Mac 68K emulator

The Mac 68k emulator was a software emulator built into all versions of the classic Mac OS for PowerPC. This emulator permitted the running of applications and system code that were originally written for the 680x0 based Macintosh models. The emulator was completely seamless for users, and reasonably seamless for programmers. It is thought that this aspect of the Mac OS is the first time that such a dual hardware architecture operating system had ever been successfully implemented.

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 later version, using dynamic recompilation, 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 an even faster PowerPC 68k emulator.

All versions of this emulator emulated the "user" subset of the 68EC040 instruction set with a 68020/68030 exception 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 Motorola 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.

...
Wikipedia