MASM -UNIT-III

This class notes is meant for M.Sc Physics of RU and other computer science students

1. 1

UNIT III – MASM86
The Microsoft Macro Assembler (MASM) is an x86 assembler that uses the Intel syntax for MS-
DOS and Microsoft Windows. The earlier versions were released in the year 1981 .Turbo
Assembler (TASM) is also an assembler package developed by Borland which runs on and
produces code for 16- or 32-bit x86 MS-DOS or Microsoft Windows. These softwares provide
the assembly language tools to program the 8086 processor based systems.
To program the x86 processors using in MASM , certain concepts are very important. They are
reserved words, identifiers, predefined symbols, constants, expressions, operators, data types, registers,
and statements.
Reserved Words : A reserved word has a special meaning fixed by the language. This must be
used under certain special conditions. These reserved words are
Instructions, which correspond to operations the processor can execute.
Directives, which give commands to the assembler.
Attributes, which provide a value for a field, such as segment alignment.
Operators, which are used in expressions.
MASM reserved words are not case sensitive except for predefined symbols. The assembler
generates an error if you use a reserved word as a variable.
For Ex: The following operands are reserved words. Reserved words are not case sensitive.
$ DWORD PASCAL SWORD
? FAR QWORD SYSCALL
@B FAR16 REAL4 TBYTE
@F FORTRAN REAL8 VARARG
ADDR FWORD REAL10 WORD
BASIC NEAR SBYTE ZERO?
BYTE NEAR16 SDWORD
C OVERFLOW? SIGN?
CARRY? PARITY? STDCALL

2. 2

Predefined Symbols :Unlike most MASM reserved words, predefined symbols are case
sensitive.

Registers :
AH CS DX SI
AL CX EAX SP
AX DH EBP SS
BH DI EBX ST
BL DL ECX TR3*
BP DR0 EDI TR4*
BX DR1 EDX TR5*
CH DR2 ES TR6
CL DR3 ESI TR7

Identifiers : An identifier is a name that you invent and attach to a definition. Identifiers can be
symbols representing variables, constants, procedure names, code labels, segment names, and
user-defined data types such as structures, unions, records, and types defined with TYPEDEF.
Identifiers longer than 247 characters generate an error.

The first character of the identifier can be an alphabetic character (A–Z) or any of these
four characters : @ , _ , $ , ?
@CatStr* @Environ* @Model*
@code @fardata @SizeStr*
@CodeSize @fardata? @stack*
@Cpu @FileCur* @SubStr*
@CurSeg @FileName @Time*
@data @InStr* @Version
@DataSize @Interface* @WordSize
@Date* @Line* @Model*

3. 3

The other characters in the identifier can be any of the characters listed above or a
decimal digit (0–9).

Predefined Symbols : The assembler includes a number of predefined symbols (also called
predefined equates). These symbols can be used at any point in the code to represent the equate
value. For example, the predefined equate @ FileName represents the base name of the current
file. If the current source file is TASK.ASM , the value of @File Name is TASK.
Integer Constants and Constant Expressions : An integer constant is a series of one or more
numerals followed by an optional radix specifier. For example, in the following statements
mov ax, 25
mov bx, 0B3h
the numbers 25 and 0B3h are integer constants.

Operators : Operators are used in expressions. The assembler evaluates expressions that contain
more than one operator according to the following rules.
Operations in parentheses are performed before adjacent operations.
Binary operations of highest precedence are performed first.
Operations of equal precedence are performed from left to right.
Unary operations of equal precedence are performed right to left.
Examples of the operators are : + ,– (unary) , &, *, /, MOD, SHL, SHR , +, – (binary) , EQ,
NE, LT, LE, GT, GE NOT , AND , OR, XOR , OPATTR, SHORT, .TYPE
Data Types : A “data type” describes a set of values.For example ,BYTE, SBYTE, WORD,
SWORD, DWORD, SDWORD, FWORD, QWORD, and TBYTE.
Registers : The 8086 family of processors have a set of 16-bit registers. They are AX,
BX,CX,DX (General purpose registers) segment registers like ,SS.CS.DS.ES and pointer
registers like SP,BP and Index registers like DI and SI etc…
Statements : Statements are the line-by-line components of source files. Each MASM statement
specifies an instruction or directive for the assembler. Statements have up to four fields,
[[name:]] [[operation]] [[operands]] [[;comment]]

