Stack architecture

In computer science, computer engineering and programming language implementations, a stack machine is a type of computer. In some cases, the term refers to a software scheme that simulates a stack machine. The main difference from other computers is that most of its instructions operate on a pushdown stack of numbers rather than numbers in registers. A stack computer is programmed with a reverse Polish notation instruction set. Most computer systems implement a stack in some form to pass parameters and link to subroutines. This does not make these computers stack machines.

The common alternatives to stack machines are register machines, in which each instruction explicitly names specific registers for its operands and result.

A "stack machine" is a computer that uses a last-in, first-out stack to hold short-lived temporary values. Most of its instructions assume that operands will be from the stack, and results placed in the stack.

For a typical instruction like "Add," the computer takes both operands from the topmost (most recent) values of the stack. The computer replaces those two values by the sum, calculated by the computer when it performs the "Add" instruction. The instruction's operands are 'popped' off the stack, and its result(s) are then 'pushed' back onto the stack, ready for the next instruction. Most stack instructions have only an opcode commanding an operation, with no additional fields to identify a constant, register or memory cell. The stack easily holds more than two inputs or more than one result, so a richer set of operations can be computed. Integer constant operands are often pushed by separate Load Immediate instructions. Memory is often accessed by separate Load or Store instructions containing a memory address or calculating the address from values in the stack.

For speed, a stack machine often implements some part of its stack with registers. To execute quickly, operands of the arithmetic logic unit (ALU) may be the top two registers of the stack and the result from the ALU is stored in the top register of the stack. Some stack machines have a stack of limited size, implemented as a register file. The ALU will access this with an index. Some machines have a stack of unlimited size, implemented as an array in RAM accessed by a 'top of stack' address register. This is slower, but the number of flip-flops is less, making a less-expensive, more compact CPU. Its topmost N values may be cached for speed. A few machines have both an expression stack in memory and a separate register stack. In this case, software, or an interrupt may move data between them.

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