Transcript In Class Programs in Assembly

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
console)
• Explain why the output is “1234”
Procedure
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Get template
Make necessary changes.
Assemble and Link using make16
Run and debug using Codeview
Template for Assembler Programs (Spring, 2003)
;***********************************************
; Program Name
*
;***********************************************
title
INCLUDE Irvine16.inc
;***********************************************
; Data Segment
*
;***********************************************
.data
; 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
main endp
end main
;int 21h function 4c
;return to DOS
;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
Int 21h
; use the mov instruction to
; store the interrupt 21h
; function number in register AH
; use int instruction to process
; interrupt
Moving Data from Memory to DL
• MOV destination, source
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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
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MOV
reg, reg
MOV
mem, reg
MOV
reg, mem
MOV
mem, imm
MOV
reg, imm
In real address mode
– MOV
– MOV
r/m16, sreg
sreg, r/m16
Moving Data from Memory to DL
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Mov dl, list
Mov dl, list + 1
Mov dl, list + 2
Mov dl, list + 3
; AL = 31h
; AL = 32h
; AL = 33h
; 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)
INCLUDE irvine16.inc
.data
array db 31h,32h,33h,34h
;use db to define each element as 1 byte
.code
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
int
21h
mov
dl, array+2
int
21h
mov
dl, array+3
int
21h
mov
ax, 4c00h
int
21h
main
endp
end
main
;copy 2nd byte to dl - uses direct-offset to access list elements
;copy 3rd byte to dl
;copy 4th byte to dl
;terminate program and return to DOS
Can INC instruction be used to access
different elements of the list ?
• The INC instruction adds 1 to a single
operand
– 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 32bits long
• In real mode, an offset is always 16-bits
long.
Change Code to …
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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
– Add bx,4
;for word size data
;for Doubleword size data
LOOP Instruction
• LOOP provides a simple way to repeat a
block of statements a specific number of
times.
• CX (ECX) is automatically used as the
counter
• 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.
INCLUDE irvine16.inc
.data
array db 31h,32h,33h,34h
;use db to define array
COUNT = ($-array)
;The $ operator gives the value of the location counter.
.code
main
LP1:
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
mov
dl, [bx]
;LP1 is a label
int
21h
inc
bx
loop
LP1
mov
ax, 4c00h
int
21h
main
endp
end
main
;decrement cx; if cx not =0,loop back to label LP1.