OperandStack

An operand stack is a last-in, first-out data structure used by many programming language runtimes to hold intermediate values during the evaluation of expressions. It is typically part of a function or method call frame and stores operands and results produced by instructions or bytecode. Values placed on the stack can include integers, floating-point numbers, references, and other primitive types, with multi-word values occupying multiple stack slots in some implementations.

Operations commonly supported by an operand stack include pushing values, popping values, peeking at the top element, and stack-manipulation operations such as duplicating or swapping the top elements. The stack is usually thread-local to a single call frame and grows and shrinks as instructions execute. Proper management is essential to prevent underflow (removing from an empty stack) or overflow (exceeding the allocated space). In many runtimes, 64-bit values occupy two stack slots, and certain instructions operate on the top of the stack only.

Usage examples include the Java Virtual Machine and WebAssembly, which implement code using an operand or value stack to hold intermediate results during instruction execution. In a JVM frame, the operand stack holds results of operations until they are consumed by other instructions. The concept contrasts with register-based architectures, where operations directly use registers rather than a stack. Understanding the operand stack is fundamental to interpreting or compiling stack-based bytecode and to reasoning about performance and security implications, such as stack growth and bounds checking.