How do you relocate the Zero Page on a 65816

Question

Background information

As a modern days PHP developer with a passing interest in 8-bit technologies, I'm a little sketchy on real low level stuff. Although I've worked with the Z80 processor, and done some MIPs assembly at University, I'm least well versed in the 65x processor family.

What I'm trying to achieve

What I want to do is to relocate the zero page to 0xd300 for my application - I've had a obligatory Internet search but am not able to find a good example that I am able to understand. I'm working in 65c02 but on a 65816 processor; I have found out, thanks to a friend, that you can only relocate the zero page in native mode, not emulation mode as it's not supported by the 65c02. This isn't an issue as I can switch to native mode for what I'm doing and revert to emulation mode should I want to return gracefully to BASIC (well, not only that I know).

If someone could also provide an example for the 8502 (Commodore C128) as well please as I know that has a relocatable zero page. I know that's two questions in one, but it's related to what I want to do.

Answer 1

The Direct Page register D is a 16-bit register specifying which 256-byte page within bank 0 that should be the current Direct Page (what you call Zero Page).

There are a couple of different instructions you can use to write to D. So e.g. something like this should work:

rep #$20  ; 16-bit accumulator
lda #$100
tcd       ; transfer 16-bit accumulator to D

Note that the page is allowed to start on any byte boundary. So you could e.g. set D=2 and have a direct page at [0002, 0101].