 This distinction between explicit and implicit operands is important in code generators, especially in the register allocation and live range tracking parts.A good code optimizer can track implicit as well as explicit operands which may allow more frequent constant propagation, constant folding of registers (a register assigned the result of a constant expression freed up by replacing it by that constant) and other code enhancements.A computer program is a list of instructions that can be executed by a central processing unit (CPU).