Re: Terminal Graphics Draft 2

From: Paul Keinanen (keinanen@sci.fi)
Date: Fri Oct 09 1998 - 03:08:21 EDT


At 13:44 8.10.1998 -0700, John Cowan wrote:
>Frank da Cruz wrote:
>

>> 2421 7F DEL DT Symbol for Delete (3)
>
>[...]
>
>> (3) Not, strictly speaking, a control character, but not a visible
>> one either.
>
>DEL is a control character in every sense, despite its position at 7F.
 
The situation with Delete/Rubout is a bit complicated, depending on usage.
In the VTxxx environment it is clearly a control function (erasing the
previous character), but originally the code for Delete/Rubout was chosen
for Teletype style paper tape manual entry. If you hit the wrong key, you
manually stepped back the tape one character and overpunched all 7 holes (7F
in 7 bit+odd parity) or all 8 holes (FF in 7 bit+even parity) and the
reader/computer would then ignore the character. In that sense it is a dummy
nonspaced character. When using a Teletype with a paper tape reader, you had
to be careful to have the computer in correct mode for manual resp. paper
tape reader input.

Due to this duality, should Rubout and Delete be concidered to be two
separate characters, although they seem to have the same code point in all
ASCII based character sets ? Probably not, as we try to unify other
characters as well, but at least the Rubout functionality should also be
included in the description. You could of course argue that the Rubout
functionality is either a nonspaced dummy character just in the same way as
NUL, but as NUL is concidered a control character, thus the Rubout should
also be concidered a control character.

So indeed, there can be many interpretations.

Paul Keinšnen



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