1. In a data processing system, the combination comprising: an instruction storage mechanism for storing instructions belonging to a plurality of different programs and including an address mechanism for accessing the stored instructions; an instruction execution mechanism for receiving and executing the accessed instructions one at a time; a time slice control mechanism for controlling the sequence in which the instructions are supplied to the execution mechanism for causing the instructions from the different programs to be executed in an interleaved manner by supplying them to the execution mechanism during interleaved time slice intervals and including circuitry for issuing a program identifier for each time slice interval and for supplying each program identifier to the instruction storage address mechanism for determining the program to be accessed during a particular time slice interval; circuitry for supplying condition signals representing the occurrences of predetermined conditions in the data processing system; and a function modification mechanism responsive to the occurrence of a predetermined program identifier and one of the condition signals for modifying the function of at least one of the instructions in the program identified by such program identifier.