Delay Line Memory
The first electronic computers faced the challenge of storing their data and instructions. ENIAC had only about twenty vacuum tube registers, a prohibitively expensive solution for any substantial memory.
J. Presper Eckert then conceived an innovative concept of transforming electrical signals into sound waves traveling through a mercury tube. These vibrations, captured at the other end, became electrical again before being reinjected at the input. The loop was complete. This technique multiplied memory capacity by one hundred at equal cost, with each line capable of containing up to 500 bits.
Maurice Wilkes and his Cambridge team brought this idea to life in EDSAC in 1949. Their machine utilized 32 mercury tubes storing 512 words of 36 bits. A word circulated every 48 microseconds on each of the 128 lines, while a simple addition required 864 microseconds.
This technology addressed the needs John von Neumann had identified in his description of EDVAC in 1945. He emphasized the necessity of an “internal” memory distinct from external devices such as punched cards. This memory had to preserve intermediate results, instructions, function tables, initial conditions, and results of successive iterations.
Despite its ingenuity, this technology suffered from uncertain reliability and high costs. Manufacturers preferred magnetic drum memory. IBM 650 systems were thus equipped with drums capable of storing up to 2,000 ten-digit words, with an average access time of 2.4 milliseconds, limited by the drum’s rotation at 12,500 revolutions per minute.
EDVAC illustrated this transition: in 1954, a 4608-word drum memory complemented its delay lines. Drums remained present until the 1960s, primarily as secondary memory.
Delay line memory proved that large-capacity memories could be built separate from computing units, thus realizing von Neumann’s architecture. It was the founding concept of our current memory hierarchy.
The 1960s saw the advent of magnetic cores. Without moving parts, they offered truly random access with cycle times of approximately 6 microseconds—a thousand times faster than drums. Capacities exploded: the 1965 CDC 6600, clocked at 100 nanoseconds, had approximately 128,000 words accessible in 1 microsecond.
The delay line symbolizes the delicate balances sought by pioneers between cost and performance, reliability and capacity, complexity and feasibility. It testifies to the inventiveness of early computer architects, confronted with challenges using the technical means of the 20th century.
The principle of data circulation and regeneration it implemented is found, transformed, in contemporary DRAM memories. Its trajectory illustrates a recurring pattern in computing history: a technical innovation overcomes an obstacle before being supplanted by more efficient solutions.
Delay line memory represents the moment when computers acquired true working memory, distinct from computing units and permanent storage. This separation, which has become axiomatic in modern computer architecture, found its first concrete expression with this technology.