Comment by lifeisstillgood
15 days ago
>>> The image is not stored at any point.
The very first computers (Manchester baby) used CRTs as memory - the ones and zeros were bright spots on a “mesh” and the electric charge on the mesh was read and resent back to the crt to keep the ram fresh (a sorta self refreshing ram)
Yes, but those were not the standard kind of CRTs that are used in TV sets and monitors.
The CRTs with memory for early computers were actually derived from the special CRTs used in video cameras. There the image formed by the projected light was converted in a distribution of charge stored on an electrode, which was then sensed by scanning with an electron beam.
Using CRTs as memory has been proposed by von Neumann and in his proposal he used the appropriate name for that kind of CRT: "iconoscope".
Why didn't that catch on pre-transistor? Feels like you'd get higher density than valves and relays.
DRAM memories made with special CRTs with memory have been used for a few years, until 1954. For instance the first generation of commercial electronic computers made by IBM (scientific IBM 701 and business-oriented IBM 702) have used such CRTs.
Then the CRT memories have become obsolete almost instantaneously, due to the development of magnetic core memories, which did not require periodic refreshing and which were significantly faster. The fact that they were also non-volatile was convenient at that early time, though not essential.
Today, due to security concerns, you would actually not want for your main memory to be non-volatile, unless you also always encrypt it completely, which creates problems of secret key management.
So CRT memories have become obsolete several years before the replacement of vacuum tubes in computers with transistors, which happened around 1959/1960.
Besides CRT memories and delay line memories, another kind of early computer memory that has quickly become obsolete was the memory with magnetic drums.
In the cheapest early computers (like IBM 650), the main memory was not a RAM (i.e. neither a CRT nor with magnetic cores), but a magnetic drum memory (i.e. with sequential periodic access to data).