Zero-address instruction format: Instruction contains only opcode, zero address fields. Operand sources are implicit stack top. Architecture is stack-organized. Example: ADD, SUB, PUSH X, POP. Execution: PUSH A (A→TOS), PUSH B, ADD → pop TOS-1+TOS → push result. Advantage: short instruction, good for expression compilation; disadvantage: many stack operations, not flexible for random access.