Hello, Guest the thread was called759 times and contains 4 replays

last post from mega65 at the

Illegal opcodes

  • Hello,

    Just looked at the new website, looks great and I don't say that often about websites that are "mobile superscrollers".

    The one point that raised my eyes blink was the "illegal opcodes" in C64 feature. Allthough Paul-Gardner Stephen already showed that he understands it right: C64 software can be fixed for compatibility and enhanced to take advantage of Mega65 features. Rewriting for C65-mode requires economically much more investment.

    Support for illegal opcodes raises the concern again wether the Mega65 would become "too compatible" with the Commodore 64 and run in the Commodore 128 trap that there is no advantage buying one because of lack of native software.

    So therefore (perhaps needless) I ask the question: Would illegal opcodes support mean that it is no longer possible to use the advanced CPU features in C64 mode?

    Daniël Mantione

  • My opinion is to give programmer choice by simple switch CPU to 6502 or 65CE02...
    Something like POKE1,64=1.02/3.54MHz or POKE1,65=48MHz...
    I'm real 128er and my opinion about C64 mode is that it was wrong market decision of Commodore - examples: VIC-IIe runs only at 1MHz, or at least overclocked 1,33MHz, downclocks Z80 to 2MHz...it's not in missing native software, it's in using it as C64. The same fate can get M65! Sorry C64 fans - I hate C64 for limiting C128 and currently looking same with M65.
    Commodore 64 is not best machine ever made, C64 is best selling machine ever...

  • So, we are keeping 4510 instructions for C65 mode, but we will have an option to tell the CPU to operate in 6502-only mode. This will probably be the default in C64 mode, unless the program asks for the CPU to be super-fast, in which case we will take that as informed consent for 4510 opcodes. However, it is a bit interesting, as the c64-mode kernal uses 4510 opcodes for the floppy drive interface. As a result the whole thing will be a bit fiddly for us to get just right, so that everything works as expected. We'll probably also have a hypervisor option that allows forcing the CPU to one mode or another, so that the end user will be able to control this behaviour completely, and M65-enabled programs will always be able to use the new opcodes.

  • Good! That should be sufficient to address the concern, if you can enable the enhanced CPU. I expect the market for the C65 is big enough to make it worthwhile to hack some enhanced functionality into existing titles, and fix incompatibilities, especially considering that most buyers will have coding skills. Developing native titles is a completely different story... might still happen, but only if the existing people who still develop for C64 get enthusiastic.

    I don't know if the C64 roms could switch the cpu mode themselves? You would have to modify them, but it would be more elegant than magic hardware behaviour to auto-switch cpu mode.

    By the way, in you Blog post of 19 April you mention you want push the CPU even further to 192MHz, but the website mentions 48 MHz as target CPU speed. Is 192MHz still on the roadmap? centre