1. A method for performing reallocation, said method comprising: initially allocating from a set of physical registers one or more architected registers and a pool of multiple rename registers; allocating from said pool of rename registers an initial number of scratch registers for storing microcode operands; in response to detecting that a fetched instruction is targeted to a scratch register requiring an additional scratch register beyond said initial number, reallocating a selected physical register from among said pool of rename registers as said additional scratch register; setting a flag to indicate said rename register is allocated as said additional scratch register; and in response to determining that said additional scratch register is no longer needed, deallocating said additional scratch register and resetting said flag, such that said selected physical register returns to said pool of rename registers.