What to expect

In this chapter I look at the construction of a computer, from von Neumann and Turing to the ARM chip. Pictured above is a “mumble-tub” mercury delay line memory store from the 1951 UNIVAC computer at UPenn.

Friends” Season 2 Episode 8, “The one with the list”, first aired November 1995

Chandler: All right, check out this bad boy. Twelve megabytes of RAM, 500 megabyte hard drive. Built-in spreadsheet capabilities and a modem that transmits at over 28,000 bps.
Phoebe: Wow. What are you gonna use it for?
Chandler: Games and stuff.

We may laugh, looking back now at Chandler’s “cutting edge laptop” but it serves as a useful illustration of “Moore’s Law”. The laptop I’m writing this book on has 8GB RAM, a 500 GB hard drive and 300 Mbps wireless networking, making it roughly a thousand times more powerful while costing less than a tenth of the price of Chandler’s, which the Friends Fandom claims was a 25 MHz Compaq Contura 4/25cx


Alan Turing described the concept of a stored-program computer in 1936. John Von Neumann built on Turing’s work, explaining in 1945 how a cycle of fetch-decode-execute could allow the same memory to hold both programs and data. Freddie Williams led a team at Manchester University that built the SSEM known as “Baby” to prove his CRT memory store. Baby ran around 700 instructions per second in 1946. Success of Baby led to the Mark 1 and the Ferranti Mark 1, the first commercial computer in 1951, for which women wrote most of the programs.

Valves gave way to much faster transistors in the 1960s and this exposed the Von Neumann bottleneck, solved by the Harvard architecture of separate memories for instructions and data. Early memory stores included paper tape, magnetic tape, magnetic drum, acoustic delay lines and core rope memory until semiconductor RAM arrived in the 1960s.

Magnetic hard disk drives provided secondary storage from the 1950s onwards, with flash memory becoming popular in the 21st Century for both portable storage devices and solid-state disks (SSDs). Invented in 1979, Compact Discs (CDs) and later DVDs and Blu-Ray discs are examples of the third common storage type, the optical disk.

Computer performance is limited by the “three C’s”: clock speed, number of cores and size of cache. CPUs can be described as CISC (complex instruction set computer) or RISC (reduced instruction set computer). RISC processors have much simpler circuitry, reducing space and power consumption making them suitable for mobile devices. The ARM chip is a RISC processor in 95% of all the world’s mobile devices.

From Baby to ARM, all CPUs still contain an ALU, registers and a control unit, and perform a fetch-decode-execute cycle first described by Von Neumann in 1945.


References for architecture