4. 4

Assembly Language Development Tools :
To develop an assembly language program we need certain program development tools .
The various development tools required for 8086 programming are explained below.
1. Editor : An Editor is a program which allows us to create a file containing the assembly
language statements for the program. Examples of some editors are PC write, Word-star. As we
type the program the editor stores the ACSII codes for the letters and numbers in successive
RAM locations. If any typing mistake is done editor will alert us to correct it. If we leave out a
program statement an editor will let you move everything down and insert a line. After typing all
the program we have to save the program for a hard disk. This we call it as source file. The next
step is to process the source file with an assembler. While using TASM or MASM we should
give a file name and extension .ASM.
Ex: Sample. asm
2.Assembler : An Assembler is used to translate the assembly language mnemonics into
machine language( i.e binary codes). When you run the assembler it reads the source file of your
program from where you have saved it. The assembler generates two files . The first file is the
Object file with the extension .OBJ. The object file consists of the binary codes for the
instructions and information about the addresses of the instructions. After further processing,
the contents of the file will be loaded in to memory and run. The second file is the assembler list
file with the extension .LST.
3. Linker : A linker is a program used to connect several object files into one large object file.
While writing large programs it is better to divide the large program into smaller modules. Each
module can be individually written, tested and debugged. Then all the object modules are linked
together to form one, functioning program. These object modules can also be kept in library file
and linked into other programs as needed. A linker produces a link file which contains the binary
codes for all the combined modules. The linker also produces a link map file which contains the
address information about the linked files. The linkers which comes with TASM or MASM
assemblers produce link files with the .EXE extension.

5. 5

4. Locator : A locator is a program used to assign the specific addresses of where the segments
of object code are to be loaded into memory. A locator program called EXE2BIN comes with the
IBM PC Disk Operating System (DOS). EXE2BIN converts a .EXE file to a .BIN file which has
physical addresses.
5. Debugger: A debugger is a program which allows to load your object code program into
system memory, execute the program, and troubleshoot or debug it. The debugger allows to
look into the contents of registers and memory locations after the program runs. We can also
change the contents of registers and memory locations and rerun the program. Some debuggers
allows to stop the program after each instruction so that you can check or alter memory and
register contents. This is called single step debug. A debugger also allows to set a breakpoint at
any point in the program. If we insert a break point , the debugger will run the program up to the
instruction where the breakpoint is put and then stop the execution.
6. Emulator: An emulator is a mixture of hard ware and software. It is usually used to test and
debug the hardware and software of an external system such as the prototype of a microprocessor
based instrument.
ASSEMBLER DIRECTIVES:
Assembler directives are the directions to the assembler which indicate how an operand
or section of the program is to be processed. These are also called pseudo operations which are
not executable by the microprocessor. The various directives are explained below.
1. ASSUME : The ASSUME directive is used to inform the assembler the name of the logical
segment it should use for a specified segment.
Ex: ASSUME DS : DATA tells the assembler that for any program instruction which refers to
the data segment ,it should use the logical segment called DATA.
2.DB : Define byte. It is used to declare a byte variable or set aside one or more storage locations
of type byte in memory.
For example, CURRENT_VALUE DB 36H tells the assembler to reserve 1 byte of memory for
a variable named CURRENT_ VALUE and to put the value 36 H in that memory location when
the program is loaded into RAM .

