Day 31 – Night Session: COA – Stack Organisation & Instruction Format – ECET 2026

Computer Organization & Architecture (COA) lo Stack Organisation and Instruction Formats are very important topics. Every year ECET exams lo direct & concept-based questions vastayi. If you understand the basics with examples, you can easily score 2–3 marks.

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⚙️ Concept Notes – Stack Organisation

🔥 What is Stack?

• Stack = A linear data structure, follows LIFO (Last In, First Out).
• Operations:
  • PUSH → Insert an element into stack.
  • POP → Remove an element from stack.
• Used in expression evaluation, function calls, and interrupt handling in CPU.

👉 Example:
If we PUSH A, PUSH B, PUSH C → Stack looks like:
TOP → C, B, A
If we POP → removes C first.

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📘 Stack Organisation in CPU

• Stack can be implemented using registers or memory.
• Stack Pointer (SP) → Special register points to top of stack.
• In register stack, limited size.
• In memory stack, large but slower.

👉 Example Formula:

• After PUSH X:

M[SP] = X

After POP:

Concept Notes – Instruction Formats

🔥 What is Instruction Format?

• An instruction format specifies how bits in an instruction are divided into fields like:
  • Opcode field → Operation to perform
  • Address field(s) → Location of operand(s)
  • Mode field → Addressing mode used

📘 Common Instruction Formats

1️⃣ Zero Address Format

• Used in Stack Organisation.
• Operands implicitly taken from top of stack.
  👉 Example:
  ADD → Pops top 2 values from stack, adds them, pushes result back.

2️⃣ One Address Format

• Contains one explicit operand; accumulator is implied.
  👉 Example:
  ADD X → AC ← AC + M[X]

3️⃣ Two Address Format

• Contains 2 explicit operands.
  👉 Example:
  ADD R1, R2 → R1 ← R1 + R2

4️⃣ Three Address Format

• Contains 3 operands (2 sources + 1 destination).
  👉 Example:
  ADD R1, R2, R3 → R1 ← R2 + R3

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🔟 10 Expected MCQs – ECET 2026

Q1. Stack works on which principle?
A) FIFO
B) LIFO
C) FILO
D) Both B & C

Q2. Special register that holds address of top of stack is:
A) PC
B) IR
C) SP
D) MAR

Q3. In stack organisation, zero-address instructions are mainly used for:
A) Queue operations
B) Arithmetic operations
C) File handling
D) Memory mapping

Q4. After a PUSH operation, the stack pointer is:
A) Incremented
B) Decremented
C) Unchanged
D) None

Q5. In POP operation, the stack pointer is:
A) Incremented
B) Decremented
C) Multiplied
D) Remains same

Q6. Which instruction format uses accumulator as implicit operand?
A) Zero Address
B) One Address
C) Two Address
D) Three Address

Q7. ADD R1, R2 is an example of:
A) One Address instruction
B) Two Address instruction
C) Three Address instruction
D) Zero Address instruction

Q8. In zero-address instruction format, operands are stored in:
A) Accumulator
B) Registers
C) Stack
D) Cache

Q9. Instruction length depends mainly on:
A) Number of address fields
B) Opcode size
C) Addressing mode
D) All of the above

Q10. Which is the most flexible instruction format?
A) Zero Address
B) One Address
C) Two Address
D) Three Address

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✅ Answer Key

Q.No	Answer
Q1	B
Q2	C
Q3	B
Q4	B
Q5	A
Q6	B
Q7	B
Q8	C
Q9	D
Q10	D

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🧠 Explanations

• Q1 → B: Stack = LIFO principle.
• Q2 → C: SP = Stack Pointer.
• Q3 → B: Zero address instructions are mainly for arithmetic.
• Q4 → B: PUSH → SP decreases.
• Q5 → A: POP → SP increases.
• Q6 → B: One-address instructions use accumulator.
• Q7 → B: ADD R1, R2 is 2-address format.
• Q8 → C: Zero-address operands taken from stack.
• Q9 → D: Instruction length depends on all 3 factors.
• Q10 → D: 3-address format is most flexible.

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🎯 Why This Practice Matters

• Stack organisation is always asked in ECET (1–2 MCQs guaranteed).
• Instruction formats questions repeat every year.
• Understanding with examples makes problem solving easy in exams.

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