1. By Nasser Halasa
Assembly Language

2. Directives pseudo-operations
PTR- is a pointer that helps the compiler to define the size of data to store in the memory
BYTE PTR indicates the size of 8 bit data
MOV BYTE PTR[AX],10H
WORD PTR indicates the size of 16 bit data
MOV WORD PTR[DI],10H
OFFSET – loads a register with the offset address of a memory location NOT the contents of the memory.
MOV BX,DATAS; Copy the contents
MOV BX,OFFSET DATAS; Copy the offset address

3. Defining Storage Space in memory
DB – Define BYTE
DW – Define WORD
DD – Define Double
? – Reserve a memory space for data to be used in the future it place zero in to the location specified by ?
SEGMENT- define a memory segment contains various forms of data
Format
Segment Name SEGMENT
//Statements
Segment Name ENDS

4. ASSUME - to assign a segment to specific segment name
stack_seg segment
[ DW 100H DUP(?) ???? ]
stack_seg ends
Data_seg segment // data go's here
Data_seg ENDS
Assume SS: stack_seg, DS:DATA_SEG

5. DUP – duplicate Creates an array.
SAM 10 DUP(?) reserve 10 locations of memory for array SAM and stores Zero in each of them
Array1 10 DUP(2) reserve 10 locations of memory for array1 and initialize them to 02H
EQU – Equate, equates a number ,ASCII, or a label to another label
TEN EQU 10
ALPHA EQU 19
ORG – original, the starting address for code or data. It changes the starting offset address of the data in the data segment

6. PROC and ENDP – procedures or subroutines format
Label name PROC {FAR, NEAR}
FAR procedure is a global procedure that may reside at any location in the memory system
NEAR procedure is a local procedure that is reside in the same code segment as the program
ENDP indicates the end of the procedure
ADDPRO PROC FAR
ADDPRO ENDP
RET – return, returns to the main program from a procedure.

7. Memory Organization
.Models – specifies the size of memory needed for a program. There are many models mostly we will use
TINY model – all data and software fit into 64K-byte memory segment. It is good for small programs
SMALL model – one data segment and one code segment, for a total of 128k bytes of memory
.EXIT – return to DOS

8. Dos Int21H for in/out put

10. In out single char
Function 01 – inputting a single character, with an echo
AH = 01 ; function number
After the interrupt AL = ASCII code of the input and is echoed to the monitor
Function 02 – outputting a single character to the monitor
AH = 02 ; function number
DL = ASCII code of the character to be displayed

11. String output Function 09 – outputting a string of data to the monitor
AH = 09 ; function number
DX = offset address of the ASCII data to be displayed, data segment is assumed
The ASCII string must end with the dollar sign $

12. String input
Function 0A – inputting a string of data from the keyboard
AH = 0A ; function number
DX = offset address at which the string of data is stored (buffer area), data
segment is assumed and the string must end with <RETURN>
After execution:
DS:DX = buffer size in bytes (up to 255 char)
DS:DX+1 = number of entered characters excluding the return key
DS:DX+2 = first character input
· · ·
DS:DX+n = last character input
To set a buffer, use the following in the data segment:
Buffer DB 10, ? , 10 DUP(FF)

13. First program using segments
Write a program that can reads a key and displays it. key must ends the program
CODE_SEG SEGMENT 'CODE'
ASSUME CS:CODE_SEG
MAIN PROC FAR
MOV AH,6 ;READ KEY
MOV DL,0FFh
INT 21h
JE MAIN ;IF NO KEY
CMP ;TEST
JE MAIN1 ;IF THERE
MOV AH,6 ;DISPLAY KEY
MOV DL,AL
INT 21H
JMP MAIN
MAIN1: MOV AH,4CH ;EXIT TO DOS
MAIN ENDP
CODE_SEG ENDS
RUN

14. Same program using model
.MODEL TINY .CODE .STARTUP MAIN: MOV AH,6 ;READ KEY MOV DL,0FFh INT 21h JE MAIN ;IF NO KEY CMP ;TEST JE MAIN1 ;IF THERE MOV AH,6 ;DISPLAY KEY MOV DL,AL INT 21H JMP MAIN ;REPEAT MAIN1: MOV AH,4CH ;EXIT TO DOS END
RUN

15. Write a program that can read a string and print it out using segments
my_data SEGMENT
Assume DS:my_Data
org 100H
buffer EQU maxsize
maxsize db 21H
actsize db ?
string db 21H dup(?)
my_data ends
my_program SEGMENT
ASSUME CS:my_program
mov ah,0aH
lea dx, buffer
int 21h
mov bl, actsize
mov bh,0
mov string[bx],'$‘
; end Input string
mov ah,2
mov dl,13
int 21H
mov dl,10
mov ah,9
lea dx,string
my_program ends
RUN

16. RUN Using Modules org 100H .model tainy .data buffer EQU maxsize
maxsize db 21H
actsize db ?
string db 21H dup(?)
mov ah,2
mov dl,13
int 21H
mov dl,10
mov ah,9
lea dx,string
.exit
end
.code
.startup
mov ah,0aH
lea dx, buffer
int 21h
mov bl, actsize
mov bh,0
mov string[bx],'$'
;end input
RUN

17. Beep Using DOS 21
org 100H .model tainy .code .startup mov cx,10H l1: mov ah,2 mov dl,7 int 21H loop l1 .exit end
RUN