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A 6502-compatible 16/32 bit core - an idea
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Author:  enso [ Thu Sep 19, 2013 12:39 am ]
Post subject:  A 6502-compatible 16/32 bit core - an idea
I had a thought... It seems feasible to create a 16- or 32- bit core that is closely compatible with the 6502 instruction stream. Let's consider a 16-bit variant for this discussion.

The registers are 16-bit wide. Instructions are 16 bits; if the upper 8 bits are 0, the instruction set is directly compatible with the 6502 (or the 65C02 perhaps). 8-bit 6502 code is loaded (by a loader) a byte at a time and stored a word at a time (zero-extend instructions and sign-extend data). 8 and 16-bit instructions can be freely mixed.

The high byte is used to extend the instruction set to 16 bits. In 6502 mode (the mode is on per-instruction basis anyway) the carry flag is really bit 8 and the zero flag is for the low 8 bits of the operation. In 16-bit mode (high byte != 0), the instruction set is fully 16 bits and the flags are adjusted accordingly (or we can add 'extended' c and z flags).

Addresses can be 32 bits using the extra 2 bytes.

I kind of like this arrangement, although I am sure there are some situations where it might cause issues. It seems too obvious to be novel - has there been a previous discussion? I don't think I am describing 65Org16 - I am thinking of compatibility first. The low 64K words (or perhaps low 32K and high 32K if addresses are sign-extended) should closely mimic the 6502 space, and 6502 code (properly expanded) should just run.
Author:  enso [ Thu Sep 19, 2013 3:51 am ]
Post subject:  Re: A 6502-compatible 16/32 bit core - an idea
Thinking a bit more...

The instruction set space is basically constrained to the 6502/65C02. The high byte can generally modify the instruction to select from a set of accumulators and index registers, where that makes sense.

Accumulator 0 may be fixed as an 8-bit accumulator (and encoded as 0); selecting another accumulator makes it a 16 bit instruction... Same with index registers.

Zero page instructions can address 64K.

Branches can go +/-32K.

For shifts, the upper byte may be used for multi-byte shifts.

Unused instruction slots may be used for more interesting instructions such as multiply, etc.
Author:  BigEd [ Thu Sep 19, 2013 5:49 am ]
Post subject:  Re: A 6502-compatible 16/32 bit core - an idea
It is indeed a little different from 65Org16. That machine is word-addressed, and so any read or write is always 16 bits. Sounds like your machine would be byte-addressed. Your data bus might be 16 bits wide, or it might be 8... if it's 8, that means the core takes 2 cycles to transfer each 16-bit word. If it's 16, that means you have to decide whether all word accesses must be word-aligned, or not.

But if you allow yourself a word-addressed machine, what you have is an extension of the 65Org16. The original (simplest) variation just ignores the top 8 bits of each instruction, whereas you have ideas to make use of that. That's a perfectly reasonable thing to do, and there are many many different ways of approaching it.

The 65Org16 can talk to 8-bit peripherals, of course, in which case the high byte is ignored (for a write) or undefined (for a read.) To load data, I've always done it by sending pairs of bytes in the obvious way. To send a byte stream and have it implicitly zero-extended or sign-extended might well be possible, but might not be very simple. Did you mean to do that data expansion in hardware or in software?

Cheers
Ed
Author:  Arlet [ Thu Sep 19, 2013 6:05 am ]
Post subject:  Re: A 6502-compatible 16/32 bit core - an idea
If I understand it correctly, the difference between this and 65Org16 is that it still allows 8-bit data transfers. So, you can have a 16 bit data bus in hardware, with 16-bit operations (I would recommend only allowing aligned access for simplicity). But in addition, you have 8 bit instructions that can access 8-bit chunks. So you could do INC $01, and increment the upper byte of the first word.
Author:  enso [ Thu Sep 19, 2013 6:29 am ]
Post subject:  Re: A 6502-compatible 16/32 bit core - an idea
To clarify, it is basically a 16-bit machine. But if the upper byte of an instruction is 0, it performs operates on the low byte. You can think of it as a 6502 with 16-bit memory with only the low 8 bits in use... Does this make sense?

