Gamma 30

adapted by Jean Bellec from a paper by Claude LeBras and Jacqueline Vidal.


Compagnie des Machines BULL observed in 1960, that the Gamma 60 was too complex and too expensive to compete with the IBM 1401 that inexorably penetrated the punched card market.
Having all its resources busy in the development of Gamma 60 and in a new generation of card equipment, Bull decided to import and market the RCA 301, developed by Radio Corporation of America that belong to the same class as the 1401 (in its upper range). It could also compete with IBM 1410 that constituted the top of the medium-class of computers, well below the much more expensive IBM 7070. It could be noted that the same RCA 301 was also sold in Germany by Siemens and by ICT (1500) in United Kingdom. Each RCA licensee was working independently and no attempt was made by RCA to federate them.

Compagnie des Machines Bull installed 172 systems in its park, 125 of them had been manufactured in Angers, France. Those sales were made from 1961 to 1965.

The Gamma 30 system and software was essentially developed by RCA. Bull Engineering participation in the system was limited to the connection of some peripherals. The maintenance of software was under the responsibility of the Sales Support (Service Technico-Commercial), that also developed with RCA some parts of the software. One peculiar software effort was a not successful attempt to adapt the COBOL language and the RCA compiler to the French language. The documentation had to be translated in French and big efforts were made to hide to customers the real origin of the product.

Description of Hardware


The Gamma 30 is a character oriented machine. 6-bits characters are represented as 2 octal digits.

Instructions are dual addresses and are 60-bits long (10 characters). They include an OP-code (1 char), a complementary OP-code (1 char), A-address (4 char) , B-address (4char). Peripherals operations (commands, data transfer) are also represented in the same format.
Complementary OP-code is used to represent a length of operand (for variable length operation), the name and type) of peripheral, a counter for repeating instructions. It also specify the "mode"  in I/O transfer operations.

Instructions can be modified dynamically and the limited amount of memory caused many programmers to use (and abuse) that facility.

There was no memory protection of any kind allowing user programs to alter memory conventionally allocated to standard subroutines or to peripherals.

The system does not use the concept of program interrupts. All conditional operations requires the testing of flags positioned par previous operations.


The central unit contains a single processor. The technology is fully transistorized ("solid-state"). The Gamma 30 logic was extremely reliable for its time and the majority of maintenance problems came from the peripheral units.

Data paths are 6-bits wide.

According to the "mode" , the processor allocate to the operation a bank of specific registers. When in "normal mode", there is no parallelism of operations. Three other mode "simu", "disc", "data com" allow to share memory by transfers to or from a class of peripherals. All peripheral can also operate synchronously by normal mode operations.

A specific scientific version with floating point hardware was sold as Gamma 30S.


Central memory uses magnetic core technology.
Its minimum capacity is 10K characters (6 bits + parity). It can be extended to 20K and to 40K.
Memory is character addressable.

gamma30.gif (1295741 octets)


Description of Software

Gamma 30 software, penalized by the limited amount of memory,  did not include a general purpose operating system. The operating functions were made available as elements of libraries linked statically with the application program that was booted from tape (for production programs) or from tape.
The bootstrap was introduced from binary cards (POI "programme d'ordres initiaux)

The software library included:

The programming languages included:

Compilers and Assemblers produced binary relocatable code, allowing to run the same programs on systems of different memory, or different set of standard subroutines. A control card ELABORE specified the base address of the executable program and addresses were updated by the Loader.

The main debugging tool was, in fact, was a step by step execution of the program that was performed either in an interactive mode (at the console panel) or by printing registers and a specified part of memory at the end of each operation. That mode included also to specify an execution stop at a given instruction or after n iterations.

Data Management

The Gamma 30 software supported tape and discs sequential files. There were file labels (begin and end files). Data were stored either as fixed length blocks or as variable length blocks special-character (DA)  delineated. The data management functions, including label processing and data error detection and handling, was performed by PGF ("Programme de Gestion de Fichiers", -File management-) standard subroutines.


The Gamma 30 was introduced in shops generally equipped in Bull card equipment. Bull brought some methodology improvements in the organization of those shops. Among those, were introduced:

1401 Compatibility

RCA had developed a  special 1401 compatibility mode to handle "1401 word flags". The 1401 memory was represented by two separate images ( a data image and a flag image).
Migrations to the Gamma 30 from IBM 1401 were almost non-existent and the that   mode did not pass a real field test.

Gamma 30 folklore

A runaway Branch operation to a 0 content cell caused the instruction stored at the 0-address that contained always a character table 0123456789. That was interpreted by the processor as a READ card into address 2345. Any card staying of the card reader at that time was read into memory.

A PRINT operation specifying a "zero" or a "space" zone (also frequent in runaway programs) on  a 160 lpm printer, cause the printer fuses blowing up, all the printer hammers being selected at the same time.

Bull developed an experimental multiprogramming transaction processing system using the CMC and the Burroughs disc on a 20Kc system in 1964. The number of terminals reached half a dozen. The major problem was that discs had to be reread at the same temperature that they were written. So systematic swaps out/in were necessary on a regular basis.


While in the Bull's management mind the import of RCA 301 was originally a close-gap, the RCA successor of the 301 the RCA 3301 (a higher performance system) was announced in late 1963 as the Bull Gamma 40 (not to be confused with Bull Gamma M40 (a scientific and industrial computer). The 3301 was not planned by RCA for a long life: in early 1964, Bull was informed of the specification of the first general purpose IC (integrated circuits) RCA computer known as Spectra 7. But, in Spring of 1964, Bull was bought by General Electric, cancelled the introduction of Gamma 40, merged with the units General Electric and position the GE-400 as the replacement of the Gamma 30.


Revision : 13 juin 2002.