Transcript MCA-104-DCE Study Material-Slides-For MCA First Semester

Assembly Language Programming
Bilal Zahoor
Kashmir University North Campus
8086 Assembly language

The language is case sensitive (MOV or mov is same).

Each instruction is represented by one assembly language
statement.

There may be only one statement/line.

A statement may start in any column.

A statement is either
 An
 An
8086 instruction or
Assembler directive: provides directions to the
assembler program.
Format of 8086 Instruction

General format for an assembly language statement
[Label:] Mnemonic [Operand1 [ ,Operand2] ] [;Comment]
Start:
MOV
AX, BX
; copy BX into AX
INC
CX
; increment CX
RET
; return from procedure

The entities enclosed with in square brackets are optional.

Start is a user defined name and you only put in a label in your
statement when necessary!!!!.

The symbol : is used to indicate that it is a label.

The symbol ; is used to indicate that it is a comment.
Format of an Assembler directive

General format for an assembler directive
[Name] Pseudo-Op [Operands] [;Comment]
temp
DB
5
;define temp as a byte variable
initialized with value 5
myProcedure
PROC
;beginning of a procedure (myProcedure).
myProcedure
ENDP
;end of a procedure (myProcedure).

Name is used for Variables, Procedure name etc.

Pseudo-ops qualify Name as segment name, macro-name and so on.

There are various Pseudo-ops used by assemblers, they have different names in
different assemblers.
ALP Development

The most common assemblers for Assembly language
programming development are:

Intel ASM

Microsoft MASM (MACRO Assembler).
 Works
on Microsoft Platforms (Windows and DOS).
 Produces

only .Com and .exe files.
Borland TASM (Turbo Assembler).
 16-bit
and 32-bit.
 Supports
all platforms.
Defining Data in a program

Data is usually stored in the data segment.

You can define constants, work areas (a chunk of memory).

Data can be defined in different lengths (8-bit, 16-bit, 32-bit and so
on)

Each byte of character is stored as its ASCII value in hexadecimal
The definition for data:
<Name> DX <Expression>

Name – name of data segment.

DX – this is called the directives. It defines length of the data.

Expression – defines the value (content) of the data segment.
Defining Data in a program(CONT..)


Pseudo-op
Description
Data size (in bytes)
DB
Define Byte
1
DW
Define Word
2
DD
Define Double word
4
DQ
Define Quad-word
8
DT
Define Ten bytes
10
Examples:
var DW 1234H
;define var as word and initialize
with value 1234(in Hex).
var DB ?
;define var as Byte and its initial value is
unknown.
Defining Arrays

Initialized Array:
arr DB ‘a’, ’b’, 2
arr DB 20 dup(5)

;initialized array of 3 elements, size of
each element is 1 byte
;arr is byte array of 20 elements all
initialized to value 5
Uninitialized Array:
arr DB 20 dup(?)
;arr is uninitialized byte array of 20
elements

DUP – duplicate can be used to define multiple storages.

arr stores the first value, arr + 1 stores the second value and so on.

E.g Mov Al, [arr+1]
; [] used for dereferencing, store value at arr+1 in Al
Named constant Directive

EQU – this directive does not define a data item; instead,
it defines a value that the assembler can use to substitute
in other instructions (similar to defining a constant in C
programming or using the #define )
FACT EQU 12
;defines FACT as a named constant
MOV CX, FACT
;without dereferencing

No memory is allocated.

Strings are also possible.
User defined Data definition directives

Structure:
Struct myStruct
;declares myStruct as a structure
var1 DB 0
; Var1 data byte initialized with 0
var2 DB 1
; Var2 data byte initialized with 1
Ends myStruct

Structure variable:
structVar myStruct ?

;creates structure variable
Acceccing structure:
MOV [structVar.var1], 20
;move 20 in var1 in mystruct
Segment Directive

The SEGMENT directive identifies the start of a memory segment and ENDS
identifies the end of a segment when full-segment definitions are in use.
Syntax:
<logical-segment_name> SEGMENT
..
..
<logical-segment_name> ENDS

E.g
mySegment SEGMENT
Mov Ax,BX
..
..
mySegment ENDS
Assume Directive

The ASSUME statement tells the assembler what names have been chosen for
the code, data, extra, and stack segments.

Without the ASSUME statement, the assembler assumes nothing and
automatically uses a segment override prefix on all instructions that address
memory data.

The ASSUME statement is only used with full-segment definitions
Syntax:
ASSUME <Physical-Segment>:<logica-segment_name>
E.g:
mySegment SEGMENT
ASSUME CS : mySegment
Mov AX,BX
mySegment ENDS
;Code segment is initialized to mySegment
Memory Organization

The assembler uses two basic formats for developing software:
1.
2.
Using Memory Models:

Memory models are unique to the MASM assembler program.

