The ICL Computer Museum


ICL  System 25/1 Desktop Computer for 1 or 2 Users  c1983

This machine was based on the architecture of the Singer System Ten machines, when ICL acquired Singer Business Machines in 1976 these were developed as their upgrade replacement.

They have remarkably good emulation of a System Ten machine.  There are four different models of the System 25 machines, the S25/1, the S25/2, the S25/3 and the S25 Plus.

The S25/2 and the S25/3 were launched during 1981, the S25/1 and the S25 Plus were launched during 1983.  The S25 pioneered 'Winchester' fixed disc drives within ICL.

The S25/1 and the S25/2 have an emulation speed of 56k ips, the S25/3 has an emulation speed of 70k ips and the S25 Plus has an emulation speed of 70k ips with extended instructions (from 25 to 35).

The S25/1 is a reduced-configuration S25/2, artificially restricted to 1 or 2 users and is of the Desktop type of machine.  The S25/2, S25/3 and S25 Plus are housed as Floor Standing type and multi-user.

A unique feature of the ICL System 25 is that there is no CPU, operating system or executive. The functions normally associated with such software being provided by the basic hardware architecture.

The ICL System 25/1 can be described as a network of micros in a box, or a distribution processing system in a box. No CPU, so the 'OS', apart from DMF III, is a set of microprograms.

This machine as can be seen, looks very much like the PC's of the day, but is very much not like a PC in operation. (ICL at it's best.)

Architecture:  9MBps highway connected:  System control processor - 80bit-slice.  System Ten instruction  processor - 96bit-slice.  File storage control 8085 for disc and mag tape subs on its own bus.

Terminal/printer control 8085 also with its own bus, with re-engineered System Ten MTIOC and MDIOC interfaces. MTIOC up to 10 VTs, MDIOC up to 16 POS or factory data collection terminals.

Each IOC associated with a program partition (up to 20) on S25, each IOC micro-controlled.

This machine has both a 35MB Micropolis fixed disc and a 5.25 inch floppy drive attached.

The machine also has a three channel communications unit attached (channel's 0, 1, and 2).

It is also the only machine of its type that the Museum has ever come across.


Many thanks to Michael Knight for the information on the ICL System 25/1.

Michael Knight was System Ten/25 product line marketing manager at ICL, 1979-84.