6

You can bank out Kernal ROM in C64 using $01 register. This was preferred by game/demo coders who wants to employ self tuned routines and wants to expose ram 'underneath'.

You can do lda #$35 : sta $01 and immediately have 16k additional memory. This was a special feature of 6510 CPU. The question is: Is it possible to disable Kernal in similar manner in Commodore PET, VIC-20 or 264 Series (e.g. Plus/4, 116 or 16) Computers?

  • 2
    Why would you do that when there is no RAM underneath? – NieDzejkob Jun 18 '18 at 16:16
  • @NieDzejkob: For software to be able to do so wouldn't be very useful, but an ability to do so in hardware might be helpful for some purposes [e.g. being able to put something other than Kernel ROM at $FFFE would make it possible to speed up IRQ response time]. – supercat Jun 18 '18 at 21:19
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    Editors and reviewers: please leave “kernal” alone, it’s the name of the Commodore ROM (and yes, it irritates). – Stephen Kitt Jun 19 '18 at 9:16
  • @StephenKitt "Kernel" is not wrong though, especially when talking about the PET. – JeremyP Jun 19 '18 at 10:36
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    I prefer to keep it as Kernal as this is the original name given by Commodore. – wizofwor Jun 19 '18 at 11:47
10

You cannot do that on the VIC-20; not only is no such feature provided by the built-in hardware but there's also no ROMDIS signal on the memory expansion bus or anything else similar.

PETs with a 64kb upgrade have a register at $FFF0 for memory selection that allows the ROMs to be paged out (see e.g. "2.2 Control Register" in this document — the 8096 is an official PET with 96kb of RAM that utilises that paging scheme).

The TED machines (e.g. Plus/4, 116, or 16) permit selection of ROM or RAM at $FF13 (for more details see e.g. this summary of the TED chip, particularly the register map at the bottom).

  • I didn't know PETs had a 64kb upgrade. Guess they came out after I stopped using them. – cup Jun 18 '18 at 20:38
  • @cup 6502.org/users/andre/petindex/local/pet-io-2.txt was my source for that claim; I'll edit it in. Ditto I'll look for something for the TED assertion. I'm not sure how I would as easily prove the VIC didn't have something though. – Tommy Jun 18 '18 at 21:06
  • @Tommy: The BLK7 signal that enables the ROM is generated by UC5 based upon A13-A15 and the phi2 clock. – supercat Jun 18 '18 at 21:30
1

While you can't do this on the VIC-20 at all, nothing is stopping you from programming your own ROM. It would not be difficult to create a ROM-switching socket or adapter of some sort, à la JiffyDOS, which does exactly this trick to let you switch from the custom high-speed-disk-I/O ROM to and from the standard ROM. This trick would, of course, work on the other systems too if you did it correctly.

  • That's after removing the built-in ROM though, right? If so, any idea whether they're usually socketed? – Tommy Jun 18 '18 at 23:27
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    @Tommy It depends on the generation of the machine - older machines have higher probabilities. I think, in general, that it's pretty common. It's also not the end of the world to remove the ROM chip and install a socket, if you need. – Jim MacKenzie Jun 19 '18 at 3:16

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