6502.org

Is the byte length of 65816 instructions a function of only the addressing mode used and not which register is being accessed or the opcode, just like the 6502?

Not quite sure what you mean by "not a function of the opcode". Clearly the length of the instruction depends on the opcode since the opcode determines which instruction it is.

As well as the opcode, the byte length depends on the state of the m and x flags. For example

LDA #0 is 2 bytes (A9 00) if m=1, but 3 bytes (A9 00 00) if m=0.
LDX #0 is 2 bytes (A2 00) if x=1, but 3 bytes (A2 00 00) if x=0.

Note also that if, for instance, m=0 but x=1, then LDA #0 is 3 bytes, but LDX #0 is 2 bytes. So it does depend on the register where there are similar instructions for different registers.

Right. Dragonslayer, all 65xx instructions begin with a one-byte opcode, and in some cases that's all that's necessary. For example, ROL is an instruction that can take several forms, and the form of ROL that rotates the accumulator requires nothing more than the opcode. There's nothing further to add, because the opcode is sufficient to tell the whole story.

But ROL also has a form that rotates a memory location in Zero Page, in which case there's a different one-byte opcode and it gets followed with a byte that supplies the Zero Page address. Similarly, ROL can address an absolute memory address, in which case the relevant opcode gets followed by two bytes of address information. (And for some instructions the '816 has long modes that use three bytes of address information.) So, the address mode has an effect on the total instruction length.

In other cases, the byte(s) following the opcode contain an immediate operand that'll directly interact with one of the processor registers. And, as kernelthread has explained, the length of the registers may be either 8 or 16 bits (according to the m and x flags). Thus, the length of instructions which include an immediate operand will be 2 or 3 bytes (including the opcode).

-- Jeff

I tried putting together a table with all the 6502 opcodes as the rows and all the addressing modes as the columns and filling in the cells with the byte lengths. Someone more experienced than me at 6502 was confused why I was trying to do this and pointed out every column would have the same number all the way down (although some cells are n/a). That's what I mean; you can figure out the byte length from the addressing mode and don't need to know the opcode.

Thanks you both; so to summarize if I understand correctly, the byte length of a 65816 instruction is a function of its addressing mode as well as the m and x flags.

Yup. And, to be clear, the list of address modes (along with zero page, absolute and all the rest) also includes implied mode (such as ROL accumulator, which I mentioned) and immediate mode (which, as I said, causes the operand after the opcode to directly interact with one of the processor registers). Immediate mode is the only one for which the m and x flags have a bearing on instruction length.

jeffythedragonslayer, it might be that when you say 'opcode' you mean something closer to 'mnemonic' - you mean the three-letter abbreviation for the operation.

Whereas it's more usual to mean the instruction byte, which is indeed a combination of the mnemonic and the addressing mode (and the machine mode).

Thanks, it sounds like a lot of people get mmenmic/opcode/instruction confused. I suggested nailing down this terminology to Ville and he did a big rename in WLA.