Assembly Language for x86 Processors Lab 5 for 2013

1. Assembly Language (Lab 5)

2. Agenda
Revision
Conditional Loops
IF, While and Repeat Directives
Hands On

3. Data
Transfer
Inst.
MOV
MOVZX
/MOVSX
ADD/SUBINC/DEC
NEG

4. Tools
ReadDec/
ReadInt
Writestring
writeInt/
WriteDec
OFFSETLENGTHOF
TYPE
PTR

5. Addressing
Modes
Types
Register
Addressing
Immediate
Addressing
Direct
Memory
Addressing
In-Direct
Memory
Addressing
Direct
Offset
Addressing

6. Flags
Carry
Zero
Overflow
Sign
Parity
Auxiliary

7. Conditional Loop
LOOPE/LOOPZ and there counterpart …

8. LOOPZ / LOOPE Instructions
Loop if Zero / Loop if Equal.
They are like LOOP instruction except that they have one
additional condition:
The Zero flag must be set in order for control to transfer
to the destination label.
Loopz/Loope destination

9. They perform the following tasks:
Otherwise, no jump occurs, and control passes to the
next instruction
ECX = ECX - 1
if ECX > 0 and ZF = 1, jump to
destination

10. LOOPNZ / LOOPNE Instructions
Loop if Not Zero / Loop if Not Equal.
The Counterpart of LOOPZ and LOOPE.
The Zero flag must be Clear in order for control to
transfer to the destination label.
Loopnz/Loopne destination

11. They perform the following tasks:
Otherwise, no jump occurs, and control passes to the
next instruction
ECX = ECX - 1
if ECX > 0 and ZF = 0, jump to
destination

12. Hands On
Let’s make the best use of our time…

13. Min and Max in Array
Write an assembly program that finds the min and max
of an entered integer array.

14. arr_size equ 5
.data
arr DWORD arr_size DUP(0)
min DWORD 9fffffffh
max DWORD 0
strPrompt BYTE 'Enter array items:', 0
strMinMsg BYTE 'The min value = ', 0;a message
to be displayed to user
strMaxMsg BYTE 'The Max value = ', 0;a message
to be displayed to user

15. .code
main PROC
mov edx, offset strPrompt ;put the address of
strPrompt in edx
call WriteString;as a parameter to WriteString
MOV ESI, OFFSET arr
MOV ECX, arr_size
L1:
CALL ReadDec
MOV [ESI], EAX ;what saved frm ReadDec
ADD ESI, TYPE arr ;update esi
CMP EAX, min ;compare with min
JB updateMin ;update min if there is a new min
CMP EAX, max ;compare with max
JA updateMax ;update max if there is a new max
LOOP L1 ;this line will be hit ... when??
JMP next

16. updateMin:
MOV min, EAX ;update the min
Loop L1 ;continue loop if ECX > 0
JMP next ;GOTO the end if loop is done
updateMax:
MOV max, EAX ;update the max
Loop L1 ;continue loop if ECX > 0
JMP next ;GOTO the end if loop is done

17. next:
MOV EDX, OFFSET strMinMsg ;print the min msg
CALL WriteString
MOV EAX, min ;print the min
Call WriteDec
CALL CRLF
MOV EDX, OFFSET strMaxMsg ;print the max msg
CALL WriteString
MOV EAX, max
CALL WriteDec ;display the max
Call CRLF
exit
main ENDP

18. Student Grade
Write an assembly program that inputs a student’s score
and prints her/his grade. The student will be “Excellent”
if her/his score is between 90‐100, “Very Good” if the
score is between 80‐89, “Good” if the score is between
70‐79, “Fair” if the score is between 60‐69, and “Fail” if
the score is lower than 60. (Assume scores are integers.)

