To quote from the "NATIVE CPU" section of the Basilisk II TECH documentation:
Code:
d) 68060 systems are a small problem because there is no Mac that ever
used the 68060 and MacOS doesn't know about this processor. Basilisk II
reports the 68060 as being a 68040 to the MacOS and patches some places
where MacOS makes use of certain 68040-specific features such as the
FPU state frame layout or the PTEST instruction. Also, Basilisk II
requires that all of the Motorola support software for the 68060 to
emulate missing FPU and integer instructions and addressing modes is
provided by the host operating system (this also applies to the 68040).The salient point in all of this, which gets reiterated every time someone makes a new thread about this, is that unlike x86 CPUs, in which newer CPUs with the exception of a few model-specific hardware control instructions are essentially always superset implementations of earlier models, Motorola's CPUs essentially peaked in complexity with the three-part MC68020+68851PMMU+MC68881/2 FPU chipset and that all subsequent models have actually *trimmed down* bits of the basic instruction set (particularly supervisor-mode and FPU instructions) and replaced the missing pieces with exceptions which jump to a software support bundle which then emulates the lacking function. Thus for all practical purposes you can't go beyond the 68040 without addressing this *software issue*. The idea of trying to trap the incompatibilities with some sort of hardware FPGA in front of the CPU is pretty laughable... it would require an instruction decoder sophisticated enough that at that point you might as well just create a whole CPU which is fully 68040 compatible. (But faster obviously, which would be a hugely challenging "garage project".)
The progress the ST people have had with their Atari Coldfire project indicates it *just might* be feasible to use a software library and some patching to run Mac II/Quadra ROM code and binaries on a newer CPU, but ironically it'd probably be much easier to do this in the form of a completely new-build "Coldfire Mac" board leveraging Basilisk II-style ROM patches for *all* the peripherals than it would be to do it in the form of an accelerator card for a real Mac. Such a machine equipped with the fastest Coldfire CPUs available might run 68k code about as fast as a mid-level G3 machine. (Of course, the G3 will completely crush the Coldfire running programs that have PowerPC binaries available.) If someone is really dying to embark on a multi-year project, well... it's an idea.