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Has Apple sounded the final POST? – The Eclectic Gentle Firm

Has Apple sounded the final POST? – The Eclectic Gentle Firm

2023-05-12 07:06:14

Energy-on self-tests (POST) are extensively utilized in electronics, and one of many oldest options of non-public computer systems. Each mannequin of Mac previously has had its personal POST routines, some which have turn out to be well-known due to the sounds that end result, or what’s displayed, from the sight of a Unhappy Mac to the sound of a automobile crash. So what occurs when an Apple silicon Mac fails its POST? Does it even run them?

One catch right here is recalling that POST routines will not be run for a restart, as they usually want a ‘chilly’ begin from the Mac being shut down. The primary place to look is the place Macs usually report the outcomes of their final POST, within the Diagnostics merchandise of System Data. Whereas that does report the outcomes of the final Diagnostics take a look at run (if any), for Apple silicon Macs there’s no point out of any POST, as there may be on Intel Macs, even these with a T2 chip.


These are usually retrieved from NVRAM, however so far as I can see, Apple silicon Macs don’t have wherever of their NVRAM the place they may retailer the results of a POST.

Perhaps Apple silicon Macs do nonetheless run POST, however haven’t but discovered a great way to report it? To find whether or not that’s possible, you could evaluate what occurs of their boot sequences.

With two separate processors in every T2 Mac, there are two separate units of firmware, one EFI and the opposite generally known as iBridge or BridgeOS.


As soon as a T2 Mac has carried out its POST and initialised the SMC, the T2 sub-system establishes the extent of Safe Boot in drive, and, if that’s Full or Medium Safety, boot.efi is checked earlier than being loaded, and that leads by way of to the remainder of the boot course of.

In distinction, boot safety in Apple silicon Macs goals to offer a verified chain of belief by way of every step within the boot course of to the loading of macOS, that may’t be exploited by malicious parts. Booting an M-series Mac thus begins with the immutable Boot ROM within the {hardware}, whose most vital job is to confirm the executable for the subsequent stage, then load and run it. If that isn’t potential, then the fallback is to enter DFU mode and await a connection over USB.


As with Intel Macs, there’s no accessible file within the log of what has occurred in the course of the preliminary part of the boot course of, as log information solely start with the kernel. Previous to which can be ‘breadcrumbs’ which can be solely intelligible to Apple’s engineers. Within the occasion of early boot failure, the one recourse appears to be to desert the method, and depart the Mac in DFU mode. At that stage, the choice could be to attempt to begin up in Restoration mode, probably flashing the ability mild offered on some fashions.

A lot of the plethora of units throughout the SoC don’t load their firmware and begin operating till after the kernel has began loading. Till then, all code has been run on a single core too, and the opposite cores aren’t run up till after the kernel is busy initialising the Mac. So a POST run from the Boot ROM wouldn’t be capable of take a look at a lot of the chip, and wouldn’t have a lot {hardware} for which to point any issues it would discover.

Intel-based Mac fashions from earlier than October 2016 sign the end result of their POST part utilizing sound. A traditional startup chime signifies that reminiscence (RAM) and ROM seem sound and adequate for the boot course of to proceed. The next sounds are emitted when booting is deserted or paused:

  • if no RAM is discovered, a single tone repeated each 5 seconds;
  • if RAM is discovered however fails POST, three tones adopted by a 5 second pause, repeating
  • if the EFI firmware discovered is corrupt and is being recovered, three lengthy, three quick, and three lengthy tones (—…—, the inverse of Morse SOS)
  • if an EFI firmware replace is in progress on a Mac mannequin previous to 2012, one lengthy tone whenever you maintain the Energy button.

Current MacBook Professional and another fashions which have mute boots neither emit sounds nor flash their shows in the event that they encounter issues throughout their POST. If they’ll’t discover a legitimate EFI firmware supply, they need to mechanically recuperate the EFI firmware, however with none sounds to offer a clue that’s what is going down; as a substitute, they show a white-on-black progress bar when making an attempt to recuperate their firmware.

See Also

Apple silicon Macs are completely different of their {hardware}, in that a lot of it’s built-in into the only M-series chip, and reminiscence is constructed into the chip provider. Outdated treatments for POST failure equivalent to shutting down and reseating reminiscence modules merely don’t apply.

So there are not any accessible information of any POST going down on Apple silicon Macs, no different proof that they do, and even when they did it’s onerous to see what they may obtain. It does appear to be Apple has sounded the final POST, except you already know any higher.

Additional studying

Mac startup process on Wikipedia

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