An instruction system for issuing instructions from a first computer system (50-90) for execution by a second computer system (10, 20, 40) is described. The instruction system comprises instructions of a first type having an effect on the second computer system which is determinable by the first and second computer systems, and instructions of a second type, the effect of which is not determinable by the first computer system. The instructions of the second type each include a descriptor for aiding a user to determine the effect of the respective instruction at the first computer system.