8085 Instruction Set & Addressing Modes: Types, Examples & Real-World Use

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1. Instruction Set of 8085: Classification and Examples

The 8085 instruction set consists of 244 instructions categorized into 8 groups based on functionality. Each instruction performs a specific operation on data or controls program flow.

1.1 Classification of Instructions

The 8085 instructions are divided into the following categories:

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1.2 Worked Example: Data Transfer Instructions

Instruction: MOV B, A

• Mnemonic: Move data from register A to register B.
• Opcode: 47H
• Operation:
  • The content of A is copied to B.
  • The source register (A) remains unchanged.
  • Flags affected: None.

Example Trace: If (A) = 37H and (B) = 57H, after MOV B, A:

• (A) = 37H (unchanged)
• (B) = 37H (copied from A)

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1.3 Worked Example: Arithmetic Instruction (ADD B)

Instruction: ADD B

• Mnemonic: Add content of B to A.
• Opcode: 80H
• Operation:
  • (A) = (A) + (B)
  • Flags affected: S (Sign), Z (Zero), C (Carry), P (Parity), AC (Auxiliary Carry).

Example Trace: If (A) = 37H and (B) = 57H:

• Binary addition:

  A: 0011 0111 (55)
  B: 0101 0111 (87)
  Sum: 1000 1110 (142)
  

• Result: (A) = 8EH (142 in decimal)
• Flags:
  • S = 1 (MSB is 1, negative in 8-bit signed)
  • Z = 0 (Result ≠ 0)
  • C = 1 (Carry generated)
  • P = 0 (Even parity)
  • AC = 1 (Auxiliary carry from bit 3 to 4)

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2. Addressing Modes of 8085

Addressing modes determine how the operand is specified in an instruction. The 8085 supports six addressing modes:

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2.1 Worked Example: Immediate Addressing (MVI A, 32H)

Instruction: MVI A, 32H

• Mnemonic: Move immediate value 32H to A.
• Opcode: 3EH (for MVI A, data)
• Operation:
  • The 8-bit literal 32H is loaded into A.
  • Flags affected: None.

Example Trace:

• Before: (A) = ? (unknown)
• After: (A) = 32H (50 in decimal)

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2.2 Worked Example: Direct Addressing (STA 2100H)

Instruction: STA 2100H

• Mnemonic: Store A into memory location 2100H.
• Opcode: 32H (for STA addr)
• Operation:
  • The 16-bit address 2100H is specified.
  • The content of A is written to 2100H.
  • Flags affected: None.

Example Trace:

• If (A) = 45H and (2100H) = ? (unknown):
  • After STA 2100H: (2100H) = 45H

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2.3 Worked Example: Register Indirect (MOV A, M)

Instruction: MOV A, M

• Mnemonic: Move data from memory (pointed by H-L) to A.
• Opcode: 7EH
• Operation:
  • The 16-bit address in H-L is used to fetch data.
  • Data at (HL) is copied to A.
  • Flags affected: None.

Example Trace:

• If (H) = 20H, (L) = 00H → (HL) = 2000H
• If (2000H) = 67H and (A) = ?:
  • After MOV A, M: (A) = 67H, (2000H) remains unchanged.

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3. Real-World Applications of 8085 Instructions & Addressing Modes

3.1 eSewa (Nepal) – Transaction Processing

• Instruction Used: ADD, SUB, STA (Direct Addressing)
• How?
  • When a user pays an electricity bill via eSewa, the 8085-like processor (or modern equivalent) in the backend system:
    1. Fetches the user’s balance (LDA 3000H – Direct Addressing).
    2. Subtracts the bill amount (SUB 20H – Immediate Addressing).
    3. Stores the updated balance (STA 3000H – Direct Addressing).
  • Addressing Mode: Direct (for memory access) and Immediate (for fixed bill amount).

