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Comment by zoomablemind | Hacker News Reader

> ...Look at the ascii chart by bit pattern and the characters will suddenly make sense.

Could you elaborate on what does the bit pattern reveal?

For example, I understand that Ctrl-C generates 0x30, which stands for ETX (end of transmission), but what is it there in 011?

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I don't know if the chart helps, but

    #define CTRL(x) ((x) ^ 0100)
    // e.g. 'C' ^ 0100 is 3
    // e.g. '@' ^ 0100 is 0
    // e.g. 'M' ^ 0100 is 13 a.k.a. \r a.k.a. enter

I like this explanation better. It also explains what notation like ^C means. It's shorthand for 0100^C.

Expanding your overloaded operator:

Crtl-C is shorthand for 0100 xor ‘C’

Also, the 0 prefix means this is a base-8 (octal) number

Ctrl clears the highest two bits. E.g. @ (ascii 0x40) turns into ^@/NUL (ascii 0x00). Similarly C (0x43) -> ^C/ETX (0x03).

Older terminals (like the VT05) used bit-pairing also with shift. They just flipped some bits depending on the upper two bits. Compare column 2 with 3 in the linked graphic, and 4,5 with 6,7.

Closer to toggling in the case of ^?.

good point!

Just run your eye horizontally to see that toggling only a couple of bits move you between ^C, C, and c (back when ASCII was formalized few terminals supported lower case, and six bit character sets were common).

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