ICT 1900

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ICT 1900 is the name given to a series of mainframe computers released by International Computers and Tabulators (ICT) in the 1960s. These varied in computing power and the models included:

ICT 1901
ICT 1902
ICT 1903
ICT 1904
ICT 1905
ICT 1906
ICT 1907
ICT 1909 (this machine had a hardware floating point arithmetic engine so it was suitable for scientific use.)

The first machine was in fact the 1904 which was a Canadian design fom the Ferranti-Packard company, originally called the FP6000. The story is that this machine with 'core store memory' fired up with its program still in store after its sea freight journey from Canada.

One feature of these mainframes was the common instruction set throughout the range meaning that programs written and compiled on one machine would run unchanged on any other. In fact the hardware was different between machines. To achieve this a program termed "the executive" or exec encapsulated the hardware and supplied software routines to supplement the hardware supplied instructions.

By 1968 ICT had merged with English Electric computers and become ICL.

Enhanced versions of the 1900 series subsequently appeared with an A suffix, an E suffix, an F suffix (for floating point), a T suffix, and an S suffix, e.g.

Stevenage designed and built machines

ICT 1901A
ICT 1901S/T?
ICT 1902A
ICT 1902S
ICT 1902T
ICT 1903A
ICT 1903S

West Gorton (Manchester) built machines

ICT 1903T
ICT 1904A
ICT 1904T
ICT 1904S
ICT 1906S

The 1900 Series were 24-bit word machines (supporting 4 6-bit characters per word) and using octal for binary short-hand, as opposed to the IBM Systems using 8-bit bytes and hex. Basic memory on the smaller machines was 16K words (or 64 kilobytes equivalent), and there were even 8 kW versions - early machines using 'core-store' memory (ferite rings on a copper wire matrix) and operated on binary hand-switches on the mainframe cabinet. Despite the apparent small memory size, quite sophisticated applications were run on the equipment and computer programmers paid great attention to the efficient use (and reuse) of memory. I/O consisted of 40 column cards (with round holes) and 8 track paper tape - print was from a solid barrel line printer (120 columns wide).

Disk capacity was also very limited (early units supporting 4 or 8Mb removable multi disc packs) (1903 system first) (and Steve Hillel crashed the first unit when demonstrating it to his manager) and similar attention to ensuring the efficient use of disk space was common. Early machines used storage on reels of magnetic tape and were then augmented by direct access devices (disks) typically with disk capacities of 1.6Mb, 4Mb and 8Mb were the order of the day. By the time 30Mb packs were available they occupied a cabinet 4 feet high (MEDS - Multiple Exchangeable Disk Store).

After ICL had introduced the 2900 series of computers during the 70's the 1900 series in fact lived on as there was such a wealth of software written for it. ICL introduced the 2903 and 2904 which were designed and built in Stevenage, which were really successors to the 1902A/3A machines as they had the same target instruction set, while the 2960 machine from West Gorton, which had a new native instruction set, could also run 1900 series code as an emulation. Stevenage later designed and built the 2940/50 based on the same central processor as the 2960, but again able to emulate its predecessor 1900 series code.

Programming languages PLAN (Programming Language Assembler Nineteen-hundred) and latterly COBOL were used for the development of commercial-orientated systems. ALGOL and FORTRAN were used for scientic work.

The basic operating System was called "Executive" and supported "multiprogramming", i.e. it was capable of running as much as 4 programs concurrently. Later on (starting about 1968) advanced features (e.g. batch processing, spooling) were provided by a privileged ("trusted") program called GEORGE (GEneral ORGanisational Environment), and so GEORGE1, GEORGE2/GEORGE2S, GEORGE3 and GEORGE4 - paging etc. were born.

The University of Leeds has created a software emulator that runs George 3 on a PC, which can be found at http://www.bcs.org.uk/sg/ccs/g3 .