Preserving program context when adding probe routine calls for program instrumentation

Method and apparatus for preserving program context when causing execution of a probe routine from a target routine of an executable computer program code. Executable code for the probe routine is created such that the probe routine does not reference a first set of registers that are usable by the target routine. A modified version of the target routine is created to cause execution of the probe routine. The modified version of the target routine is performed instead of the original version when the target routine is called during program execution. A second set of registers on the processor register stack is allocated when the probe routine is invoked. The second set of registers is not manipulated by the probe routine so as to avoid changing contents of registers of the register stack that are used by the target routine.