Mac 68k emulator

The Mac 68k emulator is a emulator built into the ROMs of PowerPC-based Macintosh models. This emulator enabled running applications and system code that were originally written for the 680x0-based Macintosh models. With a few exceptions, notably Connectix's RAM Doubler, the emulator ran most classic Mac OS software with little noticeable impact other than lower performance relative to the same program when compiled for PowerPC.

Origins
The first version was written by Gary Davidian, who had originally created it for use on the Motorola 88000 CPU, used in Apple's abortive first attempt at a RISC target platform. A newer version, which used dynamic recompilation for improved performance, was developed by Eric Traut, who later worked on successful emulation projects at Connectix such as Virtual Game Station and Virtual PC. Prior to Traut's arrival there, Connectix had released Speed Doubler, which included its own faster PowerPC 68k emulator that also used dynamic recompilation.

Implementation
All versions of this emulator emulated the "user" subset of the Motorola 68EC040 with a 68020/68030  stack frame. Apple developer documents indicate that the emulator provided an operating environment most closely resembling that of the Macintosh Centris 610, a system based on the 68LC040 microprocessor. Early versions emulated it by decoding each instruction and immediately carrying out a series of equivalent PowerPC instructions. For the PCI PowerMacs, the dynamic recompilation emulator was used to boost performance. Dynamic recompilation works by "recompiling" common sections of the code into faster, PowerPC-native, sequences that were locally cached. The emulator could recognise the same sequence of 680x0 code and run the previously-cached PowerPC code to avoid doing the translation again. This emulator was theoretically capable of emulating 680x0 code faster than any real 680x0 was capable of running it. The 68LC040 had no floating point instructions, making this feat slightly simpler but no less impressive.

One reason that this emulation was so successful is that many of the APIs for the Mac OS were originally implemented as on the 680x0 processor; therefore, calling an API actually was recognised by the 680x0 as the equivalent of an error condition, which would cause it to handle that error through one of its hardware vectors. In turn, this vector would look up and run the operating system routine from ROM or RAM. In the emulator, such traps could be replaced by native PowerPC code, so the only code being emulated was the application itself, and any system API it called could be accelerated with native PowerPC code. This also allowed Apple time to port the OS to the PowerPC. At first, only time-critical aspects were rewritten in native code, leaving much of the OS emulated. Gradually most of the OS was rewritten to be native, so the OS got faster over time.

For the programmer, the transition to the PowerPC was made fairly painless, because the emulator was started and stopped automatically. This was achieved using a new type of called a Universal Procedure Pointer (UPP). For 68k code, this pointer appeared to be an ordinary pointer to code and could be used as such. However, it actually led to a data structure which contained a special trap instruction and flags indicating the of the called code. From PowerPC code, this UPP could be passed to the CallUniversalProc function to call it. The 68k emulator then dealt with details such as presenting passed parameters in the right order for the ISA in question, as well as starting and stopping the emulator as required. The compilers for Mac OS created such UPPs automatically when the proper were used, and the PowerPC system libraries contained native stubs to transparently call through to native or still-68k functions as needed. This meant that dealing with the dual architecture required very little work for the programmer, and just like the OS, applications themselves could mix and match 680x0 and PowerPC code fairly easily.

Current status
Because it was built into all PowerPC versions of the classic Mac OS, the emulator was also part of the Classic environment in Mac OS X. PowerPC Macintosh emulators such as SheepShaver therefore use the emulator as well when running the classic Mac OS. Native Mac OS X outside of Classic never used the emulator.