001
Filename
a096.htm   PRAM General Information

A Macintosh computer's Parameter Random Access Memory (PRAM), which contains 20 to 256 bytes of default information for your system, is located in the computer's clock chip. The clock chip is powered by a battery when the Macintosh is turned off so that default and user settings are present the next time you restart the Macintosh.

PRAM contains values for the following:
- validity status of the clock chip
- clock settings (alarm setting, time, date)
- node ID hint for a modem port (9600 baud, 8 data bits, 2 stop bits, no parity) and printer port
- modem port configuration stop bits
- printer port configuration and connection
- printer connection
- monitor colors
- application font number minus 1
- auto-key threshold and rate
- speaker volume
- double-click time
- caret-blink time
- menu blink
- mouse scaling
- preferred system start-up disk
- network setting (network defaults to LocalTalk)

Because you can't usually access the clock chip directly, the PRAM values are copied into memory during startup. This copying process is a hardware action -- system software is not involved. You can change the copied PRAM values using the General Controls control panel. When you shut down the computer, these values are written back to the clock chip.

Zapping the PRAM
PRAM values can be reset to the default settings in the clock chip. Resetting PRAM values, often referred to as "zapping" the PRAM, resets the computer's PRAM to the default settings from the Monitors and Network control panels, but not the default clock settings.
According to Apple Corporation, there is no value in resetting the PRAM as a troubleshooting tool for software problems. There have been a few, unconfirmed reports that resetting the PRAM helped to reset a NuBus video card when the monitor displays only static. These reports are not verifiable and there are no PRAM values or settings for NuBus slots. The only proven time resetting the PRAM may be of benefit is if your Macintosh computer fails to start up under any circumstances.

To reset PRAM in Mac OS 7.0 and later:
1. Restart the computer and press Command+Option+P+R before you see the smiling Macintosh.
2. Keep the keys depressed until you've heard the computer's restart chime three times, then release them. The first chime indicates the computer has restarted. The second chime indicates the PRAM has been zapped. Adobe recommends you wait for a third chime, indicating the PRAM has been zapped again, to ensure the PRAM has been completely reset.

You can also zap PRAM using a third-party utility such as TechTool. Check with Apple or a local retailer for more information.

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