Gamma M-40

adapted by Jean Bellec from papers from Jean-Pierre Mourieras and Pierre Labalme.


The Gamma M-40 is an exceptional venture of Compagnie des Machines Bull off its traditional market segment that was (and later remained) the business data processing.

Pierre Davous, after a career in Naval Engineering, joined Bull in 1958. Somewhat frustrated by the Gamma 60 venture, he felt that the business computer should be joined by minicomputers performing dedicated tasks.  He gathered a set of engineers, some coming from the Gamma 60 staff, others were new comers. After preliminary studies by Jean Bosset, Georges Lepicard, initially hired from CAE to study process control applications proposed to him the structure of a general purpose 24-bits computer that had state-of-the art performances and could be buit using the new SP-2 technology designed by Christian Joly.
The project did not belong to the main stream of the company business and was tolerated more than supported by the Sales operations. Only a handful of French salesmen were interested. The marketing support was initially performed by a team headed by Hervé Lhomme for automation applications and Philippe Bück for scientific ones. Upper management of the company supported the project considering that the development of M-40 was giving  Bull some leverage in the complex negotiation with the French government and CSF in 1963-1964.

The M-40 project started at the end of 1962 as a process-control computer venture. Process control was a hot subject at that time, many American start-ups (such as Digital Equipment) were founded on that market, several electronics companies involved in automation wanted their electronic computer. Computers designed for the military market may found a place in that market.
Bull considered for some time to market a RCA process control computer developed in 1960 by the company Bull was licensed from. RCA110 proves too expensive for the French market and unsuccessfully bid against its competitors in the military and energy markets, dominated in France by the CSF group company CAE with Ramo-Wooldridge computers.

Pierre Davous was heading a group of engineers (Division 8) dealing with several requirements of this, then, promising civil process control market. Maurice Bataille was responsible of process control peripherals (A/D converters, switches, displays).
Georges Lepicard started the design of a computer of the class of the Ramo-Wooldridge RW-530.

Very early, it was identified that such a computer could also be used as a medium size scientific computer. As the Gamma 60 project was winding down, a team of software engineers lead by François Sallé was allocated to give to develop on  M-40 (its project code name) a modern set of general purpose software.

The M-40 prototype was running in September 1964 and was presented in October of the same year at SICOB.

Bull succeeds to secure an order from Elf for automating the oil refinery of Feyzin. Finally, in the middle of the financial turmoil of the General Electric take-over, Bull decided an introduction under the name Gamma M-40, without changing its code name into a more consistent marketing identifier.


The Bull M-40 was built, as a classic mini-computer, around a magnetic core memory and a single processor directly controlling peripherals. An optional unit of synchronous channels was used for high thruput instrumentation devices and allowed the asynchronous control of peripheral magnetic storage devices.

M-40 was one of first implementation by Bull (in parallel with Gamma 55) of a control memory interpreting the software visible instruction code. A ROM control memory allowed a rich instruction set with a relatively simple hardware logic. It was initially envisioned to develop special versions of firmware according to customer requirements, but that was never implemented for anything other than firmware improvement releases.

The architecture at the software level had the following characteristics:

Data types were the following:

The instructions had a length of 24-bits (one word).
Operations include fixed and floating point arithmetics, character strings handling, logical operations on words, characters or bits, stack operations.

A comprehensive system of program interrupts (a first in Bull) includes priority handling (15 levels), real-time and clock interrupts, supervisor call and asynchronous channels.

A memory protection mechanism isolates the address space of a program at a 1024 words boundary.

gammam-40 system.gif (46196 octets)


Core memory

The M-40 had a magnetic core main memory   of 4096 to 32768 words of 24-bits. The cycle time was 5 µs.


The control store is made of 2048 words of 50 bits. The technology is read-only magnetic rods wired at manufacturing time.

Additions are performed in 10 µs (fixed point- 24bits) and 80µs (floating point)
Multiplications               in 50 µs                              and 185 µs
Divisions                      in 80 µs                              and 350 µs
Data move                   in 50 + n*4.6  µs


The processor controls three console specific channels,  one "normal" channel and up to 3 "simultaneous channels"

Console channels (typewriter, paper tape reader, paper tape punch) are handled by the operating system through program interrupts.

The software visibility of normal and simultaneous channels is identical. Up to 16 devices can be connected to each channel. The "normal" channel is handled by the processor firmware and suspend the execution of the program. "Simultaneous" channels operate independently and end of channel operation results in software interrupts.



The Gamma M-40 software was designed in parallel with hardware by a team belonging to the Direction Commerciale led by François Sallé. The standard software had to reserve a real-time capability customized for applications and offered a standard operating system usable interactively or via tape batch processing. As Gamma 30 and later GE-400 took over the business applications market, no COBOL nor business oriented tools were developed.

The M-40 Software was developed on a simulator running on a Control Data 3600 at Sema/SIA.

Operating System

A supervisor program resident in main memory was controlling the interrupt mechanism,  and handled I/O operations, emulating asynchronous I/O for the console devices. A monitor perform sequential execution of batch jobs. Among the operating system designers, are Claude Chemla and Pierre Blanchard.

A special time-sharing subsystem handling up to 10 IBM Typewriters and allowing each of them to enter, compile and execute LSA programs was developed by Michael Spier and André Bensoussan in 1964.



The Feyzin M-40 was entirely supported by the Direction des Etudes. Marketing the scientific systems was initially regrouped with CAB-500 group under Michel Moutou.

only 13 M-40 systems were manufactured and installed . The first shipment occured in November 1964.


In 1965, the Gamma M-40 was the hero of Jean-Luc Godard's political fantasy, the "Alphaville" movie starring Eddie Constantine and Anna Karina, one of the first movie starring an electronic computer.



This first incursion of Bull into the minicomputer field had no follow-on. Bull introduced Honeywell the Level 6 on that segment in the early 1970s. Level-6 Bull's sales were focused on business processing and the French process control was dominated by CII (prior to 1975) and SEMS (Mitra and Solar) that eventually merged with Bull.

The M-40 project contributed significantly to the eventual success of medium-sized computers designed by Bull. In fact, the original M-40 designers were the kernel of the hardware team that had designed the Honeywell Level 64.


Revision : 07 mai 2003.