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From: howard@cpocd2.UUCP (Howard A. Landman)
Newsgroups: comp.arch
Subject: Re: An old fashioned memory technology, CRT's, how'd they work?
Message-ID: <786@cpocd2.UUCP>
Date: Tue, 14-Jul-87 16:29:52 EDT
Article-I.D.: cpocd2.786
Posted: Tue Jul 14 16:29:52 1987
Date-Received: Fri, 17-Jul-87 03:59:54 EDT
References: <602@madvax.UUCP>
Reply-To: howard@cpocd2.UUCP (Howard A. Landman)
Organization: Intel Corp. ASIC Systems Organization, Chandler AZ
Lines: 38

In article <602@madvax.UUCP> cw@madvax.UUCP (Carl Weidling) writes:
>I was reading about the design of the Mark I in Manchester,England
>where it says: "He [F.C. Williams, the project's chief engineer] hit upon
>the idea of employing ordinary cathode ray tubes [to solve the problem of
>internal memory storage]...Their operating principle was quite simple;
>"guns" in the bases of the tubes shot positively and negatively charged
>electrons at the faces of the tubes, thus storing bits in the form of charge
>spots, which, by the way, were quite visible to the eye."
>	Well, I have a problem with "positively and negatively charged
>electrons", but also, with how this can be memory.  How long did the charges
>last?  were they refreshed? How was the memory read after being stored?

The description is wrong on a couple of counts, if my memory serves
me.  First, the tubes were NOT what we would today call an "ordinary
cathode ray tube", but rather storage tubes similar to those used on
certain Tektronix oscilloscopes about 10 years ago.  Once a bit (pixel)
has been lit up, it stays lit up.  Second, I don't think it was charges
but rather sustained currents; this implies no real time limit.  (I'm
not sure about this, they might have used refresh.)  Reading was
accomplished by putting a weak signal into the pixel and measuring the
output current(?), which varied depending on the state of the bit.  I'm
not sure how they erased anything!  Also, the voltage required to write
a bit resulted in a burst of intense light as the bit was written, much
brighter than the light from a bit simply staying on.  This allowed you
to watch your program's memory accesses; random access, linear scan,
and infinite loop all have their characteristic appearances.  The first
real-time, graphical program debugging tool!

You can get a similar display today on any 16 bit computer with 2 8-bit D/As
and an X-Y oscilloscope.  Hook one D/A to the upper 8 bits of the address
bus and the Y, and the other to the lower 8 bits and the X.  Adjust gain,
and voila!

-- 
	Howard A. Landman
	...!{oliveb,...}!intelca!mipos3!cpocd2!howard
	howard%cpocd2%sc.intel.com@RELAY.CS.NET
	"Sometimes a light's all shining on me, other times I can barely see"