User:Lee Cremeans/Parameter RAM

On Apple Macintosh computers, the parameter RAM or PRAM (pronounced "p-ram") is NVRAM space used by the Mac OS to store certain user preferences (set from the Control Panel), and also the current startup disk and date and time; it's analogous to the CMOS in PCs and the NVRAM in SPARCstations.

The PRAM is battery-backed, and resides on the same chip as the real-time clock. Only the first 20 bytes are defined by Inside Macintosh; the remaining space is known as extended PRAM (XPRAM), and is officially reserved by Apple for Mac OS's use (though it does contain useful information, such as the clock's UTC offset). The first Macs used a custom chip to hold the PRAM and system clock, which was accessed serially through the second VIA on the mainboard. Later Macs used a more advanced chip called CUDA, which also has power management and Apple Desktop Bus controls.

On PCI Power Macs, the amount of PRAM increased from 256 bytes to 8 KiB. This expanded PRAM, now referred to as just NVRAM (as on SPARC systems), contains partitions for the XPRAM, Open Firmware variables, and the Macintosh Name Registry (the Mac OS's working copy of the Open Firmware device tree).