Which of the following do not represent memory reference instruction?
A.

1

opcode

mode = 1, opcode = 010
B.

0

opcode

mode = 0, opcode = 111
C.

1

opcode

mode = 1, opcode = 111
D.

1

opcode

mode = 1, opcode = 110
E.

1

opcode

mode = 1, opcode = 001
Choose the correct answer from the options given below:

Correct option is A

• 1 bit: I / mode bit (0 = direct, 1 = indirect)
• 3 bits: opcode

In this model:

• Memory reference instructions → opcode = 000 to 110
• Non–memory reference instructions → opcode = 111
• If I = 0 → Register reference instruction
• If I = 1 → I/O instruction

• Opcode 010 (≠ 111) → belongs to memory reference group.
  Memory reference instruction

B. mode = 0, opcode = 111

• Opcode 111 → non–memory-reference group.
• With I/mode = 0, this is a register reference instruction.
  Not a memory reference instruction

C. mode = 1, opcode = 111

• Opcode 111 → still in non–memory-reference group.
• With I/mode = 1, this becomes an I/O instruction.
  Not a memory reference instruction

D. mode = 1, opcode = 110

• Opcode 110 (≠ 111) → still in the memory reference class (e.g., ISZ).
  Memory reference instruction

E. mode = 1, opcode = 001

• Opcode 001 (≠ 111) → also a memory reference opcode (e.g., ADD).
  Memory reference instruction

• B → non–memory reference
• C → non–memory reference

1. Memory Reference Instructions
• Use opcodes 000–110 in this architecture.
• Operate on memory (e.g., LDA, STA, ADD, SUB, ISZ, AND, etc.).
• These involve loading/storing data from memory, such as LDA, STA, ADD, SUB and JMP.
2. Register Reference Instructions
• Use opcode = 111 with I = 0.
• Act only on CPU registers (e.g., CLA, CMA, HLT, etc.), no memory address is used.
• These operate within CPU registers, such as HLT (Halt), CLA (Clear Accumulator), CMA (Complement Accumulator).
3. I/O Instructions
• Use opcode = 111 with **I = 1`.
• Used for input/output operations, again not memory reference.
4. Role of Mode Bit (I)
• I = 0 → direct addressing (address field is actual memory address).
• I = 1 → indirect addressing (address field points to another memory location containing the effective address).
• But if opcode = 111, the instruction is never memory-reference, regardless of I.
5. Direct vs. Indirect Addressing Mode:
• Mode = 0 (Direct Addressing): Refers directly to a memory location.
• Mode = 1 (Indirect Addressing): Refers to a memory location that stores another memory address.