The production run consisted of fifty-five machines, manufactured and marketed by the British company Standard Telephones and Cables, Ltd.
[1] Large parts of the code and operating systems for ZEBRA were written by deafblind mathematician Gerrit van der Mey.
Each bit of the operation field had a distinct meaning and could be used in nearly any combination, leading to many elegant tricks that today might be considered the domain of microprogramming.
The operation bits determined things as the sign of the data to be used; if the accumulator was cleared (changing an addition into a load), if a rotation was to be applied, and so on.
Since a magnetic drum does not support random access, some time is lost waiting for an instruction or piece of data become available.