An modify next sequential instruction (MNSI) instruction, when executed, modifies a field of the fetched copy of the next sequential instruction (NSI) to enable a program to dynamically provide parameters to the NSI being executed. Thus the MNSI instruction is a non-disruptive prefix instruction to the NSI. The NSI may be modified to effectively extend the length of the NSI field, thus providing more registers or more range (in the case of a length field) than otherwise available to the NSI instruction according to the instruction set architecture (ISA).