6

In the BESM-6, there is an instruction (045) to add index registers, but not to subtract them, however, there is a nearby unused opcode 047, which is made synonymous to 045 (in fairness, that's true only in the kernel mode; in the user mode it traps as illegal).

Also, there are "branch if index is zero" (34) and "branch if index is not zero" (35), but not "branch if index is less than zero" (alternatively formulated as "branch if the MSB of the index is set"). However, there is a nearby unused opcode 36 which is made completely synonymous to 34.

This makes one to think that the unimplemented functionality was intended by the initial design (no references to confirm or deny it, unfortunately), but didn't make it into the hardware for one reason or another, be it physical space constraints, timing requirements, etc.

Are there any similar Western examples of an instruction set with obviously cut corners, or is this kind of mishap specific to socialist economy?

An answer could be in the form "Here's the instruction set of CPU X with an instruction with opcode A doing foo, but no instruction to do bar, which would be obvious to include for completeness/orthogonality; at the same time there is an unused opcode B close to A".

  • 3
    I'm tempted to vote for closing this, as answers can only be opinions. After all, as soon as a machine is Turing complete, it can produce any desired output, having more instructions just simplifies individual aspects. Most obvious with the branch example given, as both existing jumps can easy be used to synthesize the third. – Raffzahn Feb 1 at 18:55
  • 2
    @Raffzahn No, not so easy. Try writing a "branch if negative" using only moves, additions, and branches if zero/non-zero in any architecture. – Leo B. Feb 1 at 19:00
  • 1
    @LeoB. Well, the first question would be about the index registers being signed values at all - are they? Not clear from the Page linked, so are they? If not, any test for negative doesn't make sense. – Raffzahn Feb 1 at 19:22
  • 2
    @Raffzahn Given that the addition operation is 2's complement, a bit vector can be looked at as signed or unsigned arbitrarily. – Leo B. Feb 1 at 19:28
  • 1
    The Data General Nova had Boolean AND but no Boolean OR. You could get around this with De Morgan's laws, but it seems like a fundamental instruction to omit. – Ken Shirriff Mar 12 at 22:53
12

Are there any similar Western examples of an instruction set with obviously cut corners

Following on your example of having addition but not subtraction, the Intel 8080 has 8 bit addition and subtraction instructions laid out neatly as you can see in this table on rows 8x and 9x.

The 8080 also has a 16 bit addition instruction DAD with opcode 0x09, but there is no 16 bit subtraction. The logical place for it in the opcode table would be one column left at 0x08, but that is a NOP on the 8080. Most of the hardware necessary for subtraction is on the die. It just wasn't implemented for some reason.

On the 8085 there's even an undocumented opcode that does implement 16 bit subtraction at 0x08. Though unlike the 16 bit addition, it doesn't update the flags properly, which I suspect is why it's undocumented. You can find a table of the 8085 opcodes here.

  • Please improve your answer by mentioning the actual opcodes, including unused slots. Pointing to the 8080 opcode table, e.g. pastraiser.com/cpu/i8080/i8080_opcodes.html would have been helpful as well. – Leo B. Feb 1 at 20:10
  • 2
    @Raffzahn Yes, absolutely. Subtraction is less important to have since it can be synthesized. With pointers, a good assembler will handle negative constants so you can just do something like DAD -2 (0xFFFE) if you want to subtract 2. Subtraction at run time would however require generating the 2's complement of the subtrahend first, or chaining 8-bit subtractions. – RETRAC Feb 1 at 20:49
  • 3
    @Raffzahn Prove - no, but aliasing opcodes instead of making the extra one a no-op or an illegal instruction is unusual enough to raise suspicion. – Leo B. Feb 1 at 20:59
  • 2
    @LeoB. The BESM-6 is far from the only processor which doesn't bother to fully decode its instructions. The original NMOS 6502 was a minimal design, and they did not bother to fully decode the opcode space to turn unimplemented opcodes into NOPs. The opcodes in question behaved in a variety of ways: some would crash the processor, some behaved unreliably, some were useless for programming, and a few actually turned out to be useful by accident. This was certainly an intentional design choice to save die area. – RETRAC Feb 1 at 21:12
  • 1
    @LeoB. It's a mix of both. Based on the encoding of the CMP instruction, one might expect CPX with Y-indexed absolute addressing with opcode F8, but that sets decimal mode. On the other hand, ASL (arithmetic shift left) with the encoding specifying an immediate simply crashes the processor and requires a reset. – RETRAC Feb 1 at 21:47
8

The 6502, released in 1975, had a missing instruction when first released: it had a ROL (ROtate bits Left) instruction, but no corresponding ROR (ROtate bits Right) instruction. This was not an oversight on the designers' part, rather that the found a bug in the circuit for the ROR instruction during final testing. Rather than delay the release of their new product (and waste all the chips they'd already manufactured), they omitted any mention of the ROR instruction from their 1975 datasheet (page 6). The next year, they'd fixed the bug in their new processors, and the ROR instruction was included from the 1976 datasheet (page 5) onward.

Some versions of Microsoft BASIC for 6502 machines had workarounds in their floating point instructions to avoid using the ROR instruction. See https://www.pagetable.com/?p=45 for an example.

  • Unlike 6502, the IC version of the BESM-6 CPU released more than a decade later, hasn't implemented the missing instructions, probably to avoid breaking code compatibility at the user level. – Leo B. Feb 5 at 1:08
3

Socialism is terrible - but there are other reasons for "cut corners" leading to missing instructions. For example, capitalism.

In the minicomputer era it was reasonably common for there two be two variants of a machine: one without floating point and one with. You paid more $$$$ for the one with.

I'm thinking specifically of the Honeywell 716 - but there were several others. One interesting thing about the 716: It wasn't a matter of the expense of the parts. The only motherboard had the parts. The different between the 716 without FP and the 716 with FP was that to add FP you cut a trace on the motherboard.

(The general customer didn't know this of course. I know it because I worked for a Honeywell division that saved money by buying 716s without and using a xacto knife to turn them into 716s with.)

This isn't quite the same, to my mind, as whether you had a FP coprocessor or not, for machines that did that, as in the 8086/8087. I mean, it is sort of. But, it also isn't sort of. (Maybe someone can help me clarify this thought...)

(You can also quibble if you wish that this doesn't really answer the OP' question. Maybe not. But where else could I explain this?)

  • 1
    Sort of like a Tesla with a battery software locked to less than full power. – manassehkatz-Reinstate Monica Feb 6 at 2:22
  • 1
    @manassehkatz - Well, that's another story! Amdahl used to sell an IBM 360/370 compatible - forget the model number - that had a "turbo" switch on the front. (Forget the marketing name for it.) Most of the month you had something like a machine comparable to an IBM 370/148 or so, and that's what you were paying for. But, you know how you have that processing crunch at the end of the month to 'close the books'? You flipped the switch, or turned the dial, and instantly you had 20% more oomph. And you paid for that boost, on a meter, for a couple of days each month. First overclocking! – davidbak Feb 6 at 2:35
  • That's not "blatantly missing", that's "deliberately put behind a paywall". :) – Leo B. Feb 6 at 18:05

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.