The Flex machine was developed at the Defence Evaluation and Research Agency in Malvern during the 1980s. It used a tagged storage scheme to implement a capability architecture, and was designed for the safe and efficient implementation of strongly-typed procedures. (It should not be confused with either the Flex programming language or an Operating System for 6800 microcomputers).

There were two incarnations of Flex, both implemented using hardware with writable microcode. The first was supplied by Logica to an RSRE design, and the second used an ICL 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.

Another 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 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.