I am quite familiar with the two’s complement method of subtracting binary numbers. Applying it to decimal arithmetic is a curious thing to do.
Looking at the history of computers, it took a while for two’s complement arithmetic to catch on. There was a good bit of history of the sign bit being used, mostly with decimal but also with binary machines. Heck, I once worked with a self-taught programmer (really an optical sciences grad student) who didn’t know about two’s complement math, and had sign bits in his 8080 code.
You can’t subtract by adding. You know how I know? 2 + 2 = 1, that’s how.
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