The modification or tuning of the micro-code in a computer that utilizes a writable control store is one method whereby a program's execution time can be improved. A method for automatically performing a microcode tuning or synthesis has been developed by Drs. Karl-gaard and Abd-Alla and is discussed in detail in [1]. Presented is an extension of this effort which allows the microcode tuning to be performed on-line on program loops. This is accomplished by gathering data on characteristics of the program during its execution, utilizing this data to generate the information required to tune the microprogram, initiating the on-line tuning procedure, and transferring to the tuned routine to complete the execution.