Converting program instructions for two-stage processors including receiving, by a preprocessing unit, a group of program instructions; determining, by the preprocessing unit, that at least two of the group of program instructions can be converted into a single combined instruction; converting, by the preprocessing unit, the at least two program instructions into the single combined instruction comprising an extension opcode, wherein the extension opcode indicates, to an execution unit, a format of the single combined instruction; and sending, by the preprocessing unit, the single combined instruction to the execution unit.