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24 votes

Was there a Western computer with blatantly missing instructions in the instruction set?

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 reputed to ...
Kaz's user avatar
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21 votes
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Was there a Western computer with blatantly missing instructions in the instruction set?

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 ...
RETRAC's user avatar
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14 votes
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Which CPUs had instructions leaving data registers in an unspecified state?

There are a lot of instructions on x86 that leave the flag(s) in an indeterminate state, which means the whole FLAGS register is also undefined. Most will have AF undefined, for example TEST The ...
phuclv's user avatar
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13 votes
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Is this a bug or an allowed Pascal behavior?

The ISO 7185 Pascal standard, section 6.4.3.5 "File-types", says (my emphasis): There shall be a file-type that is denoted by the required structured-type-identifier text. The structure of ...
dirkt's user avatar
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7 votes

What is the purpose of the ω register of the БЭСМ-6?

As a matter of fact, the page you refer to, which was authored by me a long time ago, is somewhat incorrect, and bases its notation on a colloquial use of ω. In the original documentation ω is the ...
Leo B.'s user avatar
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7 votes
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What is the purpose of the ω register of the БЭСМ-6?

I'm finding this whole business kind of bizarre and hard to grok. So my question is basically "what's going on here?". It's a way to save on special case jump instructions. Or more general, ...
Raffzahn's user avatar
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6 votes

Was there a Western computer with blatantly missing instructions in the instruction set?

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 ...
davidbak's user avatar
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6 votes
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Was dynamic type check in Pascal commonplace?

Pascal compilers/runtimes typically do some (very often, optionally, enabled by a flag) run-time type checking. A very typical run-time check supported by most Pascal compilers is array bounds ...
tofro's user avatar
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6 votes

Which CPUs had instructions leaving data registers in an unspecified state?

This is a little bit outside the scope of the question. It's not really arithmetic instructions (though they are common instructions, and some of them do involve arithmetic), and the architecture in ...
Nate Eldredge's user avatar
5 votes

Which CPUs had instructions leaving data registers in an unspecified state?

One example are the two undocumented 8085 flags that can be found by reverse-engineering the silicon (well, it's part of a register, not a complete register). They have uses for signed overflow in ...
dirkt's user avatar
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4 votes

Was there a Western computer with blatantly missing instructions in the instruction set?

The Manchester 'Baby' famously had neither 'load accumulator with value from memory' nor 'add value from memory to accumulator'. Rather, it had 'load negative' and 'subtract'. Nevertheless, these ...
dave's user avatar
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4 votes
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How can floating point addition be so slow on a BESM-6?

Compiling details pointed to in the comments by @secondperson and my own findings: It appears that the documentation is inaccurate. There are apparent typos and/or transcription errors. For example, ...
Leo B.'s user avatar
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3 votes

Which CPUs had instructions leaving data registers in an unspecified state?

And then there are data registers of various FPUs leaving one or more entries in unspecified state after an operation. Well known for example the AMD 9511 series where next to all higher FP functions ...
Raffzahn's user avatar
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2 votes

Was there a Western computer with blatantly missing instructions in the instruction set?

Fairchild F8 There is "add to accumulator" but no subtract. There is "decrement register" but no "increment register" There is "increment accumulator" but no &...
Grabul's user avatar
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