6. 6

3. DW : Define word. It tells the assembler to define a variable of type word or to reserve
storage locations of type word in memory.
4. DD(define double word) :This directive is used to declare a variable of type double word or
restore memory locations which can be accessed as type double word.
5.DQ (define quadword) :This directive is used to tell the assembler to declare a variable 4
words in length or to reserve 4 words of storage in memory .
6.DT (define ten bytes):It is used to inform the assembler to define a variable which is 10 bytes
in length or to reserve 10 bytes of storage in memory.
7. EQU : Equate It is used to give a name to some value or symbol. Every time the assembler
finds the given name in the program, it will replace the name with the value or symbol we have
equated with that name
8.ORG : Originate : The ORG statement changes the starting offset address of the data.
It allows to set the location counter to a desired value at any point in the program. For example
the statement ORG 3000H tells the assembler to set the location counter to 3000H.
9 .PROC- Procedures: It is used to identify the start of a procedure. Or subroutine.
10. END- End program .This directive indicates the assembler that this is the end of the program
module. The assembler ignores any statements after an END directive.
11. ENDP- End procedure: It indicates the end of the procedure (subroutine) to the assembler.
12.ENDS-End Segment: This directive is used with the name of the segment to indicate the end
of that logical segment.
Ex: CODE SEGMENT : Start of logical segment containing code
CODE ENDS : End of the segment named CODE.

7. 7

SIMPLE EXAMPLES :
Program: Multiplication
code segment assume cs : code
start: mov ax,10h
mov bx,2h
mul bx
mov cx,ax
int 21h
code ends
end.

Program : Display Data

data segment
datas db 'I am an Indian $'
data ends
code segment assume cs:code,ds:data
start : mov ax,data
mov ds,ax
lea dx,datas
mov ah,09h
int 21h
code ends
end start

Program : Addition

code segment assume cs: code
start : mov ax,3h
mov bx,3h
add ax,bx
mov ah,4ch
code ends
end

Program : Seven segment

data ends
code segment
assume cs: code,ds:data
loop2: mov ax,data
mov ds,ax
mov dx,0d803h
mov al,80h
out dx,al
lea bx,datas

8. 8

mov cl,10h
loop1: mov al, [bx]
mov dx,0d800h
out dx,al
call delay
inc bx
dec cl
jnz loop1
jmp loop2
delay proc near
push cx
push bx
mov cx, 07feh
back2 : mov bx,0ffffh
back1: dec bx
jnz back1
loop back2
pop bx
pop cx
ret
delay endp

Program : Seven-segment
data segment
datas db 3fh,06h,5bh,4fh,66h,6dh,7dh,07h
db 7fh,6fh,77h,7ch,39h,5eh,79h,71h
data ends
code segment
assume cs:code,ds:data
loop2 : mov ax,data
mov ds,ax
mov dx,0d803h
mov al,80h
out dx,al
lea bx,datas
mov cl,10h
loop1: mov al,[bx]
mov dx,0d800h
out dx,al
call delay
inc bx
dec cl
jnz loop1
jmp loop2
delay proc near
push cx
push bx
mov cx,07feh

9. 9

back2: mov bx,0ffffh
back1: dec bx
jnz back1
loop back2
pop bx
pop cx
ret
delay endp

program:Factorial

code segment
assume cs:code
mov ax,200
mov ss,ax
mov sp,2000
sub sp,0004h
mov ax,6
push ax
call facto
pop ax
nop
nop
nop
facto proc near
pushf
push ax
push dx
push bp
mov bp,sp
mov ax,[bp+10]
cmp ax,0001h
jne go_on
mov word ptr [bp+12],0001h
mov word ptr [bp+14],0000h
jmp exit
go_on : sub sp,0004h
dec ax
push ax
call facto
mov bp,sp
mov ax,[bp+2]
mul word ptr [bp+16]
mov [bp+18],ax
mov [bp+20],dx
add sp,0006h
exit : pop bp
pop dx

10. 10

pop ax
popf
ret
facto endp
code ends
end

Program :Message Display

assume cs:code,ds:data
data segment
message db 0dh,0ah,"mallika is the student of m.sc",0dh,0ah,"$"
;preparing string of the message
data ends
code segment
start: mov ax,data ; initializes ds
mov ds,ax
mov ah,09 ; set function value for display
mov dx, offset message
int 21h ; point to message and run
mov ah,4ch ; the interrupt
int 21h ; return to dos
code ends
end start

