Ferranti designed a variant of the system for the University of Cambridge, called Titan or Atlas. This had a different memory organization, and used an operating system developed by the Cambridge Computer Lab, which allowed time-sharing.
The Atlas system of the University of Manchester was withdrawn in 1971. The last Atlas computer was in service until 1974. Parts of the Chilton Atlas are conserved in the National Museum of Scotland, in Edinburgh.
Extracode: An interesting feature of the Atlas was the extracode, a system that allowed new instructions to be added in the software (firmware in modern terminology). There were about 250 extracodes.
The upper part of the ten bits of a 48-bit Atlas instruction machine denoted which operation should be performed. If the most significant bit was set to zero, it was a simple instruction machine executed directly by the hardware.
If the upper bit was set it was set to one, it was an Extracode and it was implemented as a special type of jump subroutine to a fixed address in the store (ROM), where that address was determined by the other nine bits. Extracode mode had its own program from the opposite direction.
Many of the extracodes were what today could be called microcodes, they were simple calculation procedures that would have been too inefficient to implement them in hardware, for example, sine, logarithm, square root. However, about half of the codes were designed as Supervisor functions, which invoked operating system procedures. Typical examples would be "Print the specified character in the specified stream" or "Read a block of 512 logical tape words N".
Extracodes were the only means by which a program could communicate with the Supervisor program.
Software: A single control program known as the Atlas Supervisor managed the processing time of the computer (which is qualified in modern terminology as an advanced task scheduler, or simply operating system).
One of the first high-level languages available in the Atlas was named AutoCode Atlas, which was contemporary to Algol60 and created specifically to address the shortcomings of that language that Tony Brooker perceived. However, the Atlas supported Algol 60, as well as Fortran and COBOL. Being a university machine, it was sponsored by a large number of the student population who even had access to a machine protection code for the development of the environment.