With the launch of Power Macintosh, the Mac 68K emulator is part of System 7.1.2 and later. This emulator uses PowerPC features and is embedded at the lowest levels of the operating system, integrated with the Mac OS nanokernel. This means that the nanokernel is able to intercept PowerPC interrupts, translate them to 68k interrupts (then doing a mixed mode switch, if necessary), and then execute 68k code to handle the interrupts. This allows 68k and PowerPC code to be interspersed within the same fat binary.
Apple launched Rosetta in 2006 upon the Mac transition to Intel processors from PowerPC. It was embedded in Mac OS X v10.4.4 "Tiger", the version that was released with the first Intel-based Macs, and allows many PowerPC applications to run on Intel-based Mac computers without modification. Rosetta is based on QuickTransit technology.[4] It has no user interface, and launches as needed with no notification of the user, which led Apple to describe it as "the most amazing software you'll never see".[5] Rosetta is optionally installable in Mac OS X v10.6 "Snow Leopard".[6] Rosetta is neither included nor supported in Mac OS X Lion (10.7) or later, which therefore cannot run PowerPC applications.[6]
Because of the greater architectural differences between Intel and PowerPC processors, Rosetta operates at a higher level than the 68000 emulator does, as a user-level program that can only intercept and emulate user-level code. It translates G3, G4, and AltiVec instructions, but not G5 instructions. Although most commercial software for PowerPC-based Macs was compatible with these requirements (G4 systems were still widely used at the time), any applications that relied on G5-specific instructions had to be modified by their developers to work on Rosetta-supported Intel-based Macs. Apple advised that applications with heavy user interaction but low computational needs (such as word processors) would be best suited to use with Rosetta, and applications with high computational needs (such as games, AutoCAD, or Photoshop) would not.[7]
Bundled Java applications or Java applications with JNI libraries that cannot be translated
Java applets in Rosetta-translated applications, meaning that a native Intel web browser application, rather than a legacy PowerPC version, must be used to load Java applets
In 2020, Apple announced Rosetta 2 would be bundled with macOS Big Sur, to aid in the Mac transition to Apple silicon. The software permits many applications compiled exclusively for execution on x86-64-based processors to be translated for execution on Apple silicon.[3][9]
To install Rosetta 2 on an Apple silicon Mac there are two ways to do it: either by using the Terminal to install the program directly, or by trying to open an application compiled for x86, which will open an installation window.
In addition to the just-in-time (JIT) translation support, Rosetta 2 offers ahead-of-time compilation (AOT), with the x86-64 code fully translated, just once, when an application without a universal binary is installed on an Apple silicon Mac.[10]
Rosetta 2's performance has been praised greatly.[11][12] In some benchmarks, x86-64-only programs performed better under Rosetta 2 on a Mac with an Apple M1 SOC than natively on a Mac with an Intel x86-64 processor. One of the key reasons why Rosetta 2 provides such a high level of translation efficiency is the support of x86-64 memory ordering in the Apple M1 SOC.[13] The SOC also has dedicated instructions for computing x86 flags.[14]
Although Rosetta 2 works for most software, some software doesn't work at all[15] or is reported to be "sluggish".[16]
Similar to the first version, Rosetta 2 does not normally require user intervention. When a user attempts to launch an x86-64-only application for the first time, macOS prompts them to install Rosetta 2 if it is not already available. Subsequent launches of x86-64 programs will execute via translation automatically. An option also exists to force a universal binary to run as x86-64 code through Rosetta 2, even on an ARM-based machine.[17]
Since macOS Ventura, users running virtual machines with Linux as a guest operating system can make use of Rosetta 2 to run x86-64 code compiled for Linux,[18] within the virtual machine. Rosetta 2 works as a runtime binary, which is required to be installed on the guest operating system. There have been instances[19] of developers installing this runtime binary on third-party hardware, provided that it includes a CPU that supports at least the ARMv8.2-A instruction set; the memory ordering will be different from native x86. Some developers have noted that it might violate macOS's licensing agreements, since the runtime is bundled[20] within Apple's Virtualization framework.
^"Rosetta"(PDF). Universal Binary Programming Guidelines, Second Edition. Apple. Archived from the original(PDF) on August 3, 2012. Retrieved September 5, 2011.
^"What Can Be Translated?"(PDF). Universal Binary Programming Guidelines, Second Edition. Apple. Archived from the original(PDF) on August 3, 2012. Retrieved September 5, 2011.
^WWDC2020 Keynote. Apple Inc. June 22, 2020. Event occurs at 1h39m37s. It translates the apps when you install them, so they can launch immediately and can be instantly responsive. Rosetta 2 can also translate code on the fly when needed.
