![]() AutoBoot – an integer determining boot behaviour when opening the case.BootAudio – a boolean indicating whether the startup chime is to be played.The 2016 MacBook Pro adds at least two more: aht-results – details of the last run of Apple Hardware Test.SystemAudioVolume – set this to %01 in other Macs to silence their startup chime.EFIBluetoothDelay – time allowed for a wireless keyboard to connect during startup.The NVRAM in my iMac17,1 contains settings for the following variables: ![]() I give two different sets of options, as Macs will display one in XML format, and the other may be less intelligible models vary as to how they behave, so you may need to try both. ![]() Which simply displays a list of variable names, and their current values. You can inspect the current data stored in your Mac’s NVRAM using the Terminal command Before you try altering any settings in your NVRAM, ensure that you know how to reset it, in case you cause a problem. If your NVRAM settings get messed up, you will need to reset the NVRAM to ‘factory’ defaults, a process detailed here. This stores the third-party kernel extension (KEXT) policy setting, which has only been introduced with High Sierra. All Macs which are upgraded to High Sierra undergo firmware update as part of the upgrade, to add another variable to their NVRAM. NVRAM varies between models, and can change with firmware updates. Among its most enduring have been audio volume settings as Mac hardware has become progressively more complex, new settings have been added, which determine the time allowed for a wireless keyboard to connect during startup, for example, and set whether a MacBook Pro should chime on startup. It contains a few settings which are used from early on during the startup process, so cannot be read from disk. NVRAM was originally much smaller, and called Parameter RAM (PRAM). Both involve changing settings in non-volatile RAM (NVRAM) – and that is what this article is about. Two tricks discovered by Dan at could customise the MacBook Pro to emit a startup chime, and stop it from starting up whenever you opened its case. Then came the 2016 MacBook Pros, which needed to have a setting changed before they gained a voice, and now the 2017 iMacs, which are forever mute. The rather tinny sound emitted by early models, through their small built-in speakers, had evolved into a sonorous chord, often heard through a hefty audio system. Until the launch of the MacBook Pro in the autumn of 2016, every Mac had chimed when it started up.
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