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In some cases, the term refers to a software scheme that simulates a stack machine. Most computer systems implement a stack in some form to pass parameters and link to subroutines. This does not make these computers stack machines. Most of its instructions assume that operands will be from the stack, and results placed in the stack. The computer replaces those two values by the sum, calculated by the computer when it performs the “Add” instruction.
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.