Accumulator unit (ENIAC)
Twenty cabinets that were memory and arithmetic in the same box — ENIAC's entire fast storage held twenty numbers, each sitting inside the unit that also added it.
Each accumulator held one signed ten-digit decimal number. The digits lived not in registers in any modern sense but in ring counters: ten flip-flop stages per digit, built from vacuum tubes, stepped along by incoming pulses. That came to roughly 550 tubes per unit. Numbers travelled between units as pulse trains on shared digit trunks — the number seven was seven pulses — and receiving a number was the same act as counting those pulses. Addition was not an operation layered on top of storage. It was the storage.
One addition time was 200 microseconds, or 5,000 additions per second. Two accumulators could be coupled for twenty-digit precision, and the multiplier, divider and square-rooter had no registers of their own — they borrowed accumulators while they worked. What is now a register file, an ALU and main memory was here the same twenty cabinets.
The limit shaped programming more than anything else did. Twenty numbers is not much, so intermediate results were punched onto cards and read back in for the next stage. When the EDVAC draft called a year later for one large, uniform, cheap store, it was answering precisely this arithmetic.