How does the Commodore 64 address more than 64 kilobytes of memory?

Question

So, the Commodore 64 has two special registers in locations $00 and $01. By writing to these registers, you can somehow turn on and off the ROMs and other things. The thing that I am not understanding is why there is a direction register at location $00.

So if you write a 1 or 0 to the first bit in location $01, somehow this controls whether you can read from Commodore BASIC ROM or from RAM at the same addresses (in the literature this is apparently known as the LORAM signal). And I'm interested in knowing what happens if you write a particular value to the same bit in location $00. Does that mean that LORAM is now an input? Can you use this to test if BASIC is on?

Answer 1

The 6510 CPU used in the Commodore 64 has an additional built-in general purpose I/O port compared to the original 6502 CPU. Address $0000 controls the direction for each of the bits of this I/O port, address $0001 can be used to read the voltage level of the corresponding pin for inputs, or set the voltage level of this pin for output.

In the Commodore 64, the pins for the lowest three bits of this port are connected to a PLA, where they control the address space layout. All three lines are also pulled up to +5V via resistors. So if you'd change the direction of any of these port bits from output to input, the line would just be held at +5V, you could read a 1 in the corresponding position at address $0001, and the PLA would behave like a 1 is output to it.

Therefore this setting serves no purpose, it's the same as if you had kept the direction to output and wrote a 1 into the data register. In particular it can't be used to test anything.

(I'll assume that this is your real question, and you are not asking about details how the Commodore 64 uses those 3 signals to control the address space layout, i.e. ROM/RAM. If you need those details, there's a table e.g. here.)

Answer 2

The C64 BASIC ROM lives at $A000, the KERNEL ROM lives at $E000, but these ROMs also have RAM wired in 'parallel' Under normal operation this RAM is unused but you can use the LORAM/HIRAM bits to enable or disable the ROMS and switch in the equivalent RAM memory.

One fun trick is to write a quick basic program to PEEK all the addresses from $A000 to $FFFF and POKE them back into the same address. You can't write to ROM, so what the 64 will do is basically copy the PEEKed value from ROM and put it into the same address in RAM. You then flip the LORAM/HIRAM bits to 'turn off the ROM' and use the RAM exclusively and you have a C64 with 'modifiable' roms. You can add new commands to the BASIC interpreter, or tweak your ROMs. But the copy in RAM is volatile and will go away when the power gets turned off.

You can use the LORAM bit to determine if the C64 is reading addresses $A000-$BFFF from ROM or from RAM, obviously if it reports ROM then BASIC is available.

Many programs take over the entire C64 by turning off the ROMS and using the available RAM for their own program code (or resources).

Dig up a copy of Mapping the C64 by Compute! for EVERYTHING you ever wanted to know about the C64 memory map.