The Flex Computer System was developed by Michael Foster and Ian Currie of

Royal Signals and Radar Establishment The Royal Signals and Radar Establishment (RSRE) was a scientific research establishment within the Ministry of Defence (MoD) of the United Kingdom. It was located primarily at Malvern in Worcestershire, England. The RSRE motto was ''Ubique ...

(RSRE) in Malvern, England, during the late 1970s and 1980s. It used a tagged storage scheme to implement a

capability A capability is the ability to execute a specified course of action or to achieve certain outcomes. As it applies to human capital, capability represents performing or achieving certain actions/outcomes in terms of the intersection of capacity an ...

architecture, and was designed for the safe and efficient implementation of strongly typed procedures. The hardware was custom and microprogrammable, with an operating system, (modular) compiler, editor, garbage collector and filing system all written in

ALGOL 68RS ALGOL 68RS is the second ALGOL 68 compiler written by I. F. Currie and J. D. Morrison, at the Royal Signals and Radar Establishment (RSRE). Unlike the earlier ALGOL 68-R, it was designed to be portable, and implemented the language of the Revis ...

. There were (at least) two incarnations of Flex, implemented using hardware with writable

microcode In processor design, microcode (μcode) is a technique that interposes a layer of computer organization between the central processing unit (CPU) hardware and the programmer-visible instruction set architecture of a computer. Microcode is a la ...

. The first was supplied by Logica to an RSRE design, and the second used an

ICL ICL may refer to: Companies and organizations * Idaho Conservation League * Imperial College London, a UK university * Indian Confederation of Labour * Indian Cricket League * Inorganic Chemistry Laboratory of the University of Oxford * Israel Ch ...

PERQ. The microcode alone was responsible for storage allocation, deallocation and garbage collection. This immediately precluded a whole class of errors arising from the misuse (deliberate or accidental) of pointers. A notable feature of Flex was the tagged, write-once filestore. This allowed arbitrary code and data structures to be written and retrieved transparently, without recourse to external encodings. Data could thus be passed safely from program to program. In a similar way, remote capabilities allowed data and procedures on other machines to be accessed over a network connection, again without the application program being involved in external encodings of data, parameters or result values. The whole scheme allowed

abstract data types In computer science, an abstract data type (ADT) is a mathematical model for data types. An abstract data type is defined by its behavior ( semantics) from the point of view of a '' user'', of the data, specifically in terms of possible values, ...

to be safely implemented, as data items and the procedures permitted to access them could be bound together, and the resulting capability passed freely around. The capability would grant access to the procedures, but could not be used in any way to obtain access to the data. Another notable feature of Flex was the notion of shaky pointers, more recently often called

weak reference In computer programming, a weak reference is a reference that does not protect the referenced object from collection by a garbage collector, unlike a strong reference. An object referenced ''only'' by weak references – meaning "every chain of r ...

s, which points to blocks of memory that could be freed at the next garbage collection. This is used for example for cached disc blocks or a list of spare procedure work-spaces. COMFLEX, a packet switching network capable of transmitting data at magnetic-disc speed, was developed alongside Flex. It made feasible the use of remote file-stores, remote capabilities, and

remote procedure call In distributed computing, a remote procedure call (RPC) is when a computer program causes a procedure (subroutine) to execute in a different address space (commonly on another computer on a shared network), which is coded as if it were a normal (lo ...

See also

References

Further reading