Apple on Wednesday released macOS High Sierra 10.13.4 for developer testing.

The fourth major software update since High Sierra’s initial release in September 2017 came with a build number of 17E139j and no new outward-facing features or enhancements.

However, Apple did mention one major addition in release notes accompanying the download: a brand new warning when a 32-bit app is launched, not unlike the similar dialog in iOS 11.

The Cupertino giant informed customers and developers last summer that High Sierra would be its last release to support 32-bit apps “without compromise.” All Mac app updates and existing apps must support 64-bit starting June 2018, the company clarified.

Here’s an excerpt from the macOS High Sierra 10.13.4 beta changelog:

To prepare for a future release of macOS in which 32-bit software will no longer run without compromise, starting in macOS High Sierra 10.13.4 a user is notified on the launch of an app that depends on 32-bit software.

The alert appears only once per app.

To make it easier for developers to test their software for 64-bit compatibility, macOS 10.13.4 provides a new 64-bit testing mode enabled through the following Terminal command:

sudo nvram boot-args="-no32exec"

After executing the command, you must restart your Mac.

This new 64-bit test mode prevents any 32-bit processes from launching.

Opening a 32-bit app or a 64-bit app that depends on 32-bit frameworks produces a notification saying the app cannot be opened. You’re strongly discouraged from enabling this mode unless you’re an experienced software developer or IT administrator.

Other types of software may fail silently in this mode, such as 32-bit versions of Dashboard and WebKit plugins, preference panes and background processes.

To disable this mode, execute the following command in Terminal, then restart your Mac:

sudo nvram boot-args=“”

“In future beta releases, 64-bit test mode may provide additional information for the developer to help test and qualify software,” notes Apple.

“If you distribute your apps outside Mac App Store, we highly recommend distributing 64-bit binaries to make sure your users can continue to run your apps on future versions of macOS,” the Cupertino company told developers back in summer 2017.