A method of reducing a code size of a program by controlling a control flow of the program using software in a computer system is disclosed. The method includes the steps of storing a first program count of a first instruction in a first buffer when an error occurs while the first instruction having an Operand including Offset and Length is being executed among a plurality of instructions loaded in the code memory, changing a current program count of the code memory to a second program count which is obtained by adding the Offset to the first program count, storing a second instruction, which is located at a position shifted from the second program count by a value of the Length, in a second buffer, replacing the second instruction with a third instruction, which is not recognized by a microprocessor, replacing the third instruction with the second instruction stored in the second buffer when an error occurs while the third instruction is being executed, and changing the current program count of the code memory to a predetermined program count next to the first program count stored in the first buffer.