3.2 Daraz (Nepal) – Order Queue Management

• Instruction Used: MOV, INR, JNZ (Register & Implied Addressing)
• How?
  • Daraz’s order processing system (simplified for 8085 logic):
    1. Initializes an order counter (MVI B, 00H – Immediate).
    2. Loops through orders (INR B – Implied, increments counter).
    3. Checks if orders are processed (CMP 10H – Immediate, compares with max orders).
    4. Jumps back if not done (JNZ LOOP – Conditional Jump).
  • Addressing Mode: Implied (for INR B) and Immediate (for fixed values).

3.3 Ncell (Nepal) – SMS Delivery System

• Instruction Used: LXI, MOV, OUT (Register Indirect & I/O)
• How?
  • When an SMS is sent via Ncell:
    1. Loads the SMS data into H-L (LXI H, 4000H – Register Indirect).
    2. Moves data to a buffer (MOV A, M – Register Indirect).
    3. Sends via I/O port (OUT 03H – I/O Addressing).
  • Addressing Mode: Register Indirect (for memory access) and I/O (for modem communication).

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4. Comparison of Addressing Modes

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5. Instruction Execution: Worked Example (Full Trace)

Problem: Write the 8085 assembly program to add two numbers stored at 2000H and 2001H, store the result at 2002H, and check if the result is zero.

Solution:

LDA 2000H    ; Load first number (Direct Addressing)
ADD 2001H    ; Add second number (Direct Addressing)
STA 2002H    ; Store result (Direct Addressing)
JNZ NOT_ZERO ; Jump if result ≠ 0 (Conditional Jump)
HLT          ; Halt if result = 0
NOT_ZERO: HLT

Step-by-Step Trace:

1. LDA 2000H

   • (A) = (2000H) (e.g., 35H)
   • Flags: Updated (S, Z, P, etc.)

2. ADD 2001H

   • (A) = (A) + (2001H) (e.g., 35H + 4AH = 7FH)
   • Flags: Updated (C=0, Z=0, S=0, P=1)

3. STA 2002H

   • (2002H) = (A) = 7FH

4. JNZ NOT_ZERO

   • Since Z=0, jump to NOT_ZERO.
   • If (A) = 00H, Z=1, so no jump (halt).

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6. Exam Tip: How to Score Full Marks

1. Define Clearly:

   • Always start with definitions (e.g., "Addressing mode is a technique to specify the operand in an instruction.").
   • Example:

     "Immediate addressing is an addressing mode where the operand is part of the instruction itself."

2. Use Examples:

   • For every instruction, provide:
     • Opcode (if asked).
     • Operation (what it does).
     • Flags affected (if any).
   • Example for ADD B:

     "The ADD B instruction adds the content of register B to A. The opcode is 80H. Flags affected: S, Z, C, P, AC."

3. Trace Execution:

   • For programs, show step-by-step register/memory changes.
   • Example:

     "After MOV B, A, if (A)=37H, then (B)=37H."

4. Compare Addressing Modes:

   • Use a table to compare speed, flexibility, and use cases.
   • Example:

     "Immediate addressing is fastest but least flexible, while register indirect is slower but more flexible for arrays."

5. Link to Real-World:

   • In short-answer questions, relate instructions to eSewa, Daraz, or Ncell.
   • Example:

     "The STA instruction (Direct Addressing) is used in eSewa to update a user’s balance in memory."

6. Avoid Common Mistakes:

   • ❌ Don’t assume flags are affected unless specified (e.g., MOV does not affect flags).
   • ❌ Don’t mix up 8-bit and 16-bit operations (e.g., DAD is 16-bit, ADD is 8-bit).
   • ❌ Don’t forget to initialize registers before use (e.g., MVI B, 00H).

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Final Checklist for Exam:

✅ Define the instruction/addressing mode. ✅ Give an example with opcode (if asked). ✅ Trace execution (register/memory changes). ✅ Mention flags affected (if any). ✅ Compare with other modes (if comparison is asked). ✅ Relate to real-world (eSewa, Daraz, Ncell).

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