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Comment by Suzuran | Hacker News Reader

On some of IBM's smaller processors, such as channel controllers and the CSP used in the midrange line prior to the System/38, the xor instruction had a special feature when used with identical source and destination - It would inhibit parity and/or ECC error checking on the read cycle, which meant that xor could be used to clear a register or memory location that had been stored with bad parity without taking a machine check or processor check.

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Interesting, since the general culture at IBM seems to have preferred SUB over XOR -- their earlier business-oriented machines didn't even have a XOR instruction, and even on later ones the use of SUB has persisted, including in the IBM PC and AT BIOS.

(There was another, now deleted, comment somewhere in this thread that mentioned IBM's preference for SUB. Source of that statement was Claude, but it seems very likely to be correct. The BIOS code I've checked myself, lots of 'SUB AX,AX', no XOR)

You may not be looking for the right thing. On the aforementioned CSP, the instruction that performed XOR was called "XR" and not "XOR". My source is firsthand knowledge; I was a CE and performed service calls on the System/34, System/36, 370, and 390.

In any case, I am describing equipment built mostly in late 60s through the late 70s at IBM Rochester and Poughkeepsie. The IBM PC was developed by an entirely different team at IBM Boca Raton, and IBM didn't design its CPU.

I don't doubt that this specific processor special-cased XOR (regardless of how it was called in the assembly language)!

Merely pointing out that where both operations were available, there seems to have been a preference to use SUB instead, with some continuity from early business-oriented mainframes, to the 360, to the PC.

You probably would prefer to use SUB with fault-checking to clear registers in general-purpose code, and only use XOR in early startup (and perhaps fault handlers), where error checking has to be suppressed. So both observations seem to align well?

Another thing I should point out is that the CSP instruction set was not documented to the customer. The CSP software was called "Microcode" and the customer was not told about the CSP's design or how it worked. The documented instruction set for the System/34 and System/36 is that of the Main Storage Processor or MSP, which was an evolution of the IBM System/3.

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