Editing Whirlwind I (section)

===The memory subsystem===
The original machine design called for 2048 (2K) words of 16&nbsp;bits each of random-access storage. The only two available memory technologies in 1949 that could hold this much data were [[delay-line memory|mercury delay lines]] and [[storage tube|electrostatic storage]].

A mercury delay line consisted of a long tube filled with [[mercury (element)|mercury]], a mechanical transducer on one end, and a microphone on the other end, much like a [[spring reverb]] unit later used in audio processing. Pulses were sent into the mercury delay line at one end, and took a certain amount of time to reach the other end. They were detected by the microphone, amplified, reshaped into the correct pulse shape, and sent back into the delay line. Thus, the memory was said to recirculate.

Mercury delay lines operated at about the speed of sound, so were very slow in computer terms, even by the standards of the computers of the late 1940s and 1950s. The speed of sound in mercury was also very dependent on temperature. Since a delay line held a defined number of bits, the frequency of the clock had to change with the temperature of the mercury. If there were many delay lines and they did not all have the same temperature at all times, the memory data could easily become corrupted.

The Whirlwind designers quickly discarded the delay line as a possible memory—it was both too slow for the envisioned flight simulator, and too unreliable for a reproducible production system, for which Whirlwind was intended to be a functional prototype.

The alternative form of memory was known as "electrostatic". This was a cathode ray tube memory, similar in many aspects to an early [[television|TV]] [[Cathode-ray tube|picture tube]] or [[oscilloscope]] tube. An [[electron gun]] sent a beam of electrons to the far end of the tube, where they impacted a screen. The beam would be deflected to land at a particular spot on the screen. The beam could then build up a negative charge at that point, or change a charge that was already there. By measuring the beam current it could be determined whether the spot was originally a zero or a one, and a new value could be stored by the beam.

There were several forms of [[Selectron tube|electrostatic memory tubes]] in existence in 1949. The best known today is the [[Williams tube]], developed in England, but there were a number of others that had been developed independently by various research labs. The Whirlwind engineers considered the Williams tube, but determined that the dynamic nature of the storage and the need for frequent [[memory refresh|refresh cycles]] was incompatible with the design goals for Whirlwind I. Instead, they settled on a design that was being developed at the [[MIT]] [[Radiation Laboratory]]. This was a dual-gun electron tube. One gun produced a sharply-focused beam to read or write individual bits. The other gun was a "flood gun" that sprayed the entire screen with low-energy electrons. As a result of the design, this tube was more of a [[static RAM]] that did not require refresh cycles, unlike the [[dynamic RAM]] Williams tube.

In the end the choice of this tube was unfortunate. The Williams tube was considerably better developed, and despite the need for refresh could easily hold 1024&nbsp;bits per tube, and was quite reliable when operated correctly. The MIT tube was still in development, and while the goal was to hold 1024&nbsp;bits per tube, this goal was never reached, even several years after the plan had called for full-size functional tubes. Also, the specifications had called for an [[access time]] of six microseconds, but the actual access time was around 30&nbsp;microseconds. Since the basic cycle time of the Whirlwind&nbsp;I processor was determined by the memory access time, the entire processor was slower than designed.