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The 6502, like many 8-bit processors, has a somewhat arcane opcode-mode restrictions. On most such processors, the restriction is a clear result of trying to pack a lot of instructions into a limited number of opcodes. On the 6502, however, the number of opcodes without a specified meaning is huge. Among other things, there are no defined opcodes at all whose bit pattern ends in 11.

Because the number of instructions that use any kind of memory operand is less than 24, and the number of instructions with only one addressing mode is less than 64, it would have been simple to say that all opcodes whose bottom two bits are not 00 are memory-format instructions, all of which would compute their addressing mode as specified by the next three bits. This would have been simpler than having different instructions compute addressing modes differently. On the other hand, it would have filled up most of the opcode space where the bottom two opcode bits are 11.

Is there any information about the design process that would indicate whether there was a deliberate design decision to leave unused the portion of the opcode map where the bottom bits are 11, and squeeze in opcodes where needed to accommodate that, or was the fact that that portion of the opcode map was unused merely design happenstance?

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The instruction decode is quite simple on the 6502.

If we call the bits in the opcode byte aaabbbcc, then one of the first things that happens is that cc, the two bits you're talking about, gets converted into a 1-of-3 signal which selects the register. This signal is called G, and is computed like this:

  • A is true if the bits are 01
  • X is true if the bits are 10
  • Y is true if the bits are 00

(It's easy to see how both A and X are true for the NMOS undocumented opcodes like LAX)

Aside from selecting the register, these two opcode bits also divide the opcode space into three clear groups:

  1. 00, mostly ALU related ones,
  2. 01, the read-modify-write instructions,
  3. 10, rag-bag of conditional branches, compares, flow-control, push/pull, clearing and setting flags and whatnot, also the bit instruction

Each of the above groups also includes load and store instruction for the associated register

So why not four groups (and maybe another register, or more powerful handling of the stack pointer etc?)? Well, each bit is fed twice into the decode logic. The bit itself and the bit inverted. But the designers saved a fair amount of space by not putting in inverted bit 0. That saves one line of decode logic, and importantly, some space.

This is (at least some of) the reason why no opcodes have the form aaabbb11.

So to directly answer your question, it seems the decision was taken to first divide the opcode space into these three groups. Making the most of the fact it's okay to leave don't-cares all over the map, the unused value of cc = 11 is a natural outcome of the odd number of groups.

There is more information about how this all works here.

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  • Having the signals that are presently driven by Op1 instead driven by (Op1 and not Op0) would cost two gates, and would allow some other simplifications. For example, the logic to select between X and Y for indexing could simply use Op1 unconditionally, simultaneously simplifying logic and making addressing modes zp,y, (zp,y), and (zp),x available. Or the decimal-mode-select signal could be controlled by opcode bit 1, eliminating the need for the D flag entirely. On the other hand, it's possible there were some anticipated uses for opcodes of the form 11 that never materialized.
    – supercat
    Sep 23, 2021 at 16:46
  • For example, many I/O related tasks could benefit from having a large easily-decoded group of opcodes which would, based on the state of a couple of inputs at the rising edge of phi0 after the instruction fetch, behave as a one-byte NOP, a two-byte NOP, or a branch. If such functionality were added and made so that a bit pattern of 11 would override everything else, the way that such instructions would be decoded if executed wouldn't matter. Obviously such functionality was never incorporated into the 6502, but maybe something like it was planned but abandoned due to time pressure.
    – supercat
    Sep 23, 2021 at 17:01
  • Is there a reason it's not a 1-of-4 signal?, and includes a register for 11?
    – Alexander The 1st
    Sep 24, 2021 at 2:55
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    @AlexanderThe1st Yes. All bits except bit 0 are fed twice into the instruction decoder. The bit itself, and the inverted bit. There is not inverted bit 0 going into the instruction decoder, which saves a little space. For this reason, G cannot be fully decoded into a 1-of-4 signal
    – Героям слава
    Sep 24, 2021 at 6:23
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The opcodes are already sorted that way. Just a bit less obvious and schoolbook-like, but optimized to allow compact decoding.