Program : Read Numbes Macro
MOV AH,01H
INT 21H
SUB AL,'0'
MOV BH,0AH
MUL BH
MOV NUM,AL
MOV AH,01H
INT 21H
SUB AL,'0'
ADD NUM,AL
ENDM

PRINTSTRING MACRO MSG
MOV AH,09H
MOV DX,OFFSET MSG
INT 21H
ENDM

DATA SEGMENT
CR EQU 0DH
LF EQU 0AH
MSG DB CR,LF,'ENTER NUMBER','$'

11. 11

MSG1 DB CR,LF,'RESULT: ','$'
NUM1 DB ?
NUM2 DB ?
RES DB ?
DATA ENDS
CODE SEGMENT
ASSUME CS:CODE,DS:DATA
START:
MOV AX,DATA
MOV DS,AX
PRINTSTRING MSG
READNUM NUM1
PRINTSTRING MSG
READNUM NUM2
MOV AH,00H
MOV AL,NUM1
ADD AL,NUM2
MOV RES,AL
MOV SI,OFFSET RES
CALL HEX2ASC
PRINTSTRING MSG1
PRINTSTRING RES
MOV AH,4CH
MOV AL,00H
INT 21H

HEX2ASC PROC NEAR
PUSH AX
PUSH BX
PUSH CX
PUSH DX
PUSH SI
MOV CX,00H
MOV BX,0AH
RPT1:
MOV DX,00
DIV BX
ADD DL,'0'
PUSH DX
INC CX
CMP AX,0AH
JGE RPT1
ADD AL,'0'
MOV [SI],AL
RPT2:
POP AX
INC SI
MOV [SI],AL

12. 12

LOOP RPT2
INC SI
MOV AL,'$'
MOV [SI],AL
POP SI
POP DX
POP CX
POP BX
POP AX
RET
HEX2ASC ENDP
CODE ENDS
END START

Program: Multibyte addition
DATA SEGMENT
N1 DB 11H, 22H, 33H
N2 DB 22 H, 33H, 44H
RESULT DB 3H DUP (00)
DATA ENDS
CODE SEGMENT
ASSUME CS: CODE, DS: DATA
START: MOV AX, DATA
MOV DS, AX
MOV SI, OFFSET N1
MOV DI, OFFSET N2
MOV BX, OFFSET RESULT
CLC
MOV CX, 0003H
MOV AX, 0000H
BACK: MOV AL, [SI]
MOV DL, [DI]
ADC AL, DL
MOV [BX], AL
INC SI
INC DI
INC BX
DEC CX
JNZ BACK
MOV AH, 4CH
INT 21H
INT 3H
CODE ENDS
END START

RESULT: 11H 22H 33H
22H 33H 44H
33H 55 H 77H



Program: Multi byte Multiplication

13. 13

DATA SEGMENT
N1 DB 03H, 05 H, 07 H
N2 DB 05 H, 03H, 02H
RESULT DB 4H DUP (00)
DATA ENDS
CODE SEGMENT
ASSUME CS: CODE, DS: DATA
START: MOV AX, DATA
MOV DS, AX
MOV SI, OFFSET N1
MOV DI, OFFSET N2
MOV BX, OFFSET RESULT
MOV CL, 03H
MOV AX, 0000H
MOV DX, 0000H
BACK: MOV AL, [SI]
MOV CH, [DI]
MUL DH
MOV [BX], AL
INC SI
INC DI
INC BX
LOOP BACK
MOV AH, 4CH
INT 21H
INT 3H
CODE ENDS
END START

RESULT: 03H 05H 07H
05H 03H 02H
0FH 0FH 0EH

References :
1. Programmer’s Guide, Microsoft MASM, Micro soft Corporation.
2.Microprocessor and Interfacing –D.V.Hall,TMH