when was memory "above" the terminal screen invented?

[Frank McConnell]

On Dec 13, 2020, at 18:37, Stan Sieler wrote:
> 
> Hi,
> 
> First, apologies if I asked this years ago (I've searched my archives, no
> hits :)
> 
> When was the concept of memory "above" the screen invented for terminals?
> 
> I.e., previously displayed data that had scrolled up and off the screen ...
> but could be retrieved (usually by scrolling down).

Printing terminals.  Just pull the printed paper up from where it has fallen
behind your Teletype or DECwriter or Silent 700 or Terminet.

> (Sometimes called "scrollback", or "offscreen memory".)
> 
> (BTW, I'm talking about terminal-local memory, not a scrollback implemented
> by the computer to which the terminal is connected.)
> 
> The HP 2640A, 1974, had (IIRC) several pages of memory available ... the
> user could scroll
> backwards and see what had been on the screen before it scrolled off (as
> long
> as it hadn't been lost by having too much subsequent output).

That was, kind of sort of, the on-screen effect, but it could vary.

Between 1977 and 1981 I used 2640B terminals which had been purchased
without many options.  They didn’t have lower case characters (these
were displayed as upper case characters, and sometimes hilarity ensued),
and they had about 1KB of display memory.

Now you may be thinking that 24 rows of 80 characters is more like 1920
characters which would require a little more than 1KB of display memory,
and you would be correct.

The tricky bit about the 264X display controller is that it is reading
display memory as a linked list of fixed-size short chunks (under 20
bytes) and the last one has an end-of-line indicator in it (it is an
ASCII terminal and byte values 0x80-0xff are interpreted as display
controls).  So a short line of text doesn’t take up as much display
memory.  Which means you can have more of them in display memory.

So your 2640B with 1KB display memory has scrollback if most lines that
you have in memory are short, but can only fill its screen halfway if
all lines are long.

> I suspect the DEV VT100, 1978, had it, but I can't find definitive proof
> online (sure, I can find VT102 emulators that have scrollback, but reading
> an old VT102 manual doesn't make it clear that it has it.)

I think the VT100 did not.  I’m not sure it matters.  The 2640A would
predate the VT100.

I wonder if the termcap da and/or db flags would turn up some older 
terminals with the same feature.  (These indicate display above and 
display below.)

-Frank McConnell