motorollin wrote:
The only way I will buy a Dragon is if it is 100% backwards compatible with 68k software and runs at least at 060 speed. Otherwise it's a downgrade.
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moto
There's more to system performance than raw MIPs. One thing the Coldfire has going for it is a much faster memory interface.
Compatibility is the bigger problem. The coldfire doesn't just fail to implement some operations, it implements them differently to the 68K family. That's a headache for systems just trying to run 68K object code with the least amount of intervention.
Given that missing support includes fairly commonly used addressing modes (at least in amiga software), any kind of trap/emulate is going to be a major performance hinderance.
One way to solve 68K compatibility in the absence of a fast/compatible enough trap/emulate layer is probably to use a hotspot based JIT and just emulate all the 68K code all of the time. Transcription will be 1:1 for all the supported opcodes, the unsupported ones could be expanded in-line.
This might sound a bit odd (emulating the 68K) but the transcription layer itself would be pretty lightweight, a lot moreso than emulating a totally different class of CPU and therefore could be implemented pretty optimally.
