English Electric KDF9 - AbsoluteAstronomy.com

KDF9 was an early British computer designed and built by English Electric

English Electric

English Electric was a British industrial manufacturer. Founded in 1918, it initially specialised in industrial electric motors and transformers...

, later English Electric Leo Marconi, EELM, later still incorporated into ICL. It first came into service in 1964 and was still in use in 1980 in at least one installation. The present article presents a synoptic overview of the architecture of the KDF9; for a more complete account, see ‘The English Electric KDF9’, and ‘The Hardware of the KDF9’, at the links below.

Architecture

The logic circuits of the KDF9 were entirely solid-state. The KDF9 used transformer-coupled diode-transistor logic

Diode-transistor logic

Diode–transistor logic is a class of digital circuits that is the direct ancestor of transistor–transistor logic. It is called so because the logic gating function is performed by a diode network and the amplifying function is performed by a transistor .- Implementations :The DTL circuit shown in...

, built from germanium diodes, about 20,000 transistors, and about 2,000 toroid pulse transformers. They ran on a 1 MHz two-phase clock. The maximum configuration incorporated 32K words of 48-bit core storage (192K bytes) with a cycle time of 6 microseconds. Each word could hold a 48-bit integer or floating-point number, two 24-bit integer or floating-point numbers, six 8-bit instruction syllables, or eight 6-bit characters. There was also provision for efficient handling of double-word, 96-bit, numbers in both integer and floating point formats. However, there was no facility for byte or character addressing, so that non-numerical work suffered by comparison. Moreover, there was no standard character set. Each I/O device type had its own more or less similar character code. Not every character that could be read from paper tape could be successfully printed, for example.

Registers

The CPU architecture featured three register sets. The Nest was a 16-deep pushdown stack of arithmetic registers, The SJNS (Subroutine Jump Nesting Store) was a similar stack of return addresses. The Q Store was a set of 16 index registers, each of 48 bits divided into Counter (C), Increment (I) and Modifier (M) parts of 16 bits each. Flags on a memory-reference instruction specified whether the address should be modified by the M part of a Q Store, and, if so, whether the C part should be decremented by 1 and the M part incremented by the contents of the I part. This made the coding of counting loops very efficient.

Instruction set

Instructions were of 1, 2 or 3 syllables. Most arithmetic took place at the top of the Nest and used zero-address
Zero address arithmetic
Zero address arithmetic is a feature of a few innovative computer architectures, whereby the assignment to a physical address space is deferred until programming statement execution time...

, 1-syllable instructions, although address arithmetic and index updating were handled separately in the Q store. Q Store handling, and some memory reference instructions, used 2 syllables. Memory reference instructions with a 16-bit address offset, most jump instructions, and 16-bit literal load instructions, all used 3 syllables.

Dense instruction coding, and extensive use of the register sets, meant that relatively few store accesses were needed for common scientific codes, such as scalar product and polynomial inner loops. This did much to offset the relatively slow core cycle time, giving the KDF9 about a third of the speed of its more famous, but much more expensive and much less commercially successful contemporary, the Manchester/Ferranti

Ferranti

Ferranti or Ferranti International plc was a UK electrical engineering and equipment firm that operated for over a century from 1885 until it went bankrupt in 1993. Known primarily for defence electronics, the Company was once a constituent of the FTSE 100 Index but ceased trading in 1993.The...

Atlas Computer.

Multiprogramming (Timesharing)

The KDF9 was one of the earliest fully hardware-secured multiprogramming systems. Up to four programs could be run at once under the control of its elegantly simple operating system, the Timesharing Director, each being locked into its own core area by BA (Base Address) and NOL (Number of Locations) registers. Each program had its own sets of stack and Q store registers, which were activated when that program was dispatched, so that context switching was very efficient. Each program could drive hardware I/O devices directly, but was limited by hardware checks to those that the Director had allocated to it. Any attempt to use an unallocated device caused an error interrupt. 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. Somewhat different was the Lock-Out interrupt, which resulted from trying to access an area of store that was currently being used by an I/O device, so that there was hardware mutual exclusion of access to DMA buffers. When a program blocked on a Lock-Out, or by voluntarily waiting for an I/O transfer to terminate, it was interrupted and Director switched to the program of highest priority that was not itself blocked. When a Lock-Out cleared, or an awaited transfer terminated, and the newly unblocked program was of higher priority than the program currently running, an interrupt to Director allowed for an immediate context switch.