It is very close to 'org16. The memory is 16 bits wide. However the accumulator/index registers selected by index 0 (resulting in a 0 upper byte) operate on the low byte only.

So the databus is 16-bit and there is no way to operate on the high byte except as part of a 16-bit operation. Or you can treat it as a 6502 with 64K. In fact you can run 8-bit code in the low 64K words, and have a 16-bit OS around it.

INC 01 would increment the first word. I think it doesn't matter if it's in 16 bit mode or 8 bit mode, the entire word can be incremented. Two distinct carry flags are maintained simultaneously. If the next instruction is an 8 bit BCS, the 8-bit carry flag is used (from the 9th bit), otherwise, the 16-bit carry (from the 17th).

A more pertinent instruction is ROR. In 8-bit mode it pulls in bit 8 from the carry, in 16-bit mode it gets bit 16 from the 16-bit carry. Some instructions just do the same thing in both modes and the high byte is just not relevant in 8 bit mode.
Author:  Arlet [ Thu Sep 19, 2013 6:35 am ]
Post subject:  Re: A 6502-compatible 16/32 bit core - an idea
Why not use odd addresses to access high byte only ? It would make it more powerful (and increase compatibility with old 6502 code), and it's not that much harder to implement.
Author:  enso [ Thu Sep 19, 2013 6:39 am ]
Post subject:  Re: A 6502-compatible 16/32 bit core - an idea
I kind of like the clarity of the 16 bit operation, a 16-bit bus, and some legacy opcodes working with the low 8 bits. Otherwise it gets really confusing.

With 8 bit instructions you get a contiguous byte space as the processor ignores the high 8 bits.
Author:  Arlet [ Thu Sep 19, 2013 6:46 am ]
Post subject:  Re: A 6502-compatible 16/32 bit core - an idea
What I'm proposing is basically how the ARM7 does it. You have a 32 bit bus, 32 bit instructions, and 32 bit data, but even though the memory is physically 32 bit wide, the logical memory addressing is done in 8-bit bytes. So, first word is at address 0, and second word is at address 4. The 32-bit instructions can only do whole, 32-bit aligned, words, but there are some 8 bit LDRB/STRB instructions that can access any of the individual bytes.

So, you still have a contiguous byte space, but without the holes. I've never found it particularly confusing.
Author:  enso [ Thu Sep 19, 2013 2:51 pm ]
Post subject:  Re: A 6502-compatible 16/32 bit core - an idea
Arlet wrote:
What I'm proposing is basically how the ARM7 does it.

While this is doable I don't like it.

I like the symmetry of what I proposed much more. I like the hidden bytes quietly extending the processor, clean 16-bit fetching, the same 6502 decoder (pretty much), not worrying about alignment.

There are two different ways to work the modes. One is to have A, X and Y as 8-bit registers and have the rest (5 or 13) be 16-bit. A,X,and Y should really sign-extend to 16 bits so they can be in the mix in 16-bit operations.

The other is to have mode bits somewhere in the instruction that tap the registers at 8 bits (you can even mix - 16 bit A and 8-bit x). Then we can stay with the same instruction set, even keep the assembly language intact and use capitalization to indicate register size. I like this option - but it needs more thought. 6502 works pretty well without all those registers (which must be stacked all the time). And we still have a 64K zero page.

P.S.I don't like the ARM architecture that much, to be honest.
Author:  enso [ Thu Sep 19, 2013 3:31 pm ]
Post subject:  Re: A 6502-compatible 16/32 bit core - an idea
I guess what I like the most is the ability to cleanly virtualize a 6502 in a 16 (or 32) bit environment. I suppose having extra registers in 16-bit mode is nice.

Does anyone have a clean 65org16 'starting point' code? I don't have much time to work on this yet, but it would be fun to mess around.

Is the original 65org16 here https://github.com/BigEd/verilog-6502 what I want?
Author:  BigEd [ Fri Sep 20, 2013 8:53 am ]
Post subject:  Re: A 6502-compatible 16/32 bit core - an idea
Hi enso, yes that's the minimal and furthest upstream version of 65Org16.
Cheers
Ed
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