Why a Computer Hangs on Checking the NVRAM

by Elizabeth Mott
Intel-based iMacs use NVRAM, whereas earlier Macs used PRAM.


Kevork Djansezian/Getty Images News/Getty Images

The RAM your Mac uses to support its OS and applications purges its contents every time you shut down your computer as well as when you unplug a desktop system or remove the main battery from a laptop. By its very definition, non-volatile RAM, or NVRAM, retains settings even when your system shuts down or loses power. NVRAM-related problems can increase your computer's startup time and even stop it from booting up correctly.

NVRAM Settings

When you press the power button and your Mac begins to boot, it reads basic settings from NVRAM before you hear the system chime and uses these settings to determine where to find its operating system files. NVRAM also tells your computer the screen resolution you use and how loud you've set your system audio volume. If you recently experienced a kernel panic -- an operating system malfunction typically caused by software problems -- NVRAM also stores information about it. A small battery cell connected to a Mac's motherboard supports the contents of NVRAM when you remove or lose electrical power.

NVRAM Problems

Your Mac's startup sequence takes about the same amount of time each time you press the power button, except after you install OS updates or software that makes changes to your operating system. If your boot time seems unusually long, with a pause at the beginning of the process, you may have problems related to the settings preserved in NVRAM. For example, if you formerly booted your computer from an external drive that you've since disconnected, your Mac can spend time looking for it before it falls back to an OS installation on its internal hard drive.


An NVRAM reset clears the contents of NVRAM so you can change its startup settings. To perform a reset, press and hold the keyboard combination "Command-Option-P-R" the moment your computer starts to boot and before its startup chime sounds. Release the keys after your Mac chimes twice. When you see the Finder desktop, open the Apple menu and choose "System Preferences" to reset your startup disk and screen resolution.

Other Considerations

A failing NVRAM backup battery can disable your Mac's ability to store and use NVRAM settings, necessitating a replacement, which typically requires technician assistance. Intel Macs also contain a System Management Controller, or SMC, that regulates power and video performance. You may mistake some of its malfunctions for problems related to NVRAM, including high fan speeds or slow system performance under light usage conditions, keyboard backlights or indicator lights that don't perform properly, failure to boot or wake from sleep, or applications that launch slowly or don't respond correctly. Resetting NVRAM won't address these conditions. How you reset the SMC (see Resources) depends on whether you use a desktop or portable Mac. For notebook computers, the procedures depend on whether your Mac uses a battery you can remove.

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