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Extended Parameter RAM
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Extended Parameter RAM (xPRAM)

Most user settings that need to be present at system startup are stored in parameter RAM. Parameter RAM takes up 256 bytes of battery-powered RAM: 20 bytes are documented, other 236 bytes are known as extended parameter RAM. Much of the information stored in parameter RAM is used exclusively by the system software.

Note that user can clear the current settings in the parameter RAM and restore the default values by holding down the Command-Option-P-R keys at system startup. When system software detects this key combination, it resets parameter RAM to the default values and then restarts the computer again. Clearing the current settings in the parameter RAM also causes system software to change other settings not stored in parameter RAM to default values. These settings include the desktop pattern and the color depth of the default monitor.

The default values for parameter RAM vary depending on the version of the system software. Do not rely on any one default value being the same for all machines.

location/length related to
$01 byte used by _InternalWait (_SetTimeout/_GetTimeout, $A07F)
$08 4 bytes looks like the last 4 bytes of the regular PRAM
$10 16 bytes looks like the first 16 bytes of the regular PRAM
$76 byte has something to do with the RAM disk size
$78 4 bytes default startup device info (DefStartType record)
$7C 2 bytes SysBeep ID -- ID of the corresponding 'snd ' resource in the System file
$7E byte used by _InitProcMenu ($A808)
$7F byte apparently it has something to do with the way windows and dialogs appear on the screen
$80 2 bytes some default video settings (used by _GetVideoDefault, $A080)
$82 6 bytes hilite color, apparently in the RGB format (see the Color control panel)
$8A byte memory/cache control flags (see the Cache control panel and the Memory control panel):
  • %1xxx xxxx    68040 Cache is OFF
  • %0xxx xxxx    68040 Cache is ON
  • %xxxx x1x1    32-bit addressing is ON
$AF byte has something to do with the RAM disk size
$BD 33 bytes used and set by some system routines in ROM; BTW, the beginning of the field at $BD looks like the name of default AppleTalk zone
$DE 2 bytes
$E0 4 bytes network info: is AppleTalk active, the selected network (say, LocalTalk or EtherTalk) access (see the Chooser and the Network control panel)
$E4 8 bytes latitude/longitude of the place the Mac is at (see the Map control panel)


The total power on hours is also in the xPRAM. It is a 2-byte value. That contains the total time the computer has been on in 5 minute intervals.

Supposedly there is also some creation stuff in their to that tells when the machine was made, but I can not make sense of it.