[pwg]

> by essentially subtracting 64 (which is almost but not quite equivalent to masking out a bit)

Subtracting 64 is exactly the same as "masking out a bit". The difference between the control characters and their upper case equivalents is that bit 6 (counting from zero) of the byte is zero for the control characters and is one for the upper case equivalents.

On old terminals, the 'control' key more often than not worked by simply forcing bit six of the character mapped to a given key to zero (i.e., it was a direct hardware change).

Another seemingly little known fact, for ASCII, the difference between upper case and lower case letters is again a single bit. Letter A is 65, letter a is 97, the difference there is bit 5 (counting from zero) is zero for "A" and one for "a".

[saurik]

It isn't the same as "masking out a bit" when you are subtracting 64 from 63 to get 127 (the behavior of C-?). I think it is equivalent to Xor-ing it, but I didn't want to go into that for various reasons, including it just being overly complicated (and, in fact, edited that statement three times in an attempt to find something that was all of true, expository, and easy to understand; but, I apparently didn't optimize for "something everyone would appreciate why it was said the way it was said if they liked adding information but hadn't paid as much as attention to all of the examples as I had been forced to while drafting the comment").