1. A microprocessor having a macroinstruction set in its macroarchitecture that includes a macroinstruction that instructs the microprocessor to perform a read/modify/write operation on an operand in memory, the microprocessor comprising: an instruction translator, configured to translate the macroinstruction into exactly three microinstructions to perform the read/modify/write operation on the memory operand, wherein the three microinstructions are first, second, and third microinstructions; wherein the first microinstruction instructs the microprocessor to calculate a source address and to load the memory operand into the microprocessor from memory at the source address and to calculate a destination address; wherein the second microinstruction instructs the microprocessor to perform an arithmetic or logical operation on the loaded memory operand to generate a result; wherein the third microinstruction instructs the microprocessor to write the result to memory at the destination address calculated by the first microinstruction; a first execution unit, configured to receive the first microinstruction, to responsively calculate the source address and to load the memory operand into the microprocessor from the memory at the source address; and a second execution unit, configured to also receive the first microinstruction, to responsively calculate the destination address, wherein the first and second execution units are distinct execution units within the microprocessor.