BCL Molecular 18

The BCL Molecular 18 was a range of 18-bit computers designed and manufactured in the UK from 1970 until the late 1980s.

The machines were originally manufactured by Systemation Limited and serviced by Business Mechanisation Limited. The two companies merged in 1968 to form Business Computers Limited - a public limited company. Business Computers Ltd subsequently went into receivership in 1974.

It was purchased from the receiver by Computer World Trade, maintenance of existing machines was by a subsidiary of CWT called CFM, manufacturing was passed to ABS Computer in the old BCL building and sales rights were sold to a team from the old Singer Computers by 1976 trading as Business Computers (Systems) Ltd selling the Molecular.

BC(S) Ltd subsequently went public in 1981 to form Business Computers (Systems) Plc. Servicing and manufacturing was gradually taken over by Systemation Services/ Systemation Developments Ltd. BC(S)Plc was eventually taken over by Electronic Data Processing (EDP)

The Molecular was an 18 bit, dual accumulator, programmable computer, with a typical minicomputer architecture.

The Molecular could address up to 64K words of memory. In later models, memory extensions were made available by the use of bank switching. The top 32K memory could be switched between four banks, giving an overall memory limit of 160K words. There was a complex (for the time) instruction set, a simple interrupt system, multiple input/output ports, and Direct Memory Access or Data Channel for high speed peripherals.

No industry standard operating system was supported, but in later years an standard control program was installed on all machines. The control programs produced, LOS and OS, gave the machine it's multi-user function. Note that the task scheduling was co-operative, pre-emptive multi-tasking was never developed.

There was a gradual progression in the models available, although the basic instruction set didn't change over the years. (Actually the Mk 1 machines didn't support multiple shift and rotate instructions.)

The primary use for the Molecular was in commercial systems, particular distribution companies - hence the name change to Distributor - one of which is shown alongside.

Launched in 1972, the BCL Molecular 18 remained in production until the early 1990s. Although the physical construction changed over the years, the logical design remained very much the same.

The Leicester Operating System

LOS has been developed by BC(S)L to meet the multi-programming needs of Molecular Mk I, 6M, Mk III and Series IV users. At December 1978 it was in use at about three dozen installations, ranging in size from two input stations and one printer (running in 16K word core) up to eight input stations and two printers (running in 32K word core). Larger installations are planned, including remote peripherals linked by the GPO Datel service.

The Molecular word size is 18 bits, and the basic memory is 32K words.  The machine was generally known as the Molecular-18 for this reason. The MSB (Bit 18) was a parity bit, so for all intents and purposes it was a 17 bit machine.

The minimal CPU contained the following registers:

PC - Program counter
MA - Memory address
REGA - Register A
REGB - Register B
C - Carry Flag
GT - Greater Than Flag

Unfoirtunately we just have the keyboard unit

Manufacturer: BCL
Date: 1972

BCL Molecular 18 Articles:

Item Manufacturer Date
Memories - BCL SUSIE MARK 2 27-07-2010

Other Systems Related To BCL Molecular 18:

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BCL Susie BCL 1967
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This exhibit has a reference ID of CH30776. Please quote this reference ID in any communication with the Centre for Computing History.


BCL Molecular 18

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