The TASM assembler also uses memory models, but they differ
somewhat from the MASM models.

The models are easier to use for simple tasks.
Using full-segment definitions:

The full-segment definitions are common to most assemblers,
including the Intel assembler, and are often used for software
development.

The full-segment definitions offer better control over the assembly
language task and are recommended for complex programs.
Simplified Segment Directives
.MODEL

The .MODEL statement followed by the size of the memory system
designates the Memory Model.
E.g
.MODEL SMALL
;select small model
.CODE

Designates the beginning of the CODE segment in the program.
.DATA

Designates the beginning of the DATA segment in the program
.STACK

Defines STACK segment in the program.

Syntax : .STACK [memory-size]

Default memory size for stack segment is 1KB.

Initializes Stack Segment(SS), Stack Pointer(SP) and Base Pointer(BP).
;memory-size is optional
Simplified Segment Directives
.STARTUP

Designates the start of the execution of the problem (similar to main() in C).

Loads DATA segment and EXTRA segment.

If the .STARTUP directive is used, the MOV AX,@DATA followed by MOV DS,AX
statements can be eliminated.

The .STARTUP directive also eliminates the need to store the starting address
next to the END label.
.EXIT

The .EXIT 0 directive returns to DOS with an error code of 0 (no error).

If no parameter is added to .EXIT, it still returns to DOS, but the error code is
not defined.
END:
End of the FIle.
Memory Model
MODEL
Description
TINY
All data and code must fit into a single 64K-byte memory
segment. Tiny programs assemble as DOS.COM files and
must use an origin at 0100H for the code.
SMALL
A two-segment model with a single code segment and
a single data segment. Small programs generate DOS.EXE
files and have an origin of 0000H.
FLAT
The flat model uses a single segment of up to 4G bytes
in length. Flat programs are programs that will only
function in Windows with an origin of 00000000H.

All models except TINY model assemble as .exe

Data segment(DS) and Extra Segment (ES) need to be initialized explicitly by
programmer
Standard I/O

DOS function calls are used for Standard input/output in Assembly language(8086).

To use a DOS function call in a DOS program,
1.
Place the function number in AH (8 bit register) and other data that might be necessary
in other registers.
2.
Once everything is loaded, execute the INT 21H instruction to perform the task.

After execution of a DOS function, it may return results in some specific registers.

01H: Read the Keyboard

This function waits until a character is input from the keyboard.

Returns ASCII key code of character in AL register.
E.g: MOV AH,01H
INT 21H
;load DOS function number in AH
;access DOS
;returns with AL = ASCII key code
Standard I/O (CONT..)

02H: Write to Standard Output device

This function displays single character on the video display.

ASCII key code of the character to be displayed must be loaded in DL
register.
E.g MOV DL, ‘A’
;load ASCII key code of Character ‘A’ in DL
MOV AH,02H
;load DOS function number in AH
INT 21H
;access DOS
Standard I/O (CONT..)

09H: Display a character String

This function displays a character string on the video display.

The character string must end with an ASCII of symbol ‘$’ (24H).

The character string can be of any length and may contain control
characters such as carriage return (0DH) and line feed (0AH).

DX must contain address of the character string.
E.g Buf DB “Hello World$”
;define character string
MOV DX, offset Buf
;load address of the string in DX, offset gives
address of the Buf.
MOV AH,09H
;load DOS function number in AH
INT 21H
;access DOS
Standard I/O (CONT..)

0AH: Buffered keyboard input

This function continues to read the keyboard (displaying data as typed) until either
the specified number of characters are typed or until the enter key is typed.

The first byte of the buffer contains the size of the buffer (up to 255).

The second byte is filled with the number of characters typed upon return.

The third byte through the end of the buffer contains the character string typed,
followed by a carriage return (0DH).
Buffer e.g: Buf DB 13, 10, “Welcome$”
E.g Buf DB 10, ?, 10 dup(0)
MOV DX, offset Buf
MOV AH,0AH
INT 21H
;declare a buffer.
;load address of the buffer in DX, offset gives
address of the Buf.
;load DOS function number in AH
;access DOS
Procedure/Subroutines

Procedures can be declared within the .CODE (code segment). However,
ensure that a procedure is not executed without explicit invocation.

Use CALL instruction to call the procedure.
Eg. CALL procedure-name
Example:
.MODEL SMALL
.DATA
msg DB “Hello World$”
.STACK
.CODE
myProc PROC
MOV DX, offset msg
MOV AH, 09H
INT 21H
RET
myProc ENDP
.STARTUP
CALL myProc
.EXIT
END
Procedure/Subroutines

The PROC and ENDP directives indicate the start and end of a procedure
(subroutine).

