The KDF-9 Computer System

Abstract

The English Electric KDF.9 computing system has a number of unusual features whose origins are to be found in certain decisions reached at an early stage in the planning of the system. At this time (1958--59) simplified and automatic programming procedures were becoming established as desirable for all programming purposes, whereas previously they had been regarded as appropriate only to rapid programming of one-off problems because of the drastic reductions of machine efficiency which seemed inevitable.Many early interpretive programming schemes aimed to provide an external three-address language, and for a time it appeared that a machine with this type of internal coding approached the ideal. Increasing interest in translating programs, particular for problem languages such as ALGOL and FORTRAN, showed the fallacy of this assumption. It became evident that efficient translation could only be achieved on a computer whose internal structure is adapted to handle lengthy algebraic formulae rather than the artificially divided segments of a three address machine.The solution to the difficulty was found in the use of a system of working registers. This consists of a number of associated storage positions forming a magazine in which information is stored on a last in, first out basis. It is shown that this basic idea leads to development of a computer having an order code near to the ideal for evaluation of problems expressible in algebraic form.A number of other significant advantages arise directly from the nesting store principel, chief among them being a striking reduction in the program storage space required. This is due to elimination of unnecessary references to main store addresses and to the implicit addressing of operands in the nesting store itself. Many instructions are therefore concerned with specifying only a function, requiring many fewer bits than those instructions involving a main store address. Instructions are therefore of variable length to suit the information content necessary and on average three instructions may occupy a single machine word (48 bits). This again reduces the number of main store references, allowing the use of a store of modest speed while still allowing adequate time for simultaneous operation of a number of peripheral devices on an autonomous main store interrupt basis.Fast parallel arithmetic facilities are associated with the nesting store, both fixed and floating-point operations being provided. A further nesting store system facilitates the use of subroutines, and a third set of special stores is associated with a particularly comprehensive set of instruction modification and counting procedures.Operation of the machine is normally under the control of a Director program. A number of different Directors cover a variety of operating conditions. Thus a simple version is used when only a single program is to be executed and more sophisticated versions may be used, for example, to control pseudo-off-line transcription operations in parallel with a main program, operation of several programs simultaneously on a priority basis, etc.