Editing Clean (programming language) (section)

=== The ABC machine ===
The ABC code mentioned above is an [[intermediate representation]] for an [[abstract machine]]. Because machine code generation for ABC code is relatively straightforward, it is easy to support new architectures. The ABC machine is an [[Imperative programming|imperative]] abstract [[graph rewriting]] machine.<ref>{{cite thesis|type=PhD|last1=Koopman|first1=Pieter|title=Functional Programs as Executable Specifications|date=10 December 1990|publisher=Katholieke Universiteit Nijmegen|isbn=90-9003689-X|page=35}}</ref> It consists of a graph store to hold the Clean graph that is being rewritten and three stacks:
* The A(rgument)-stack holds arguments that refer to nodes in the graph store.
* The B(asic value)-stack holds basic values (integers, characters, reals, etc.). Although these values could be nodes in the graph store, a separate stack is used for efficiency.
* The C(ontrol)-stack holds return addresses for flow control.
The [[runtime system]], which is linked into every executable, builds a <code>Start</code> node in the graph store and pushes it on the A-stack. It then begins printing it, evaluating it as needed.