The "loop and a half"

[bartc]

On 09/10/2017 06:15, Gregory Ewing wrote:
> Grant Edwards wrote:
>> Which took it from RSX-11.  Or probably more specifically from
>> FILES-11.  I woldn't be surprised if the enineers at DEC got it from
>> somewhere else before that.
> 
> Quite possibly it goes back to the very earliest DEC OS
> that had files, whatever that was.
> 
> The reason for it was that the file system only kept track
> of file sizes in blocks, not bytes, so some way was needed
> to mark the end of a text file part way through a block.

Ctrl-Z was just code 26 or [what I've always called] ETX or end-of-text.

I'd used DEC quite a bit (including rsx-11m) but only became aware of 
ETX when using CP/M. (Where it had a annoying habit, IIRC, of copying 
files in text mode so that it stopped at the first ETX byte.)

And actually, until recently, I added ETX (followed by 0 for good 
measure) as a sentinel in a function reading a file into a block of 
memory (now I just use 0 for that purpose).

But I don't consider that these internal uses, some of which are 
archaic, belong in a user interface.


[Looking it up now, actual ETX is code 3, or Ctrl-C, and EOT is code 4. 
So how 26 - apparently code SUB, whatever that means - ended up being 
used to mark the end of text files, I don't know.]

-- 
bartc