EDSAC order code

{{stub}}{{language|EDSAC order code}} The Electronic Delay Storage Automatic Calculator (EDSAC), which went into operation at Cambridge in 1949, was one of the first stored-program computers. It was also the machine on which the first programming book—The Preparation of Programs for an Electronic Digital Computer (1951), by Wilkes, Wheeler, and Gill—was based.

The EDSAC featured a memory of 512 17-bit words, later upgraded to 1,024, and a rather sophisticated loader (Initial Orders) in read-only memory. The machine's 'order code' (instruction set) was designed to be partly mnemonic, subject to the restriction that each 'order' was encoded using a single character: so A stood for 'add', S for 'subtract', etc. The Initial Orders also provided for relocatable code, with the character θ denoting 'offset from base address'.

The original EDSAC was retired in 1958, but simulators exist allowing EDSAC programs to be run under Windows, GNU Linux, or OS X, and also in the browser. These simulations additionally permit the use of comments, newlines, and whitespace, to make the orders more readable; and they employ ASCII characters in place of the handful of Greek letters used in EDSAC code (@ instead of θ, for instance). Work is underway at the National Museum of Computing in Bletchley Park to construct a full working replica of the EDSAC.