1. A system for a millicode store access checking instruction, the system comprising: an operand access control register (OACR) including a test modifier indicator; an instruction unit subsystem for fetching and decoding instructions, wherein the instructions include a millicode instruction with an operand defining an address to check for a store access exception; and an execution unit to execute the millicode instruction, the execution unit performing a method comprising: receiving the millicode instruction from the instruction unit subsystem; testing for the store access exception at the address as if the test modifier is set without regard to the state of the OACR; and outputting a result of the testing for the store access exception.