Essentials of Computer Organization and Architecture, 5th Edition

5.2   INSTRUCTION FORMATS

We know that a machine instruction has an opcode and zero or more operands. In Chapter 4, we saw that MARIE had an instruction length of 16 bits and could have, at most, one operand. Encoding an instruction set can be done in a variety of ways. Architectures are differentiated from one another by the number of bits allowed per instruction (16, 32, and 64 are the most common), by the number of operands allowed per instruction, and by the types of instructions and data each can process. More specifically, instruction sets are differentiated by the following features:

• Operand storage (data can be stored in a stack structure or in registers or both)

• Number of explicit operands per instruction (zero, one, two, and three ...