In instruction-set terminology, which statement best characterizes Storage-to-Storage instructions based on where their operands reside and where results are placed?

Introduction / Context:
Instruction-set architectures (ISAs) classify instructions by where operands are fetched and where results are written. Understanding Storage-to-Storage (memory-to-memory) operations helps distinguish them from Register-to-Register and Register-to-Storage formats, which impact performance, code density, and CPU design.

Given Data / Assumptions:

• Operands can reside in registers or main memory.
• Some ISAs (for example, classic CISC) support memory-to-memory operations.
• We need the definition specific to Storage-to-Storage instructions.

Concept / Approach:
Storage-to-Storage instructions fetch both source operands directly from memory and typically store the result back to memory, minimizing explicit register use. This differs from Register-to-Storage (one operand in a register, one in memory) and Register-to-Register (both operands in registers).

Step-by-Step Solution:

Verification / Alternative check:
Examining an ISA manual that lists “MOVS”, “CMPS”, or block memory ops confirms that some instructions read two memory locations and update memory directly without explicit general-purpose register destinations.

Why Other Options Are Wrong:

• Register and memory, result in register: describes Register-to-Storage, not Storage-to-Storage.
• Two registers, result in register: describes Register-to-Register.
• All of the above: Mutually exclusive; cannot all be S–S.
• None of the above: Incorrect because S–S is well defined as memory-to-memory.

Common Pitfalls:
Assuming all modern CPUs forbid memory-to-memory ALU ops. Some CISC ISAs retain them; RISC designs prefer register-based operations for pipeline simplicity.

Final Answer:
Have both their operands in the main store.