| Sussman and Steele | December 22, 1975 | 27 | Some Implementation Issues |
slicing the interpreter among several processes. One can even model this in substitution semantics by supposing that there can be more than one expression, and at each step an expression is randomly chosen to perform a reduction within. (On the other hand, synchronizing of the processes is very hard to model using substitution semantics!)
Since our value cells effectively solve the readers and writers problem (i.e. reads and writes of variables are indivisible) no more than our side effect primitive is necessary to synchronize our processes [Dijkstra] [Knuth] [Lamport]. However, the techniques for achieving synchronization using only := are quite cumbersome and opaque. It behooves the implementor to make things easier for the user by introducing a more tractable synchronization primitive (e.g. P+V or monitors or path expressions or ...). Machine language programmers have long known that the easiest way to synchronize processes is to turn off the scheduling clock during the execution of critical code. We have introduced such a primitive, EVALUATE!UNINTERRUPTIBLY, (which is a sort of "over-anxious serializer", because it locks out the whole world) into our interpreter.
