Assembly Language Programming of 8085

Assembly Language Programming of 8085

Topics

• 1. Introduction 

• 2. Programming model of 8085 

• 3. Instruction set of 8085 

• 4. Example Programs 

• 5. Addressing modes of 8085 

• 6. Instruction & Data Formats of 8085

1. Introduction

•  A microprocessor executes instructions given by the user
•  Instructions should be in a language known to the microprocessor
•  Microprocessor understands the language of 0’s and 1’s only
•  This language is called Machine Language

• For e.g.

01001111 – Is a valid machine language instruction of

• 8085 – It copies the contents of one of the internal registers of 8085 to another
• A Machine language program to add two numbers

eg:

    Copy value 2H in register A 00000010 00000110 

    Copy value 4H in register B 00000100 10000000 

         A = A + B

Assembly Language of 8085

•  It uses English like words to convey the action/meaning called as MNEMONICS

• For e.g.

      MOV to indicate data transfer 

       ADD to add two values 

       SUB to subtract two values

Assembly language program to add two numbers

MVI A, 2H ;          Copy value 2H in register A

 MVI B, 4H ;         Copy value 4H in register B 

ADD B ;                A = A + B

Note:

• Assembly language is specific to a given processor
• For e.g. assembly language of 8085 is different than that of Motorola 6800 microprocessor

• Microprocessor understands Machine Language only!
•  Microprocessor cannot understand a program written in Assembly language
• A program known as Assembler is used to convert a Assembly language program to machine language

2. Programming model of 8085

• Accumulator(8-bit)
• ALU
• Flags(8-bit)
• Instruction Decoder
• 16-bit Address Bus Register Array
• Memory Pointer Registers
• 8-bit Data Bus
• 8-bit Data Bus
• Timing and Control Unit
• Control Bus

Overview: 8085 Programming model 

1.  Six general-purpose Registers
2.  Accumulator Register 
3.  Flag Register
4.  Program Counter Register 
5.  Stack Pointer Register

1. Six general-purpose registers

                     –B, C, D, E, H, L – Can be combined as register pairs to perform 16-bit operations (BC, DE, HL) 

2. Accumulator – identified by name A

                    – This register is a part of ALU – 8-bit data storage – Performs arithmetic and logical operations 

                    – Result of an operation is stored in accumulator

3. Flag Register

– This is also a part of ALU – 8085 has five flags named

• • Zero flag (Z)
• • Carry flag (CY)
• • Sign flag (S)
• • Parity flag (P)
• • Auxiliary Carry flag (AC)

 These flags are five flip-flops in flag register

• Execution of an arithmetic/logic operation can set or reset these flags
• Condition of flags (set or reset) can be tested through software instructions
• 8085 uses these flags in decision-making process

4. Program Counter (PC)

        – A 16-bit memory pointer register – Used to sequence execution of program instructions – Stores address of a memory location

• where next instruction byte is to be fetched by the 8085 – when 8085 gets busy to fetch current instruction from memory
•  PC is incremented by one
•  PC is now pointing to the address of next instruction

5. Stack Pointer Register

        – a 16-bit memory pointer register – Points to a location in Stack memory – Beginning of the stack is defined by loading a 16-bit address in stack pointer register

3.Instruction Set of 8085

• Consists of – 74 operation codes, e.g. MOV – 246 Instructions, e.g. MOV A,B

• 8085 instructions can be classified as 

1. Data Transfer (Copy) 

2. Arithmetic

3. Logical and Bit manipulation

4. Branch 

5. Machine Control

1. Data Transfer (Copy) Operations

• 1. Load a 8-bit number in a Register 
• 2. Copy from Register to Register 
• 3. Copy between Register and Memory
• 4. Copy between Input/Output Port and Accumulator 
• 5. Load a 16-bit number in a Register pair
• 6. Copy between Register pair and Stackmemory 

