8085 Two Pass Assembler – Sample Program

Assembler is a program which is used for converting mnemonics into machine code. For more explanation refer “Systems programming and operating systems By Dhamdhere “book. Here i have given very simple example to understand the Intel 8085 two pass assembler.

General Functions of an assembler

Pass one

• Assign addresses to all statements in source code

• Save values (addresses) assigned to labels for use in pass two

• Process directives

Pass two

• Translate instructions

• Convert labels to addresses

• Generate values defined by BYTE and WORD

• Process the directives not done in pass one

• Write object code to output device

Sample Program : Addition of two numbers

Sample Data (In Hex Format):

(4000H) = 14H

(4001H) = 89H

Result  = 14H + 89H = 9DH

Source program

ORG 2000H

START:      LXI H 4000H          : HL points 4000H

MOV A, M                                         : Get first operand

INX H                                                 : HL points 4001H

ADD M                                               : Add second operand

INX H                                                 : HL points 4002H

MOV M, A                                         : Store result at 4002H

HLT                                                     : Terminate program execution

JMP START

Pass1 :

a) Symbol Table :   NIL

START    2000

b) Literal Table

4000   14H

4001   89H

c) Opcode / Mnemonics Table

Address    Mnemonics        Opcode       Operand

2000              LXI H,4000H       21          00  40

2003              MOV A, M             7E

2004              INX H                     23

2005              ADD M                   8E

2006              INX H                     23

2007              MOV M, A             77

2008              HLT                        76

2009              JMP START         32

Pass2:

1. Generating Hexadecimal / Object Code

Address  Opcode

2000      21

2001      00

2002        40

2003      7E

2004      23

2005      8E

2006      23

2007      77

2008      76

2009      32

200A      00

200B      20

2. Machine Code

00100001

00000000

01000000

01111110

00010011

10001110

00100011

01110111

01110110