On 2023-01-23, Mark Jamsek <mark@jamsek.com> wrote:

> Oh whoops, I've been doing it wrong! I always thought it was [1,80] not
> [1,80)  :)

There was a historical reason to stay clear of the 80th column.

A traditional video terminal had 80 columns.  Let's say you write
79 characters from the start of the line.  Now your cursor is on
the final column.  Then you write another character.  What happens
next?

In early video terminals, the cursor would wrap around and advance
to the first column of the next line.  If you then wrote a newline
to move one line down, the cursor would be _two_ lines down.  Oops.
Also, if you wrote to the final column of the bottommost line, the
cursor would wrap around and the screen would scroll up one line.
Which means you could never fill the screen completely.

Some terminals introduced a modified behavior: When you write to
the final column of a line, the cursor stays there.  It will only
wrap around if you write one more character; effectively the cursor
position jumps by two characters.  The termcap(5) property "xn"
marks a terminal with this behavior.  All DEC terminal starting
from the VT100 had it.  So does xterm, etc.

I think the last terminal I ran into that didn't have this was some
earlier iteration of the FreeBSD console, but we're a few rewrites
beyond that, too.  So, yeah, I don't think it's a concern nowadays.

Anyway, that's the historical reason why people tended to avoid
writing to the final column.

-- 
Christian "naddy" Weisgerber                          naddy@mips.inka.de