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Instruction Format

✨ What is an Instruction Format?

  • An Instruction Format is the layout or structure of the bits in a computer instruction.
  • It tells the CPU what to do and how to do it.
  • Every instruction contains different parts like:
    • Operation (what operation to perform)
    • Operands (data to operate on)
    • Address (location of data)

👉 Simple Definition:

“Instruction format defines how an instruction is divided into different fields to be understood and executed by the CPU.“

🛠️ Key Fields in an Instruction Format

FieldPurpose
OpcodeSpecifies the operation (add, subtract, load, store, etc.)
Operand(s)Specifies the data or addresses on which operation is to be performed
Address FieldSpecifies the memory address or register where data is stored
Mode FieldSpecifies how to interpret the address (direct, indirect, immediate, etc.)

📑 Basic Structure of an Instruction

+-----------+--------------------+----------------+
|   Opcode  |   Operand 1         |   Operand 2    |
+-----------+--------------------+----------------+

or

+-----------+----------------+
|   Opcode  |   Address       |
+-----------+----------------+

✅ Opcode → Operation to perform (e.g., ADD, SUB)
✅ Address/Operand → Data or memory location involved in the operation

📚 Types of Instruction Formats

Depending on CPU design, instructions can have different formats:

📌 1. Zero Address Instruction (Stack-based)

  • No operand field.
  • Operands are implied (taken from stack).
  • Example: ADD (adds top two values of stack)

📌 2. One Address Instruction

  • Only one operand field.
  • Second operand is implied (usually accumulator).
  • Example: LOAD A (load value of A into accumulator)

📌 3. Two Address Instruction

  • Two operand fields.
  • One operand can be source and destination.
  • Example: ADD A, B (A = A + B)

📌 4. Three Address Instruction

  • Three operand fields.
  • Very flexible.
  • Example: ADD A, B, C (A = B + C)

🎯 Example of Three Address Instruction Format

Instruction: ADD R1, R2, R3

Meaning:
R1 = R2 + R3

Fields:
- Opcode: ADD
- Operand1: R1 (destination)
- Operand2: R2 (source 1)
- Operand3: R3 (source 2)

⚙️ Diagram: Simple Instruction Format Layout

|  Opcode  |  Source Operand 1  |  Source Operand 2  |  Destination Operand |
|  (bits)  |      (bits)         |      (bits)        |        (bits)         |

For example, a 32-bit instruction may have:

  • 8 bits for Opcode
  • 8 bits for Operand 1
  • 8 bits for Operand 2
  • 8 bits for Operand 3

🚀 Importance of Instruction Format

✅ CPU Decoding:

  • CPU needs a standard format to decode and execute instructions correctly.

✅ Efficiency:

  • Well-designed instruction formats increase processing speed and reduce complexity.

✅ Program Size:

  • Compact instruction formats save memory space.

✅ Performance:

  • Affects the number of clock cycles needed for execution.

📢 Factors Affecting Instruction Format Design

  • Size of memory (address length)
  • Number of CPU registers
  • Number and type of operations supported
  • Instruction length (fixed or variable)

🏆 Summary:

“Instruction Format is the structure of bits in an instruction, divided into fields like opcode and operands. It is essential for the CPU to correctly understand and execute operations. Different formats (zero, one, two, three address) exist based on the architecture and complexity of operations.“

📌 Quick Memory Tip:

Instruction Format = "HOW" CPU reads "WHAT" it has to do!