Later operating systems, including Eldon 2 at the University of Leeds, and COTAN, developed by UKAEA Culham Laboratories with the collaboration of several KDF9 universities, were fully interactive multi-access systems, usually with PDP-8

PDP-8

The 12-bit PDP-8 was the first successful commercial minicomputer, produced by Digital Equipment Corporation in the 1960s. DEC introduced it on 22 March 1965, and sold more than 50,000 systems, the most of any computer up to that date. It was the first widely sold computer in the DEC PDP series of...

front ends to handle the terminals.

The Kidsgrove and Whetstone Algol 60 compilers were among the first of their class. The Kidsgrove compiler stressed optimization; the Whetstone compiler produced an interpretive object code aimed at debugging. It was by instrumenting the latter that Brian Wichmann obtained the statistics on program behaviour that led him to devise the Whetstone

Whetstone (benchmark)

The Whetstone benchmark is a synthetic benchmark for evaluating the performance of computers. It was first written in Algol 60 in 1972 at the National Physical Laboratory in the United Kingdom and derived from statistics on program behaviour gathered on the KDF9 computer, using a modified version...

benchmark for scientific computation, which inspired in turn the Dhrystone

Dhrystone

Dhrystone is a synthetic computing benchmark program developed in 1984 by Reinhold P. Weicker intended to be representative of system programming. The Dhrystone grew to become representative of general processor performance...

benchmark for non-numerical workloads.

Reminiscence

Machine code programming used an unusual form of octal, known locally as 'bastardized octal'. It represented 8 bits with three octal digits but the first represented only two bits, whilst the others, the usual three.

Within English Electric, its predecessor, DEUCE

English Electric DEUCE

The DEUCE was one of the earliest British commercially available computers, built by English Electric from 1955.It was the production version of the Pilot ACE, itself a cut down version of Alan Turing's ACE....

, had a well-used matrix package. The unreliability of valve machines led to the inclusion of a sum-check mechanism to detect single errors in matrix operations. The package used fixed-point arithmetic, in which the sum-checks were precise. However, when the corresponding package was implemented on KDF9, it used floating point, a new concept that had only limited mathematical analysis. It quickly became clear that sum checks were no longer precise and a project was established in an attempt to provide a usable check. (In floating point (A + B) + C is not necessarily the same as A + (B + C) i.e. the + operation is not associative.) Before long, however, it was recognized that error rates with transistor machines was not an issue - they either worked or did not! Consequently the idea of sum checks was abandoned. The initial matrix package proved a very useful system testing tool as it was able to generate lengthy performance checks well before more formal test packages which were subsequently developed.

Legend has it that the KDF9 was developed as project KD9 (Kidsgrove Development 9) and that the 'F' in its designation was contributed by the then Chairman after a long and tedious discussion on what to name the machine at launch - "I don't care if you call it the .......". (See also KDF8

KDF8

KDF8 was an early British computer designed and built by English Electric, later English Electric Leo Marconi, EELM, later still incorporated into ICL.-Background:During the late 1950s English Electric embarked on two major computer projects...

for the parallel development and use of a commercially-oriented computer.)

The Egdon operating system was so named because one was going to UKAEA Winfrith

Winfrith

Winfrith was a United Kingdom Atomic Energy Authority site near Winfrith Newburgh in Dorset. It covered an area on Bovington Heath to the west of the village of Wool between the A352 road and the London Waterloo to Weymouth railway line....

: in Thomas Hardy's book The Return of the Native Winfrith Heath is called Egdon Heath. Their Fortran was called Egtran. Eldon was so named because Leeds University's computer was located in a converted Eldon chapel.

During the "foot and mouth" trauma of 1968, the Grand National

Grand National

The Grand National is a world-famous National Hunt horse race which is held annually at Aintree Racecourse, near Liverpool, England. It is a handicap chase run over a distance of four miles and 856 yards , with horses jumping thirty fences over two circuits of Aintree's National Course...

was run on English Electric's Bureau KDF9 in Hartree House, Queensway

Queensway (London)

Queensway is a bustling cosmopolitan street in the Bayswater district of west London. It contains many restaurants , pubs, letting agents, and high street stores...

, London

London

London is the capital city of :England and the :United Kingdom, the largest metropolitan area in the United Kingdom, and the largest urban zone in the European Union by most measures. Located on the River Thames, London has been a major settlement for two millennia, its history going back to its...

with Raymond Glendinning leaning over the back of the line printer, reading the printout to the world on BBC radio.

External links

The source of this article is wikipedia, the free encyclopedia. The text of this article is licensed under the GFDL.