Abstract: A microprocessor is provided which is configured to detect the presence of segment override prefixes in instruction code sequences being executed in flat memory mode, and to use the prefix value or the value stored in the associated segment register to control the selection of a bank of registers to use for the instruction operands. Each bank of registers includes the full complement of AMD 80x86 Series registers. Additional registers are available to a program other than the AMD 80x86 Series architecture specifies, but the instruction encoding is unchanged. Having more registers available to a program allows for more operands to be stored in the registers. Since registers are accessible in a shorter period of time than memory, operand access time is decreased.