It is all about space saving.

Real chip space and (potential) transistors that is. It's well known that the 6500 design was all about cost saving and the most important factor in chip production cost is its size. Smaller chips mean more chips per wafer run and fewer defects at the same time.

Bits not present need not be decoded. Each bit needs two lines within the Instruction Decoder ROM (inverted and not inverted), spaced at a distance to hold a gate. So two bits save PLA lines stretching across the whole chip, enabling to shorten the chip in Y direction by this, as easy visible in this annotated die shot by Visual6502.

(6502 die photo)

[..] simple to say that all opcodes whose bottom two bits are not 00 are memory-format [... and so on ...]

Well, that is what the 6500 actually does.

  • X... ..XX Group - Two bits plus the top bit define the instruction format/group.
  • ...X XX.. Addressing Mode - Three bits define the addressing mode used.
  • .XX. .... Instruction - Two bits determine what instruction it is.

The Group Bits define the basic function/structure:

  • 0..00 'oddities'
  • 1..00 index register instructions (STY/LDY/CPY/CPX)
  • x..01 the regular read/write ones related to A as source/target
  • 0..10 Shifting RMW
  • 1..10 Counting RMW plus STX/LDX
  • X..11 unused

Implied instructions fill holes in the 00 and 01 types.

It's all about easy decoding. Filling is not random, but creates sub groups, which becomes obvious when sorting the microprogram accordingly. It also shows that instructions using index registers have the register in the third lowest bit encoded - within their group that is, or target/source for Txx in 2^5.

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  • How much of this is preserved in the later Western Design 65C02? Or to ask another way, didn't they add new opcodes to the 65C02 and 'fill' in some empty spots, effectively breaking this pattern for newer opcodes?
    – Geo...
    Sep 23, 2021 at 17:50
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    @Geo... Well, no and yes. No, as the R6502 (which had the new instructions even before the 65C02) had filled all new instruction into the holes left in, so they could have gone without, but yes, as they added those useless bit instructions as well. Then again, being CMOS the CPU had to be redone in quite substantial way, while at the same time a shrink was possible. Not sure about the first generation, but with the WDC 65C02, the CPU despite needing way more transistors smaller than the minimum size for 40 pins, so there was empty space all around anyway.
    – Raffzahn
    Sep 23, 2021 at 17:59
  • Addressing mode isn't determined solely by bits 2-4. Indeed, every bit other than bit 5 plays a role in addressing mode determination, since y-based indexing mode is selected by both xxx11001 and 10x11110. But your mention of the R6502 hints at part of what I was aiming at with the question: if the 6502 had put some useful opcodes into the bit patterns with xxxxxx11 that could have made support for things like RMB etc. more difficult. Further, some processors had groups of instructions which were intended to be handled by outside hardware, the most famous of which was...
    – supercat
    Sep 23, 2021 at 18:03
  • ...may be the family of opcodes handed by the 8087. If a future variant of the 6502 had e.g. included two or three pins to say e.g. "treat current opcode normally", "treat current opcode as one-byte NOP", "tread current opcode as branch not taken", and "treat current opcode as branch taken", then external hardware could have easily decoded instructions ending in 11 and used them to perform I/O operations much faster than would be possible using loads and stores, something that wouldn't have been practical if useful opcodes occupied those slots.
    – supercat
    Sep 23, 2021 at 18:08
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    @supercat you're not turning that into an What-If question. You know they are absolute off topic. The whole point for MOS was that keeping columns unused will save chip real estate to lower the CPU price. Rockwells market was a different one, they sold to customers with way more expensive builds. Regarding your addressing mode point: That doen't matter, it has only to be consistent within one group, as a single full width decoder ROM is used, not a series of staggered decoders per function. So selection can be made group specific. it's a decoding tree, not a flatt decoding.
    – Raffzahn
    Sep 23, 2021 at 18:09

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