A number of settings on a Mac are stored in Non-Volatile RAM, or NVRAM. NVRAM has a number of keys that contain values that define how the hardware is configured on a device that aren’t otherwise stored in the settings of an operating system.
You can list all of the variables available using the -p option, as follows:
Variables in the available key pairs are managed using the nvram command as well. Each setting, which we’ll provide a list of below, can then be edited provided you have elevated privileges to do so, without any options defined. Most values are true and false and will need to be supplied as %01 for true and %00 for false. As an example, the following command will turn on Location Services on a Mac:
nvram LocationServicesEnabled =%01
And the following will turn it back off:
Some settings won’t apply to certain models if the hardware doesn’t support that setting, but the following is a list of settings available on most models:
- act-results: The results of the last Apple Diagnostics or Apple Hardware Test.
- ALS_Data and Test_ALS_Data
- AutoBoot: Defines how the system boots (e.g. %00-%03)
- blacklight-level: Defines the level of the backlight (e.g. %d9%0a)
- bluetoothActiveControllerInfo: Shows information about the active Bluetooth Controller (e.g. %8f%82%ac%05%02%00%00%003%14dv%ba%b5%c3A)
- bluethoothInternalControllerInfo: Shows information about the internal Bluetooth Controller (e.g. %00%ac%05%00%00%00%00%00%00%8c%85%90@%a4k)
- boot-args: Provides for arguments for use when booting the kernel (or instructions sent to the kernel such as don’t bring any chicken, er, I mean don’t allow 32-bit software to be run or disable rootless (sudo nvram boot-args=“rootless=0″). Also includes:
- -pbmkp rtc=1
- -v: boots in verbose mode
- BootAudio: Indicates whether the startup chime will be played on models from 2017 or earlier (e.g. %00 or %01)
- csr-active-config: Shows the policy on third party kernel extensions (e.g. %ff)
- csr-data: Defines a policy on third party kernel extensions
- display-config: Resolution for supported displays.
- EFIBluetoothDelay: Time system waits for a wireless keyboard during boot (useful in data centers)
- efi-Apple-payload0, eft-apple-payload0-data, efi-apple-payload1, and efi-apple-payload2: UUID, EFI path, etc (e.g. <array><dict><key>IOMatch</key><dict><key>IOProviderClass</key><string>IOMedia</string><key>IOPropertyMatch</key><dict><key>UUID</key><string>7CD53879-001B-4AC9-8D3C-13768C6883B9</string></dict></dict><key>BLLastBSDName</key><string>disk0s1</string></dict><dict><key>IOEFIDevicePathType</key><string>MediaFilePath</string><key>Path</key><string>\EFI\APPLE\UPDATERS\MULTIUPDATER\ThorUtil.efi</string></dict></array>)
- efi-apple-recovery: Path to firmware information
- efi-backup-boot-device-data: path to a backup efi file
- efi-backup-boot-device-data-data: Same as above but encoded (I think).
- efi-boot-device: Path with efi but with last booted (BLLastBSDName)
- efi-boot-device-data: Just the path to the efi file with the UUID directory
- Eos-fdr-cache-uuid: The UUID, but not the useful one used elsewhere
- fmm-computer-name: The name given to the computer at boot time
- gpu-policy: Enables the GPU on some models of computers
- HW_BOOT_DATA: Hardware boot data
- LocationServicesEnabled: Enables (e.g. %01) or disables (e.g. %00) location services
- multiupdater-0: Encoded multi updater used in efi-apple-payload0
- previous-system-uuid: Previous uuid used, if available.
- prev-lang:kbd: The previously selected keyboard language (e.g. en-US:0, en:2, GB:2)
- security-mode: Used on some models to enable firmware passwords
- security-password: When security-mode is used, used to set the firmware password
- SmcFlasherResult (e.g. %00%00%00%00%00%00%00%00%00%00%00%00%00%00%00%00)
- SystemAudioVolume: How loud the audio volume is (e.g. 7)
- SystemAudioVolumeDB: Audio volume encoded %de which decodes to 222
- ThorUpdateResult: Thunderbolt updater (e.g. %00%00%00%00%00%00%00%00)
The above list is not complete as I’ve not managed to change all of these in a useful manner. If you know what one is that I didn’t define, let me know! If you get in over your head, never fear, the following article will allow you to clear the values back to defaults:
Cisco, NetApp, Qlogic, the former Xserve RAID and a number of other appliances I’ve used over the years also use nvram to store similar variables. I’ll try and write up the similarities and discrepancies at some point. And good luck if you’re mucking around with this stuff.