Central to instruction set design is the issue of the instruction format. We discuss some common format encoding techniques, and introduce a method of representing high-level language parse trees by means of formats that cover successively larger portions of a tree. We then introduce variations on the method that represent directed acyclic graphs as well as simple parse trees, and that encode constants in a special fashion. For a particular representation, we measure the number of times each format is executed to run a sample program to completion. Copyright © 1985 by The Institute of Electrical and Electronics Engineers, Inc.