Example Data Transfer (Copy) Operations / Instructions 

MVI B, 4FH :Load a 8-bit number 4F in register B

MOV A,B :Copy from Register B to Register A

LXI H, 2050H :Load a 16-bit number 2050 in Register pair HL

MOV M,B :Copy from Register B to Memory Address 2050

OUT 01H IN 07H :opy between Input/Output Port and Accumulator

2. Arithmetic Operations

1. Addition of two 8-bit numbers 

2. Subtraction of two 8-bit numbers 

3. Increment/ Decrement a 8-bit number

Example Arithmetic Operations / Instructions


ADI 32H :Add a 8-bit number 32H to Accumulator

ADD B :Add contents of Register B to Accumulator

SUI 32H :Subtract a 8-bit number 32H from Accumulator

SUB C :Subtract contents of Register C from Accumulator

INR D :Increment the contents of Register D by 1

DCR E :Decrement the contents of  Register E by 1

3. Logical & Bit Manipulation Operations

1. AND two 8-bit numbers

 2. OR two 8-bit numbers 

3. Exclusive-OR two 8-bit numbers 

4. Compare two 8-bit numbers

 5. Complement 

6. Rotate Left/Right Accumulator bits

Example Logical & Bit Manipulation Operations / Instructions

ANA H : Logically AND Register H with Accumulator

ORA L :Logically OR Register L with Accumulator

XRA B :Logically XOR Register B with Accumulator

CMP C :Compare contents of Register C with Accumulator

CMA RAL :Rotate Accumulator Left

4. Branching Operations

These operations are used to control the flow of program execution 1.Jumps

• Conditional jumps

• Unconditional jumps 2.Call & Return

• Conditional Call & Return

• Unconditional Call & Return

Example Branching Operations / Instructions


JC 2080H : Jump to a 16-bit Address 2080H if Carry flag is SET

JMP 2050H CALL 3050H :Unconditional Jump

RET CNC 3050H : Return back from the Call

RZ :Call a subroutine with its 16-bit Address if Carry flag is RESET

5. Machine Control Instructions

These instructions affect the operation of the processor.

 For e.g. HLT Stop program execution 

              NOP Do not perform any operation

4. Writing a Assembly Language Program

• Steps to write a program

–Analyze the problem

 –Develop program Logic 

–Write an Algorithm 

–Make a Flowchart

 –Write program Instructions using Assembly language of 8085 

Program 8085 in Assembly language to add two 8- bit numbers and store 8-bit result in register C.
1. Analyze the problem

– Addition of two 8-bit numbers to be done 

2. Program Logic

– Add two numbers – Store result in register C – Example

10011001 (99H) A +00111001 (39H) D 11010010 (D2H) C

3. Algorithm Translation to 8085 operations

1. Get two numbers

• Load 1st no. in register D

• Load 2nd no. in register E

2. Add them

• Copy register D to A

• Add register E to A

3. Store result

• Copy A to register C

4. Stop

• Stop processing

4. Make a Flowchart

own


5. Assembly Language Program

1. Get two numbers a) Load 1st no. in register D b) Load 2nd no. in register E

2. Add them a) Copy register D to A b) Add register E to A

3. a) Store Copy A result to register C

4. Stop a) Stop processing

MVI D, 2H MVI E, 3H

MOV A, D ADD E

MOV C, A

HLT

4. Addressing Modes of 8085 

• The various formats of specifying operands are called addressing modes

• Addressing modes of 8085 

1. Register Addressing 

2. Immediate Addressing 

3. Memory Addressing 

4. Input/Output Addressing

1. Register Addressing

• Operands are one of the internal registers of 8085

• Examples-

MOV A, B ADD C

2. Immediate Addressing

• Value of the operand is given in the instruction itself

• Example-

MVI A, 20H LXI H, 2050H ADI 30H SUI 10H

3. Memory Addressing

• One of the operands is a memory location

• Depending on how address of memory location is specified, memory addressing is of two types

– Direct addressing – Indirect addressing

3(a) Direct Addressing

• 16-bit Address of the memory location is specified in the instruction directly

• Examples- LDA 2050H ;load A with contents of memory

location with address 2050H STA 3050H ;store A with contents of memory

location with address 3050H

3(b) Indirect Addressing

• A memory pointer register is used to store the address of the memory location

• Example- MOV M, A ;copy register A to memory location whose address is stored in register pair HL


30H A 20HH

50HL

30H 2050H

30H 2050H

4. Input/Output Addressing

• 8-bit address of the port is directly specified in the instruction

• Examples-

IN 07H OUT 21H

5. Instruction & Data Formats

8085 Instruction set can be classified according to size (in bytes) as

1. 1-byte Instructions 

2. 2-byte Instructions 

3. 3-byte Instructions

1. One-byte Instructions

• Includes Opcode and Operand in the same byte

• Examples-

Opcode Operand Binary Code Hex Code

MOV C, A 0100 1111 4FH ADD B 1000 0000 80H HLT 0111 0110 76H

2. Two-byte Instructions

• First byte specifies Operation Code

• Second byte specifies Operand

• Examples-

Opcode Operand Binary Code Hex Code

MVI A, 32H 0011 1110 0011 0010

3EH 32H MVI B, F2H 0000 0110 1111 0010

06H F2H

3. Three-byte Instructions

• First byte specifies Operation Code

• Second & Third byte specifies Operand

• Examples-

Opcode Operand Binary Code Hex Code

LXI H, 2050H 0010 0001 0101 0000 0010 0000

21H 50H 20H LDA 3070H 0011 1010 0111 0000 0011 0000

3AH 70H 30H