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The Manchester Small-Scale Experimental Machine used two Williams–Kilburn tubes to hold its registers: one stored a 32-bit accumulator, while another stored the current 32-bit instruction and the current instruction address, or program counter. This was aside from the 32×32 bit Williams–Kilburn tubes used as main memory and display. The Manchester Mark 1 and the derived Ferranti Mark 1 also used the same technology for memory.
The largest proposed thermionic valve memory storage unit was the RCA Selectron at 4096 bits. RCA were never able to produce this reliably in quantity, and by the time the simplified 256 bit Selectron was ready for market, magnetic core had already taken most of the market.