Classic Computing



The Molecular18 computer system.

The Molecular was an 18 bit, dual accumulator, programmable computer, with a typical von Neumann 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.

Product Information Programming and Restoration Progress
Summary of models LOS Programming Manual
FAQ page on the molecular Programming guide
What made the Molecular special Joe Templeman's LOS Manual
Molecular users pages Molecular 18 Assembler
Molecular 18 Emulator (Coming soon)
Restoring a Mark 4


Specification (Taken from Computing 16th August 1973)

Supplier Model Cycle Speed in us Word size Memory range (k) Typical Cost Software Available
Business Computers Ltd Molecular 18 1.6 18 4K - 32K 4,400 - 70,000 Assembler, Macro Processor, Multiprogramming operating system, RPG II, and applications.
Digital Equipment Co. Ltd PDP8/E 1.2 12 4K - 32K 15,000 Algol, Basic, Dibol, Focal, OS, and Fortran
Allied Business Systems GR1-99 1.76 16 4K - 32K 7,500 File Management, VDU handlers, Disk operating system, full real time executive.


I would suggest that the list of software available for the Molecular 18 is somewhat exaggerated!

I have included the entry for the PDP8 to allow some comparison with a machine that is more generally known.

I have included the GR1-99 machine from Allied Business Systems, because I think that machine shares a great deal with the M18.   Prior to the Mark 5, the M18 range was manufactured by ABS.

(C) Kevin Murrell, January 2000