Understanding a few iTerm2 default keycode mappings

Question

What are the following used for in iTerm2? For example:

• ^2 through ^9 and ^- --> Send Hex Code 0x00 .. 0x7f ?
• ⌥↑ and ⌥↓ --> Send Hex Codes: 0x1b 0x1b 0x5b 0x41...

To put it all into one here are the ones in question in a nicely formatted way:

Answer 1

Short answer: For compatibility with old terminals. Real terminals that were dedicated pieces of hardware, not applications in a window system!

About the ^0...^9 ones: Terminals would typically use 7-bit ASCII codes, with character codes ranging from 0 to 127, inclusive. Codes 32-126 where used for letters, numbers and punctuation (as they still are in Unicode). 127 was usually the DELETE key (though sometimes that key would send code 8, "backspace" instead, leading to problems which persist in FAQ lists to this day. CTRL-A to CTRL-Z would generate the corresponding ASCII code minus 64, i.e. 1-26. The remaining codes could be a bit harder to generate. CTRL-@ usually gave you a 0, but sometimes that was on CTRL-space instead. The 27-31 range was usually at CTRL plus the "ASCII minus 64" position, so for example CTRL-] would give you 0x1D, but there were some terminals where those codes were mapped to CTRL-number instead, and those mappings in iTerm2 seem to be there to cater to people used to such terminals.

As for alt-arrow giving 1b 1b 5b 41, that is ESC ESC [ A . Now ESC [ A is a fairly common sequence for the up arrow alone, and prefixing it with an extra escape probably makes Emacs users happy, because it makes the Alt key work as a Meta key for them. I have not looked at the other multi-byte sequences, but I guess they have similar explanations.