8051
ASSEMBLY
PROGRAMMING

Structure of
Assembly
Language
8051
ASSEMBLY
PROGRAMMING
􀂉 Assembly language instruction includes
􀂾 a mnemonic (abbreviation easy to remember)
􀂃 the commands to the CPU, telling it what those
to do with those items
􀂾 optionally followed by one or two operands
􀂃 the data items being manipulated
􀂉 A given Assembly language program is
a series of statements, or lines
􀂾 Assembly language instructions
􀂃 Tell the CPU what to do
􀂾 Directives (or pseudo-instructions)
􀂃 Give directions to the assembler

Structure of
Assembly
Language
ORG 0H ;start(origin) at location
0
MOV R5, #25H ;load 25H into R5
MOV R7, #34H ;load 34H into R7
MOV A, #0 ;load 0 into A
ADD A, R5 ;add contents of R5 to A
;now A = A + R5
ADD A, R7 ;add contents of R7 to A
;now A = A + R7
ADD A, #12H ;add to A value 12H
;now A = A + 12H
HERE: SJMP HERE ;stay in this loop
END ;end of asm source file
8051
ASSEMBLY
PROGRAMMING
􀂉 An Assembly language instruction
consists of four fields:
[label:] Mnemonic [operands] [;comment]
Mnemonics
produce
opcodes
The label field allows
the program to refer to a
line of code by name
Comments may be at the end of a
line or on a line by themselves
The assembler ignores comments
Directives do not
generate any machine
code and are used
only by the assembler

ASSEMBLING
AND RUNNING
AN 8051
PROGRAM
􀂉 The step of Assembly language
program are outlines as follows:
1) First we use an editor to type a program,
many excellent editors or word
processors are available that can be used
to create and/or edit the program
􀂃 Notice that the editor must be able to produce
an ASCII file
􀂃 For many assemblers, the file names follow
the usual DOS conventions, but the source file
has the extension “asm“ or “src”, depending
on which assembly you are using

ASSEMBLING
AND RUNNING
AN 8051
PROGRAM
(cont’)
2) The “asm” source file containing the
program code created in step 1 is fed to
an 8051 assembler
􀂃 The assembler converts the instructions into
machine code
􀂃 The assembler will produce an object file and
a list file
􀂃 The extension for the object file is “obj” while
the extension for the list file is “lst”
3) Assembler require a third step called
linking
􀂃 The linker program takes one or more object
code files and produce an absolute object file
with the extension “abs”
􀂃 This abs file is used by 8051 trainers that
have a monitor program

ASSEMBLING
AND RUNNING
AN 8051
PROGRAM
(cont’)
4) Next the “abs” file is fed into a program
called “OH” (object to hex converter)
which creates a file with extension “hex”
that is ready to burn into ROM
􀂃 This program comes with all 8051 assemblers
􀂃 Recent Windows-based assemblers combine
step 2 through 4 into one step

ASSEMBLING
AND RUNNING
AN 8051
PROGRAM
Steps to Create
a Program
EDITOR
PROGRAM
ASSEMBLER
PROGRAM
LINKER
PROGRAM
OH
PROGRAM
myfile.asm
myfile.obj
myfile.abs
myfile.lst
Other obj files
myfile.hex

ASSEMBLING
AND RUNNING
AN 8051
PROGRAM
lst File
􀂉 The lst (list) file, which is optional, is
very useful to the programmer
􀂾 It lists all the opcodes and addresses as
well as errors that the assembler detected
􀂾 The programmer uses the lst file to find
the syntax errors or debug
1 0000 ORG 0H ;start (origin) at 0
2 0000 7D25 MOV R5,#25H ;load 25H into R5
3 0002 7F34 MOV R7,#34H ;load 34H into R7
4 0004 7400 MOV A,#0 ;load 0 into A
5 0006 2D ADD A,R5 ;add contents of R5 to A
;now A = A + R5
6 0007 2F ADD A,R7 ;add contents of R7 to A
;now A = A + R7
7 0008 2412 ADD A,#12H ;add to A value 12H
;now A = A + 12H
8 000A 80EF HERE: SJMP HERE;stay in this loop
9 000C END ;end of asm source file
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