zp, zp+1 would contain a pointer to the base of a list of pointers again 2 bytes each, that's why 2X, to address up to 256 of them. This indexed pointer is taken and postindexed by Y.

edit(1):
I was thinking of 8 bit index registers and 8 bit data bus width. Just plain 6502

GaBuZoMeu:
I certainly agree the factor of two is warranted if X and Y are both 8-bit index registers. When X or Y is a 16-bit register, then the addressing mode is better thought of as base + offset. The base is in the register and the offset is in the instruction. I assume from the answer above, if X is a 16-bit register, the factor of 2 is not required on the inner post-indexed (by X) indirect read.

I realized that extending the X and Y registers to 16-bits changed the effect of the index registers in the indexed addressing modes of the 6502/65C02. For example, zp,X, (zp,X), abs,X, and (abs,X) could be used to support base pointer relative addressing of variables on the stack. If X is assigned the value of the stack pointer immediately after entry into a subroutine, it functions as a base pointer providing easy access to the parameters passed to the subroutine with positive offsets, and the local variables of the subroutine with negative offsets. These offsets are fixed and easily calculated by a compiler or assembler. With a 16-bit index register, the pre-indexed (by X) addressing modes of the 6502/65C02 become quite powerful, and I expect, require significantly less programming effort to manage split stack push and pop operations as discussed by 6502/65C02 FORTH programmers to improve performance. Even the post-indexed by Y addressing modes are affected positively by extending the Y register to 16 bits.

For stack pointer relative addressing, the X and S registers (S is also extended to 16-bits) are swapped using a prefix instruction in my extended 65C02-compatible core. S could be copied to X using TSX, and Y used as the base pointer, but given the dynamic nature of expression evaluation stacks for which built-in stack pointer relative addressing is almost required, I felt that it would be more natural to swap the S and X registers using a prefix instruction. (Note: Without the siz prefix instruction TSX only transfers the least significant 8-bits of S to X. For transparent compatibility with 6502/65C02 code, an 8-bit transfer from X to S using TXS automatically sets the system stack to page 1 where it normally resides. When the system stack is in page 1, the stack size is automatically limited to 256 bytes. If S is set outside of page 1 (or page 0) using a 16-bit transfer, the stack is limited only by available memory.)

The 6502 overcomes the limitations of its 8-bit index registers by providing more powerful addressing modes. The 16-bit index register of the 6800 processor provides that processor some advantage over the 6502/65C02, but having a limited number of addressing modes limits the utility of its 16-bit index register. The 6809 with two 16-bit index registers is a tough competitor. As Brad Rodriguez, Camel FORTH developer, demonstrated in his "Starting FORTH" series of articles. the 16-bit index registers of the 6809 enable a very efficient FORTH VM.

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Michael A.

I merely took the TO's question and add this double indirect post indexed variant. Others (like stack relative) were already mentioned.

I did not consider Forth nor 16 bit index registers at all.

16 bit index registers would impact the addressing modes a lot if 16 bit would cover the full address range - otherwise not.

It struck me that an addressing mode that I'd've loved to have on the 6502 is "from stack". Quite often you have a value on the stack, and perhaps you want to add it to something. Currently you have to make sure that the 'something' is in memory so you can write:

But how much nicer it would be if you already had that 'something' in the accumulator, and you could just write:

I guess it could have been a 4 cycle addressing mode, just like pull. The ALU operation at the end would be performed at the same time as the next opcode fetch.

The 65816 has stack-relative addressing modes which do exactly that. It can be done on the '02 as well, although with some limitations. The way to do it is that at the beginning of the section of code, you do a TSX, then use indexed addressing like ADC 101,X. You don't have to stop at just the byte at the top of the stack. You can operate on other ones in the stack, without pulling off the later-added bytes in order to get to what you want to access. Just increase the base number; for example,

The subroutine below multiplies two unsigned 16-bit numbers and produces a 32-bit result, without using any variables, only the page-1 hardware stack. It takes Bruce Clark's improvement on my commented bug fix on the UM* multiplication in fig-Forth, and it modifies it for I/O on the hardware stack. The structures are per my structure-macro article and source code linked there. They assemble exactly the same thing you would by hand, but make the conditions and branches in the source code clearer.

If you want to preserve X, you can do the usual PHX, PLX.

This and lots more about stacks (plural, not just the page-1 hardware stack) is in my 6502 stacks treatise, at http://wilsonminesco.com/stacks/ .

_________________
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Indeed, quite right Garth, though it would be nice to be able to do it without having to sacrifice X in the process - we've only got three registers to start with!

Just from my own programming of late, I find it quite irritating that the STX and STY instructions are so limited in addressing modes - Absolute, Zero Page and ZP,X. Of those three the only one that isn't fixed at compile time is ZP,X and it still restricts you to ZP only.

This is particularly strange given that the matching LDX and LDY instructions have a,y and a,x respectively which make it very easy to use them for virtual stack manipulation loads, but the corresponding store has to go through A. Even STZ, which isn't actually all that useful, has a,x mode!

Actual 65C02 code:


Simplification if STY a,y existed:


Three instructions shorter, doesn't disturb A at all and, in my opinion, easier to understand what's going on. idk it just seems like an obvious oversight, and its not as though the 65C02 is lacking in spare opcodes.

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Yes, I've often thought it would be nice to have a second X register (or a second hardware stack with its automatically incrementing and decrementing pointer, or [...], kind of like playing chess and wishing your pieces could move in directions they're not allowed to). Again though, the '816 largely takes care of that with its stack-relative addressing modes.

As mentioned earlier, Forth uses a data stack in ZP and uses X as its pointer. In my '02 ITC Forth, I very seldom have to save and restore X. It's almost exclusively for doing the looping controls. In my '816 Forth, the only thing I save X for is to synthesize the non-existent LDA (X),Y instruction for the ROLL and PICK instructions. (I have not searched all my INCLude files to see if there's any other I'm forgetting.) The things that use X so much in assembly language tend to use the data stack in Forth, which is one reason why the sometimes-ridiculed (ZP,X) addressing mode is valuable. It's like you're giving X a different job, rather than an additional job. I have carried this over into assembly language a little bit, and will probably do it a lot more in the future.

When I'm beating my head against a wall trying for force a certain solution, I usually find that it means I'm looking for the wrong solution, or need to back up and take a different approach to solve the problem or see if maybe I'm misunderstanding the problem itself. I've never needed to push Y onto a virtual stack. Would I, if I had to solve the same problems you were when you came to that? Maybe. I don't know. BDD did say above, "However, I can't recall any time recently when I wished I had STX <addr>,Y and STY <addr>,X available to me."

I like Ed's observation: "With 6502, I suspect more than one beginner has wondered why they can't do arithmetic or logic operations on X or Y, or struggled to remember which addressing modes use which of the two. And then the intermediate 6502 programmer will be loading and saving X and Y while the expert always seems to have the right values already in place."

BTW, I do use STZ a lot, probably mostly for clearing flag variables.

There's lots of interesting discussion here. Thanks.

_________________
http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?