[Info-vax] VTxxx/VMS term driver: bug or feature?

[Johnny Billquist]

On 2011-07-01 22:26, VAXman- @SendSpamHere.ORG wrote:
> In article<iul1m2$qb7$6 at online.de>, helbig at astro.multiCLOTHESvax.de (Phillip Helbig---undress to reply) writes:
>> In article<00AB1ACF.761B5A23 at SendSpamHere.ORG>, VAXman-
>> @SendSpamHere.ORG writes:
>>
>>> Didn't my "ASCII" art demonstrating what I see on the screen not
>>> appear in the posting you quoted?
>>
>> Yes, and what was in your post was what I saw when running the test
>> program.  So I know the answer but am wondering what the question is.
>
> Initially, I would have thought that the local nowrap, terminal driver
> /WRAP combination should have appears the same.  Strangely enough, the
> terminal emulator I was checking didn't show this.  While testing it,
> I was performing the same tests on 2 VT terminals (VT220 and VT525, as
> well as, a DECterm).  I was simple taken aback when I saw differences
> that were shown on the VTs and DECterm but not this terminal emulator.
> I am, of course, convinced now that the terminal emulator got it flat
> out wrong.

Ah. Yes, the terminal driver gets this wrong because it do not 
internally parse your CSI 15;10H, and so it does not know that now you 
are actually at column 10, and not at column 1. So it will output 80 
printable characters before doing the wrapping.

It would be almost impossible for a terminal driver to get this right, 
as understanding what might change the cursor position is very dependent 
on terminal type in combination with what sequence is sent. The 
permutations are endless. The terminal driver understands about CR, LF, 
BS, HT (and I think it assumes a HT moves to the next tab column, which 
is every 8th position), and printable characters. It also understands 
escape sequences enough to not count those as printable characters, even 
though they contain printable characters after the ESC.
And then it's just a question of inserting a CR+LF when the driver 
believes you have reached the right margin (based on counting, and 
comparing to the terminal width information).

The bug is those darned terminal emulators who tries to be more clever, 
and don't behave like a VT100, in a situation which should be easy to do 
right.

	Johnny

-- 
Johnny Billquist                  || "I'm on a bus
                                   ||  on a psychedelic trip
email: bqt at softjar.se             ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol