ORG 100H             ;Tell to the compiler that you want to get a *.com program.
;Create File:
MOV AH,3CH         ;Use INT 21H and AH=3CH to createe a file and get it's file handle.
MOV CX,0            ;0-Normal,1-ReadOnly,2-Hide,3-ReadOnly&Hide,4-System,5-System&ReadOnly,6-System&Hide,7-System&ReadOnly&Hide
MOV DX,FILENAME;Pass the string of the filename to DX.It means the program passes the filename to DS:DX.
INT 21H
;If you just want to open the file and not create the file,you can replace the CREAT FILE CODE with the following code.
;Open File:
;MOV AH,3DH         ;Use INT 21H and AH=3DH to open a file and get it's file handle.
;MOV AL,1             ;When AL=0,it will read a file.
                          ;When AL=3,it will read or write a file.
;MOV DX,FILENAME ;Pass the string of the filename to DX.It means the program passes the filename to DS:DX.
;INT 21H               ;Do as we command:open the file,and ask for the file handle.
                         ;If we succeeded,we can get the file handle from AX.
                         ;If we failed,we can get the ERROE CODE from AX.
MOV BX,AX          ;Now,we had better copy the file handle to BX.Because we will write a file soon and BX's function is storing the file handle.
;Write File:
MOV AH,40H        ;Use INT 21H and AH=40H to write a file.
MOV CX,11          ;This is the length of the string.
MOV DX,STR1      ;Pass the string yo DX.It means the program passes the string to DS:DX and write it to your file.
INT 21H              ;Do as we command:write the string to the file we want to write.
                         ;If we succeeded,we can get the real length of the string from AX.
                         ;If we failed,we can get the ERROE CODE from AX.
;Close File:
MOV AH,3EH        ;Use INT 21H and AH=3EH to close a file.
INT 21H              ;Do as we command:close the file.
                         ;If we failed,we can get the ERROE CODE from AX.
;Exit:
MOV AX, 4C00H   
INT 21H              ;Exit
 
FILENAME DB 'D:\TEST.TXT',0
STR1 DB 'HELLO WORLD'