Day 8 Night – Embedded: Instruction Set Basics

Why This Topic is Important for ECET?

In ECET 2026 ECE, Embedded Systems questions often test your knowledge of Instruction Set Architecture (ISA) basics.

• ISAs define how a processor understands instructions (add, move, jump, etc.).
• Without understanding instructions, you can’t solve questions related to microcontrollers, programming, or automation.
• Direct questions appear in ECET on types of instructions, addressing modes, and program execution flow.

So, mastering instruction sets = easy scoring in the exam! 💯

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📘 Concept Notes

1. What is an Instruction Set?

• An Instruction Set is the collection of all commands (instructions) that a processor understands and executes.
• Examples: ADD, SUB, MOV, JMP, CALL, AND, OR, CMP.

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2. Types of Instructions

1. Data Transfer Instructions → Move data between registers or memory.
   • Example: MOV A, B (copies B into A).
2. Arithmetic Instructions → Perform math operations.
   • Example: ADD A, B (A ← A + B).
3. Logical Instructions → Perform logic operations.
   • Example: AND A, B.
4. Branch Instructions → Change program flow.
   • Example: JMP LABEL (jump to LABEL).
5. Control Instructions → Manage execution.
   • Example: HLT (halt execution).

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3. Instruction Format

Each instruction has opcode + operand(s).

• Opcode → Operation code (ADD, MOV, etc.)
• Operand → Data or address on which operation is performed.

Example: ADD R1, R2

• Opcode = ADD
• Operands = R1, R2

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4. Addressing Modes (How operands are accessed)

• Immediate: Data is part of the instruction. (MOV A, #05)
• Register: Operand in a register. (ADD A, R1)
• Direct: Address of operand is given. (MOV A, 2000H)
• Indirect: Register holds address. (MOV A, @R0)

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5. Example – Execution Flow

Instruction: MOV A, #05H → Accumulator = 05H
Instruction: ADD A, #03H → Accumulator = 08H
Instruction: JMP LOOP → Program jumps to label LOOP

This is how assembly-level programming works in embedded systems.

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⚙️ Formulas (Related to Instruction Sets)

Even though instruction sets are more conceptual, some related formulas in embedded/microprocessor context are useful:

• CPU Execution Time:

Effective Address (EA) in Direct Mode:

Effective Address (EA) in Register Indirect Mode:

Program Counter (PC) Update:

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🔟 10 MCQs

Q1. Which part of instruction tells the CPU what operation to perform?
a) Operand
b) Opcode
c) Address field
d) Program counter

Q2. MOV A, #25H is an example of which addressing mode?
a) Immediate
b) Register
c) Direct
d) Indirect

Q3. In ADD A, R1, the operation performed is:
a) R1 ← A + R1
b) A ← A + R1
c) A ← R1
d) R1 ← A

Q4. Which of the following is a branch instruction?
a) MOV A, B
b) ADD A, B
c) JMP LOOP
d) AND A, B

Q5. If CPI = 3, N = 200 instructions, clock = 10 MHz, find execution time.

Q6. Which instruction stops program execution?
a) CALL
b) HLT
c) JMP
d) RET

Q7. Direct addressing mode means:
a) Operand is given in instruction
b) Address of operand is given in instruction
c) Operand is inside CPU register
d) None

Q8. If Program Counter (PC) = 2000H, after fetching one instruction (size = 2 bytes), new PC will be:
a) 2001H
b) 2002H
c) 2003H
d) 1FFFH

Q9. Which instruction set architecture uses fewer, simple instructions?
a) RISC
b) CISC
c) VLIW
d) DSP

Q10. Which instruction is used to return from a subroutine?
a) CALL
b) RET
c) HLT
d) NOP

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✅ Answer Key

Q No	Answer
Q1	b
Q2	a
Q3	b
Q4	c
Q5	60 µs
Q6	b
Q7	b
Q8	b
Q9	a
Q10	b

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🧠 Explanations

• Q1: Opcode tells CPU which operation → (b).
• Q2: Immediate mode → operand given directly → (a).
• Q3: ADD A, R1 means A ← A + R1 → (b).
• Q4: JMP changes program flow → (c).
• Q5:
  = 200 × 3 × (1/10 MHz) = 200 × 3 × 0.1 µs = 60 µs → (correct).
• Q6: Halt instruction = HLT → (b).
• Q7: Direct addressing = address of operand given → (b).
• Q8: Instruction size 2 bytes → PC = 2000H + 2 = 2002H → (b).
• Q9: RISC = Reduced Instruction Set Computer → fewer, simple instructions → (a).
• Q10: RET is return from subroutine → (b).

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🎯 Motivation / Why Practice Matters

Instruction set basics are highly scoring in ECET:

• Usually 1–2 direct questions appear on opcodes, addressing modes, or execution time.
• Practicing these questions improves speed and accuracy since they are direct formula or concept-based.
• Strong ISA understanding helps in microprocessors, microcontrollers, and embedded applications.

Practicing daily ensures you won’t waste time in the exam on simple instruction-related problems. 🚀

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