Low on ROM space

[rikardn]

Since we are getting to the limit of available ROM space we need to do something. I see that we have at least two different interests:

1. The possibility to customize the ROMs with various modern features
2. To be compatible with the original C64 ROMs

To be able to cater to both of these and to be able to fit as much as possible my suggestion is to do the following (and I think the first would take us the furthest):

1. Make BASIC optional

To be able to cater for both of these I think we should make the BASIC optional. I mean the parts of the BASIC that are needed to make BASIC programs, not the commands needed to operate the machine, like for example LOADand RUN. We could have more fine grained options in the future with including or excluding specific basic functions (for example excluding trigonometric functions), but I think splitting it in two pieces would be enough for now.

2. Try to fit the BASIC functions

In many cases we know how much space a BASIC or KERNAL function takes. We could document this and try to optimize the functions so that they fit. If we don't do this it might even become impossible to fit everything that the original ROMs contains.

3. Make fixed addresses optional

This might be harder to do, but it would be good to allow functions to not have to be put at a certain address. This will reduce compatibility, but more might fit, and it might not be needed for some machines. This would also be useful if we also want to support machines like the VIC-20, the Apple II etc. For this even different fixed addresses could be necessary for the same function. One solution to this would be to put the fixed address inside the file in the header and tie it to a certain configuration instead of having it in the file name.

Any comments or further ideas would be much appreciated!

[FeralChild64]

1. IMHO for now it would be best to disable the trigonometric functions - they are not referenced by anything within the ROM, and they don't even work due to division still being buggy. Another one is CHRGET, until it is being utilized by BASIC - as currently it duplicates internal mechanisms.

2. Yes - in my opinion the first part to look into is the string compression; it complicates the code and I'm not sure if it brings us any benefit overall. But we definitely don't want the BASIC commands to be encoded the same way as on the C64, that would induce quite a lot of binary similarity.

3. You can try, but I don't think this would bring much improvement.

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One other idea would be to accept that C64 won't get the full-featured ROM and focus more on the MEGA65, which has much more ROM space (AFAIK it got a hardware floating point support, this could be utilized to get rid of some ROM code). Long time ago there were some vague ideas to have extra ROM space on the Ultimate 64 (for modded original ROMs), but I'm not sure if this eventually got implemented, this would have to be checked.

[rikardn]

Thanks for interesting suggestions.

1. Ok. I could add a config option to turn off all transcendental functions in basic. This would then also include POLY1, POLY2, log and exp.

2. Ok. I might try reducing the size of some function and see if it is possible and what happens with the similarity.

3. Ok. I might try, but I feel this is low prio.

Finally on C64 vs MEGA65. I think it would be good if this project could continue to cater for multiple machines and configurations. That would allow for making it interesting to more people. My current interest is compatibility with C64 software and in the future potentially also other systems currently not supported like the VIC-20 and the C128.