1. A computer program product for extending an effective number of registers available to instructions of a program in a computer having a plurality of registers, each of said registers comprising a high order portion and a low order portion, each of said instructions comprising an opcode specifying an operation, each of said instructions specifying at least one register for holding a corresponding register operand, the computer program product comprising tangible storage medium readable by a processing circuit and storing instructions for execution by the processing circuit for performing a method comprising: executing said instructions comprising: responsive to an instruction being an instruction of a first subset of said instructions, accessing, by the processing circuit, a register operand consisting of only the high order portion of the corresponding register; and responsive to an instruction being one of a second subset of said instructions, accessing a register operand comprising at least in part the low order portion of the corresponding register.