Quote:
Originally Posted by Photon
Backwards compatibility is easy, forwards compatibility is harder. Now possibly made impossible by not supporting an instruction that has been in every 680x0 CPU! Please tell me they are reconsidering.  |
Some 68k instructions are baggage. They are poorly encoded (slows decoder for all instructions) or poorly implemented and don't work well in a superscaler environment. Not many programmers used them because they were too slow. The 68060 designers could see that it wasn't practical to support them in hardware. They trapped the less common ones for backward compatibility but this made them even slower. Reusing the encoding space is not the preferred choice but there isn't a lot of encoding space left without using A-line or F-line for integer instructions but doing that has it's down sides also. We are paying attention to which instructions were used on the Amiga and that's why MOVEP, TAS and most of the BCD instructions will most surely be supported. We are open to reconsidering removals if you can show us examples where CAS, CAS2, CMP2 and CHK2 are used in a legitimate way on the Amiga while being faster than using other instructions and being common enough that patching the executable to remove them would be a major problem. Note that these removals do not affect 68000 compatibility which, interestingly enough for this thread, should be excellent.
Quote:
Originally Posted by Photon
Also: betcha I can make CAS work fine on accelerated Amigas. Trapping for patches is ...*oof* fine I guess... *grumble* *suspicious look*
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I bet you can too but I bet you can't find an advantage to using it on the Amiga because of it's slow timing (on the 68040 and 68060 at least). How many instructions can the 68060 execute in:
CAS 19 cycles
TAS 17 cycles (in memory)
You can do the same faster with replacement code. If it was faster in any case on the Amiga then someone would have used it but it's not and that's why it's gone unused. CAS is useless without a working read-modify-write cycle that's slow on a modern processor.