The KDF9 was designed for, and used almost entirely in, the mathematical and scientific processing fields – in 1967, nine were in use in UK universities and technical colleges.
The KDF9 was an early example of a machine that directly supported multiprogramming, using offsets into its core memory to separate the programs into distinct virtual address spaces.
Several operating systems were developed for the platform, including some that provided fully interactive use through PDP-8 machines acting as smart terminal servers.
The KDF9 used transformer-coupled diode–transistor logic, built from germanium diodes, about 20,000 transistors, and about 2,000 toroid pulse transformers.
The maximum configuration incorporated 32K words of 48-bit core storage (192K bytes) with a cycle time of 6 microseconds.
Up to four programs could be run at once under the control of its elegantly simple operating system, the Timesharing Director, each being confined to its own core area by BA (Base Address) and NOL (Number of Locations) registers.
A similar interrupt resulted from overfilling (or over-emptying) the Nest or SJNS, or attempting to access storage at an address above that given in the NOL register.
Later operating systems, including Eldon 2[4] at the University of Leeds, and COTAN, developed by UKAEA Culham Laboratories with the collaboration of Glasgow University, were fully interactive multi-access systems, with PDP-8 front ends to handle the terminals.
Within English Electric, its predecessor, DEUCE, had a well-used matrix scheme based on GIP (General Interpretive Programme).
The unreliability of valve machines led to the inclusion of a sum-check mechanism to detect errors in matrix operations.
[nb 1] The scheme used block floating-point using fixed-point arithmetic hardware, in which the sum-checks were precise.
However, when the corresponding scheme was implemented on KDF9, it used floating point, a new concept that had only limited mathematical analysis.
The truth is more mundane: the name was chosen essentially at random by a marketing manager.