Both the PROC and ENDP directives require a label to indicate the name of
the procedure.
Syntax:
<Procedure-name> PROC
..
;starts procedure
;body of the procedure
..
RET
<Procedure-name>ENDP

;close of the procedure
When returning from procedures, use RET instruction.
Macros

A macro is a group of instructions that perform one task, just as a procedure
performs one task.

The difference is that a procedure is accessed via a CALL instruction,
whereas a macro, and all the instructions defined in the macro, is inserted in
the program at the point of usage.

Creating a macro is very similar to creating a new opcode, which is actually a
sequence of instructions, in this case, that can be used in the program.

You type the name of the macro and any parameters associated with it, and
the assembler then inserts them into the program.

Macro sequences execute faster than procedures because there is no CALL or
RET instruction to execute.

The instructions of the macro are placed in your program by the assembler at
the point where they are invoked.
Macros (CONT..)

The MACRO and ENDM directives delineate a macro sequence.

The first statement of a macro is the MACRO instruction, which contains the
name of the macro and any parameters associated with it.

An example is MOVE MACRO A,B

which defines the macro name as MOVE. This new pseudo opcode uses two
parameters: A and B.

The last statement of a macro is the ENDM instruction, which is placed on a
line by itself.

Never place a label in front of the ENDM statement. If a label appears before
ENDM, the macro will not assemble.
Syntax:
<Macro-name> MACRO [<Arg 1> <,Arg 2>…<,Arg n>]
..
ENDM
Macros (CONT..)
Example; Adding two numbers using Macro
.MODEL SMALL
;define Memory Model
SUM MACRO X,Y
;Macro definition SUM, X and Y are parameters
MOV AX, X
;Move value of X in AX
MOV BX, Y
;Move Value of Y in BX
ADD AX, BX
;Add values of X and Y
ENDM
;end of macro
.DATA
;start of data segment
.STACK
;start of stack segment
.CODE
;start of code segment
.STARTUP
SUM 5,10
;start of execution
;call Macro with Parameters 5 and 10
.EXIT
;return to DOS
END
;end of program file
Modular Programming

Many programs are too large to be developed by one person. This means
that programs are routinely developed by teams of programmers.

The Linker program is used so that programming modules can be linked
together into a complete program.

The assembler program converts a symbolic source module (file) into a
hexadecimal object file.

The linker program, reads the object files that are created by the
assembler program and links them together into a single execution file.

An execution file is created with the file name extension EXE.

If a file is short enough (less than 64K bytes long), it can be converted
from an execution file to a command file (.COM).

The command file is slightly different from an execution file in that the
program must be originated at location 0100H before it can execute.
Modular Programming (CONT..)
Object file1
.obj
Program
.asm
Assembler
Object file2
.obj
Object file3
.obj
Linker
Executable file
.exe
Modular Programming (CONT..)

The PUBLIC and EXTRN directives are very important to modular
programming because they allow communications between modules.

We use PUBLIC to declare that labels of code, data, or entire
segments are available to other program modules.

EXTRN (external) declares that labels are external to a module.

The PUBLIC directive is placed in the opcode field of an assembly
language statement to define a label as public, so that the label can
be used (seen by) by other modules.

The label declared as public can be a jump address, a data address,
or an entire segment.

When segments are made public, they are combined with other public
segments that contain data with the same segment name.
Modular Programming (CONT..)
Placing MACRO Definitions in Their Own Module:

Macro definitions can be placed in the program file, or they can be
placed in their own macro module.

A file can be created that contains only macros to be included with
other program files.

We use the INCLUDE directive to indicate that a program file will
include a module that contains external macro definitions.

When macro sequences are placed in a file (often with the extension
INC or MAC), they do not contain PUBLIC statements as does a library.
Modular Programming (CONT..)
Example
MyFun.inc file
.MODEL SMALL
SUM MACRO X,Y
.DATA
MOV AX, X
MOV BX, Y
ADD AX, BX
.STACK
.CODE
INCLUDE MyFun.inc
.STARTUP
SUM 5,10
.EXIT
END
;include myFun file
ENDM
Conditional Assembly (or Controlled Expansion)

Conditional assembly language statements allow portions of a program to be
assembled if a condition is met.

These are useful for tailoring software to an application

Following table shows the forms used for IF statement in the conditional
assembly.
Statement
Function
IF
If the expression is true
IFB
If argument is blank
IFE
If the expression is not true
IFDEF
If the label has been defined
IFNB
If argument is not blank
IFNDEF
If the label has not been defined
IFIDN
If argument 1 equals argument 2
IFDIFWWW
If argument 1 does not equal to
argument 2