Firmware engineering must be among the least glamorous of the specialities, but it’s literally among the most fundamental. As regular users seldom need to come into contact with firmware and its versions, how those are numbered is kept as a dark secret. This article reveals what I have gathered.

Although you will come across different usage elsewhere, I here draw the distinction between:

• True ROM, including Macintosh ROM (Classic Macs) and Apple silicon Boot ROM, is set in the factory and normally can’t be changed;
• Firmware that can be updated in place includes Mac OS ROM (PowerPC), EFI Boot ROM or bootloader, and LLB/iBoot (Apple silicon);
• The operating system including any kernel, in Mac OS, Mac OS X, OS X and macOS, which are usually updated repeatedly.

PowerPC: Open Firmware

Classic Macs using Motorola 68K processors have their own Macintosh ROM with opaque hexadecimal versions. After the introduction of PowerPC models in 1994, that changed with the Power Macintosh 9500 of 1995 to support Apple’s version of Open Firmware.

Their ROM and firmware version numbering is elaborate, with a ROM revision, here shown as $77D.45F6, a Boot ROM version of $0004.25f1, and a Mac OS ROM file version of 8.4, for this Power Mac G4 running Mac OS 9.2.1 in 2002. Of those, Mac OS ROM file updates were provided as needed, and appear to have followed a plain major and minor numbering system.

Intel: EFI

When Apple transitioned from PowerPC to the Intel architecture in 2006, Open Firmware was replaced by the Extensible Firmware Interface (EFI), already on its way to become Unified as UEFI. Each Intel model thus has an EFI version number, from 1.0 upwards, and a firmware version number such as IM41.0055.B08, that of the early 2006 iMac.

Back in late 2017, this iMac17,1 was reported as running Boot ROM version IM171.0105.B26.

This consists of the following elements:

1. A model type given as 2-3 letters, such as IM for iMac, MM for Mac mini, MBP for MacBook Pro, or XS for Xserve.
2. An individual model within that type given as 2-3 decimal digits, here 41 for the two iMac variants introduced in early 2006, while the mid-2006 iMac is 42, and the late-2006 iMac is 51 or 52. Those are commonly combined in a more accessible model identifier, such as iMac4,1 or iMac17,1.
3. A major version number given as four hexadecimal digits.
4. A minor version number given as three hexadecimal digits. With High Sierra, Apple ceased using the minor version number, leaving it set to either B00 or 00B.

Just for fun, many models also have a completely different SMC firmware version that was updated separately.

These were replaced in October 2018, in Macs that have been updated to macOS 10.14.1, High Sierra Security Update 2018-002, or Sierra 2018-005. Those updated EFI firmware to use a different version numbering system, with five decimal numbers separated by dots, such as 1037.140.50.0.0. Those differ between individual model types, making conversion from firmware version to model identifier impossible. The only way to determine whether any given firmware version is the most recent for a given model is thus to use a lookup table.

Intel: T2

Apple introduced the first MacBook Pro models with T1 chips in 2016-2017, but it wasn’t until the T2 models at the end of 2017 that there was any change in the format of firmware version numbers. Therefore MacBook Pro models with T1 chips continue to report just a plain EFI firmware version.

Intel Macs with T2 chips report two complete firmware versions, that for their Intel side as an EFI firmware version, and an iBridge or BridgeOS version for the firmware installed in the T2 chip. For example, 1037.147.1.0.0 (iBridge: 17.16.16065.0.0,0) gives 1037.147.1.0.0 as its new-style EFI firmware, and 17.16.16065.0.0,0 as that for the T2. However, in return for that added complexity, all Intel Macs with T2 chips give the same firmware versions when running the same version of macOS, so there’s no need for a model-based lookup table.

At this stage, Intel Macs can have one of three firmware versions:

• old-style EFI firmware, such as IM171.0105.B26, when they haven’t had a firmware update from October 2018 or later;
• new-style EFI firmware, such as 1037.140.50.0.0, when their firmware dates from October 2018 or later, and they don’t have a T2 chip;
• T2 models with EFI and iBridge firmware, such as 1037.147.1.0.0 (iBridge: 17.16.16065.0.0,0).

Apple silicon

Strictly speaking, Apple silicon Macs have two distinct firmware version numbers, one for the Low-Level Bootloader (LLB, stage one) and the other for iBoot (stage two), but I can’t recall ever seeing the two differ in any release version of macOS. Both bring the sanity of a standard numbering system consisting of three numbers [major].[minor].[patch], and so far all models have remained in synchrony with the same numbers for each release of macOS since Big Sur 11.

Version numbers are composed of:

1. A major version number of 4-5 decimal digits set by the current major macOS. These have been 67xx (macOS 11), 74xx (12), 84xx (13), 10151 (14), 11881 (15), and for macOS 26 it’s currently 13822.
2. A minor version number in decimal that increments for each minor version of macOS. Although this varies between major versions, a recent sequence might run: 1 (macOS x.0), 41 (x.1), 61 (x.2), 81 (x.3), 101 (x.4), 121 (x.5) and 141 (x.6, if it precedes the next major version of macOS).
3. A patch version number in decimal that usually varies from 1-10, and has even reached 96.

For example, 13822.81.10 is installed in macOS 26.3, 15.7.4 and 14.8.4, which were released simultaneously.

Most recently, version numbers reported by beta-test releases of 26.4 have been completely different, of the form mBoot-18000.100.10.0.1. It’s not yet known whether this is a bug, or the harbinger of yet another change in firmware version numbering.

What firmware engineers might lack in glamour they seem to compensate for by turning what should be simple and consistent into great complexity.

Further reading

A brief history of Mac firmware
Pre-2018 Intel firmware version numbering