Stack machine

In computer science, computer engineering and programming language implementations, a stack machine is a real or emulated computer that uses a pushdown stack rather than individual machine registers to evaluate each sub-expression in the program. A stack computer is programmed with a reverse Polish notation instruction set.

The common alternative to stack machines are register machines, in which each instruction explicitly names the specific registers to use for operand and result values.

Practical expression-stack machines

A stack machine implements registers with a stack. The operands of the arithmetic logic unit (ALU) are always the top two registers of the stack and the result from the ALU is stored in the top register of the stack. "Stack machine" commonly refers to computers which use a Last-in, First-out stack to hold short-lived temporary values while executing individual program statements. The instruction set carries out most ALU actions with postfix (Reverse Polish notation) operations that work only on the expression stack, not on data registers or main memory cells.