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My question is about bankswitching behaviour on the Commodore 64.

I'm writing a C64 game for a 16kb cartridge with EXROM and GAME pulled low, so the available bankswitching modes are (according to c64-wiki):

Bank Switching table summarised https://www.c64-wiki.com/wiki/Bank_Switching

The VIC-II display registers are memory-mapped into $D000-$D3FF. On boot up, CHAREN, HIRAM and LORAM will all be 1, so the system will start in mode 7, with I/O access enabled within that memory region.

Does CPU access to the VIC-II registers fall under the category of I/O?

If I switch to mode 4 to allow access to the 'full' ~64kB of RAM, will I still be able to access VIC-II registers from the CPU, or will I have to switch to mode 5 to put I/O back into the memory map?

(Assume my code has been copied to some location in RAM first (e.g. $6000) at that point so it won't be affected by any mode changes.)

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2 Answers 2

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Does CPU access to the VIC-II registers fall under the category of I/O?

Yes. To access the RAM below the I/O, you have to use a function copied to RAM which does exactly that. A common use for that area however is to put a custom character generator and/or sprites there, because in the $4000..$7fff and $c000..$ffff area, the VIC always reads from RAM.

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  • " in the $4000..$7fff [...] area, the VIC always reads from RAM." ? Err, what else should it read then RAM?
    – Raffzahn
    Commented Feb 11, 2018 at 12:48
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    In the $1000..$1fff area, the VIC reads out the CHAR ROM, regardless of the CHAREN/HIRAM setting. Because the VIC only decodes 14 address bits, one could assume it's the same for $5000..5fff and $d000..$dfff, but in the odd 16K pages, it's always RAM.
    – Janka
    Commented Feb 11, 2018 at 13:05
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    From CPU view, there's nothing but RAM in the $1000..$1fff either. But the PLA does make a distinction between CPU and VIC access. For the VIC, the RAM/ROM select depends on the CIA2 Bit 0 and A12/A13 only. So, for the CPU, the RAM at $d000..$dfff is hard to use, because it overlaps the I/O area. But for the VIC it doesn't matter so it's a good idea to put your RAM character generator/sprites there.
    – Janka
    Commented Feb 11, 2018 at 13:23
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    Feel free to downvote, edit, delete, whatever, my answer.
    – Janka
    Commented Feb 11, 2018 at 13:27
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    And I thought it was a good answer.
    – Janka
    Commented Feb 11, 2018 at 14:09
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Does CPU access to the VIC-II registers fall under the category of I/O?

Yes

If I switch to mode 4 to allow access to the 'full' ~64kB of RAM, will I still be able to access VIC-II registers from the CPU,

No, as you switched it off.

or will I have to switch to mode 5 to put I/O back into the memory map?

Exactly. Under on address(region) only one component can be reached at one time. So if you want to use the I/O, the 4 KiB at $Dxxx have to be switched to I/O (mode 5/6/7 of that table), and the RAM at this address will be invisible. If the RAM is accessible there (Mode 0/1/4), you won't see any registers.

Assume my code is running from $6000 at that point so it won't be affected by mode changes.

It's always a good idea not to switch of the active programm, isn't it?

BTW: $6000 is not part of any cardridge ROM.

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    > BTW: $6000 is not part of any cardridge ROM. - Yes, I know. :) If I want to access ram at $D000-DFFF, I will have to page the cartridge ROM out too: there's no mode where both the cartridge is active and the IO is switched out. So I'll have to copy some of my code to somewhere in RAM first.
    – E. Smith
    Commented Feb 10, 2018 at 18:39
  • @E.Smith Well, if space is such a premium with your application, it may be more apropriate to think about the programm structure than doing much fliping just for 4 KiB.
    – Raffzahn
    Commented Feb 10, 2018 at 18:52
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    You should use the RAM at $1000-$1ffff and $9000-$9fff for program code, because the VIC always reads the character generator ROM in these regions. In the ancient past, a cartridge would had it's own character generator at $9000-$9fff, but it's such a huge waste of space when it's not compressed on the cartridge not even Commodore did it that way in the later 4in1 cartridges.
    – Janka
    Commented Feb 10, 2018 at 18:52

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