1. A method of decoding instructions of a computer program at a decode unit, the method comprising: receiving a first instruction at the decode unit; determining, at the decode unit, if the first instruction was invoked by a non-incremental change to a program counter; in response to determining that the first instruction was invoked by a non-incremental change to the program counter, requesting a second instruction from a fetch unit, the second instruction being an instruction immediately preceding the first instruction in the computer program; receiving the second instruction at the decode unit from the fetch unit; analyzing the second instruction at the decode unit to determine if the first instruction is a base instruction subject to modification by at least one other instruction; and in response to determining that the first instruction is a base instruction, invoking exception handling code.