A system and a method for efficient reliable execution on a simultaneous multithreading machine are provided. A processor is placed in a reliable execution mode (REM) to detect possible errors during execution of a mission critical software application. Only two threads may be configured to operate in this mode. Floating-point store and integer-transfer unary instructions may be converted to new binary instructions. Each new instruction has two source operands, each one corresponding to a different thread is specified by a same logical register number as a single source operand of the original unary instruction. All other instructions are replicated, wherein the original instruction and its twin are assigned to different threads. Simultaneous multi-threaded (SMT) floating-point logic may only be able to provide lockstep execution when it communicates using the new instruction with instantiated integer independent clusters. The new instruction cannot begin until both source operands are ready, which are subsequently compared to determine any mismatches or errors.