[GTALUG] CRT memories [was Re: IBM - cache skirmish story.]

D. Hugh Redelmeier hugh at mimosa.com
Tue Apr 24 13:33:11 EDT 2018

| From: James Knott via talk <talk at gtalug.org>

|   However, that system was based on a Data
| General Nova 800 and used on dumb terminals (made by VST) that used
| delay line memory.

Video terminal "VDT" development was very much gated by developments
of memory technology.  CRTs need constant refresh so there needs to be
some kind of backing store for the image.

Many different solutions were developed.

Tektronix developed a CRT technology that retained an image once it
was written.  The trouble was that the only kind of erasing was total
image erasing.  Think of an etch-a-sketch.  I used one of these with a
PDP-8 in the late 1960s.  It had a lot of advantages over the Teletype
Model 33 ASR.  Think of the output as going through more(1): after a
page of output, you had to type a control character to request the
next page.

Remember, the PDP-8 was a computer costing $10000 or more and only
having 4k words of main memory (12 bits/word) (1967).  A frame buffer
for a black and white 640x480 screen would require 25k words!  The
tail would be wagging the dog.  In those days RAM was implemented as
core memory.  Each bit was a little torus of ferite, with three or so
wires running though it.  Assembled by hand.  It cost roughly a buck a

A little earlier, IBM made the 2250 display.  It was a vector display:
the screen was painted via vectors.  For graphs, this was a very dense
representation.  It cost more than a house.  Both University of
Waterloo and University of Toronto had one, highly subsidized by IBM.
I think that it was developed for NASA.

The next step was to store characters in a buffer: much more compact
than a pixel buffer. Refreshing would by by raster scan, but a
character generating ROM would on-the-fly generate pixels for each
character.  The buffer would be about 25x80 = 2000 bytes.  Even this
was expensive so different kinds of implementations were used:

- magneto-restrictive delay lines (eg. in the IBM 2260 or the VST)

- shift registers (logically similar to delay lines, but using
  semiconductors) (e.g. DataPoint terminals)

- finally: RAM

Until RAM was used, terminals often did not allow editing the middle
of a screen.  up, down, etc. were not implemented.  These were the bad
old days.

One early CRT that I worked with was the product of an MASc thesis at
UofT.  It used a slow-decay orange phosphor.  The refresh was the duty
of the program in the attached computer (IBM 1710).  The output was
encoded the same way as plotter output was encoded.  As long as the
program output the same stuff every quarter (?) second or so, it was
visible.  (The attached computer was about a hundred thousand times
slower than current machines.)

In the mid-1970s, the Dynamic Graphics Project at U of T commissioned a
couple of displays: a large monochrome vector and a more modest colour
raster display with one byte per pixel and a 256 entry colour mapping
table.  Each cost about $20k.

Eventually RAM became cheap enough that a colour frame buffer was
affordable for individuals.  For example, The Atari ST (1985)
supported only 16 colours at a time for a resolution of 320x200 --
somewhat usable.  I preferred mine in monochrome at 640x400 but my
kids preferred colour.

Now GPU cards come with 4G or more of RAM!

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