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					            In Class Program
• Write, assemble and test a program:
  – Use the DB directive to define the following list
    of numbers and name it array:
     • 31h, 32h, 33h, 34h
  – Write instructions to load each number into DL
    and display it on the console. (use Int 21h,
    function 2h to display the byte in DL on the
• Explain why the output is “1234”
•   Get template
•   Make necessary changes.
•   Assemble and Link using make16
•   Run and debug using Codeview
              Template for Assembler Programs (Spring, 2003)
; Program Name                                 *

; Data Segment                                 *
; put data here
; Code Segment                                 *
.code main proc
            mov ax,@data
            mov ds,ax                              ;initialize DS register

; put code here

         mov ah,4Ch                                ;int 21h function 4c
         int 21h                                   ;return to DOS
main endp
end main                                           ;end of program
            Defining Data
• .data
• List    byte 31h, 32h, 33h, 34h
    Writing to Standard Output
• Interrupt 21h (MS-DOS Services)
  – Function 2
     • Write character stored in DL to standard out
       (Appendix C-2, page 652 of Irvine 4th Edition)
• We have to get the data from memory to
  register DL and then
  Mov ah,2         ; use the mov instruction to
                   ; store the interrupt 21h
                   ; function number in register AH
  Int 21h          ; use int instruction to process
                   ; interrupt
Moving Data from Memory to DL

• MOV destination, source
  –   The source operand is unchanged!
  –   Both operands must be the same size
  –   Both operands cannot be memory operands!
  –   CS and IP cannot be destination operands
  –   An immediate value cannot be moved to a
      segment register
              MOV Instruction
•   MOV        reg, reg
•   MOV        mem, reg
•   MOV        reg, mem
•   MOV        mem, imm
•   MOV        reg, imm
•   In real address mode
    – MOV    r/m16, sreg
    – MOV    sreg, r/m16
Moving Data from Memory to DL
•   Mov dl, list       ; AL = 31h
•   Mov dl, list + 1   ; AL = 32h
•   Mov dl, list + 2   ; AL = 33h
•   Mov dl, list + 3   ; AL = 34h
  Using Direct-Offset Operands
• The data in list (31h, 32h, 33h, 34h) can be
  accessed using the label (list).
• Putting brackets around the source operand
  does not affect the outcome. They are not
  required by MASM.
  – List+1 == [List+1]
• Use List+2, List+4, etc… for WORD lists
• Use List+4, List+8, etc… for DWORD lists
Title Writing_1234_to_Screen (1234_2.ASM)
          array db 31h,32h,33h,34h          ;use db to define each element as 1 byte
main      proc
          mov          ax,@data             ;copy the address of the data
          mov          ds,ax                ;segment(@data) into the DS register
          mov          ah,2                 ;int 21 function 2 displays the character in DL
          mov          dl, array            ;copy 1st byte of array to dl
          int          21h
          mov          dl, array+1          ;copy 2nd byte to dl - uses direct-offset to access list elements
          int          21h
          mov          dl, array+2          ;copy 3rd byte to dl
          int          21h
          mov          dl, array+3          ;copy 4th byte to dl
          int          21h
          mov          ax, 4c00h            ;terminate program and return to DOS
          int          21h
main      endp
end       main
Can INC instruction be used to access
   different elements of the list ?
 • The INC instruction adds 1 to a single
    – INC reg/mem
 • So can we use
    – INC array (example program)
 • No! The only practical way to handle an
   array or list is to use a register as a pointer
   and change the register value to point to
   different elements of the list.
          OFFSET Operator
• Returns the offset of a data label.
• In protected mode, an offset is always 32-
  bits long
• In real mode, an offset is always 16-bits
            Change Code to …
•   Mov bx, OFFSET list
•   Mov dl, [bx]
•   Int 21h
•   Inc bx
•   Mov dl, [bx]
•   Int 21h
•   Inc bx

NOTE: Inc works because the data is bytes. Use:
    – Add bx,2    ;for word size data
    – Add bx,4    ;for Doubleword size data
           LOOP Instruction
• LOOP provides a simple way to repeat a
  block of statements a specific number of
• CX (ECX) is automatically used as the
• LOOP destination is put at the end of a
  section of code to be repeated. First, CX is
  decremented, then CX is compared to zero.
• If CX is not equal to zero a jump is taken to
  the label identified by destination.
         array db 31h,32h,33h,34h    ;use db to define array
         COUNT = ($-array)           ;The $ operator gives the value of the location counter.
main     proc
         mov      ax, @data          ;copy the address of the data segment
         mov      ds, ax             ;@data into the DS register
         mov      bx, offset array   ;the offset operator returns the 16-bit offset of a label
         mov      cx, COUNT          ;set up cx register as a counter register.
         mov      ah, 02             ;use function 2 of int 21h - display char stored in dl on screen
LP1:     mov      dl, [bx]           ;LP1 is a label
         int      21h
         inc      bx
         loop     LP1                ;decrement cx; if cx not =0,loop back to label LP1.
         mov      ax, 4c00h
         int      21h
main     endp
end      main

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