19. New Directives
This problem can be done using CMP && JMP, but here
we will introduce new directives to do the same
functionality

20. .IF
.IF cond1
Statements
[.ELSEIF cond2
Statements]
[.ELSE
Statements]
.ENDIF
Boolean expressions
involving two
operands and a
conditional
operator within
them.
Possible operators
are ==, !=, >, >=,
<,
<=, && and ||

21. .IF
.data
uint1 dword 5
uint2 dword 10
int1 sdword -1
mov eax, 0
.IF eax > -1
mov eax, uint2
.endif
mov eax, 0
.IF eax > int1
mov eax, uint2
.endif
Generated Code:
CMP eax, -1
JBE @C0001
mov eax, uint2
@C0001:
Generated Code:
CMP eax, int1
JLE @C0002
mov eax, uint2
@C0002:
VS

22. Student Grade
Write an assembly program that inputs a student’s score
and prints her/his grade. The student will be “Excellent”
if her/his score is between 90‐100, “Very Good” if the
score is between 80‐89, “Good” if the score is between
70‐79, “Fair” if the score is between 60‐69, and “Fail” if
the score is lower than 60. (Assume scores are integers.)

23. .data
prompt byte "Enter a student score: ",0
;print the output msg
promptA byte "Excellent",0
promptB byte "Very Good",0
promptC byte "Good",0
promptD byte "Fair",0
promptF byte "Fail",0
promptE byte "Error!!",0

24. .code
main PROC
mov edx, offset prompt
call writestring
call readdec
.IF eax > 100 || eax < 0
mov edx, offset promptE
.ELSEIF eax >= 90
mov edx, offset promptA
.ELSEIF eax >= 80
mov edx, offset promptB
.ELSEIF eax >= 70
mov edx, offset promptC
.ELSEIF eax >= 60
mov edx, offset promptD
.ELSE
mov edx, offset promptF
.ENDIF

25. call writestring ;print the output msg based on
the prev selection
call crlf
exit
main ENDP

26. Grades Counter
Write an assembly program that accepts multiple
students’ scores, and then prints number of students in
each grade. Grades and their score ranges are defined in
the previous exercise. The program should also print an
error message if the entered score is less than 0 or
greater than 100.

27. scores_cnt equ 5
.data
prompt byte "Enter 5 scores: ",0
promptA byte "Studens Have Excellent ",0
CntA DWORD 0
promptB byte "Students Have Very Good ",0
CntB DWORD 0
promptC byte "Students Have Good ",0
CntC DWORD 0
promptD byte "Students Have Fair ",0
CntD DWORD 0
promptF byte "Students Have Fail ",0
CntF DWORD 0
promptE byte "Error!!",0

28. main PROC
mov edx, offset prompt
call writestring
MOV ecx, scores_cnt
L1:
call readdec
.IF eax > 100 || eax < 0
JMP errorState
.ELSEIF eax >= 90
INC CntA
.ELSEIF eax >= 80
INC CntB
.ELSEIF eax >= 70
INC CntC
.ELSEIF eax >= 60
INC CntD
.ELSE
INC CntF
.ENDIF
LOOP L1
JMP next

29. errorState:
MOV edx, OFFSET promptE
call writestring
call crlf
JMP endState
next:
MOV edx, OFFSET promptA
call writestring
MOV eax, CntA
call writeDec
call crlf

30. MOV edx, OFFSET promptB
call writestring
MOV eax, CntB
call writeDec
call crlf
MOV edx, OFFSET promptC
call writestring
MOV eax, CntC
call writeDec
call crlf

31. MOV edx, OFFSET promptD
call writestring
MOV eax, CntD
call writeDec
call crlf
MOV edx, OFFSET promptF
call writestring
MOV eax, CntF
call writeDec
call crlf
endState:
exit

32. What If the Problem …
takes a student score, prints its grade then asks user if
s/he wants to enter another score
Repeat OR While

33. .REPEAT & .WHILE
The .REPEAT directive executes the loop body before
testing the runtime condition following the .UNTIL
directive. However, the .WHILE directive tests the
condition before executing the body loop
.REPEAT
Statements
.UNTIL cond
.WHILE cond
Statements
.ENDW
Do then
check
Check
then
DO

34. .data
prompt byte "Enter a student score: ",0
;print the output msg
promptA byte "Excellent",0
promptB byte "Very Good",0
promptC byte "Good",0
promptD byte "Fair",0
promptF byte "Fail",0
promptE byte "Error!!",0
stragain byte "Do you want to enter another
score (Y/N)? ",0

35. .code
main PROC
.REPEAT
mov edx, offset prompt
call writestring
call readdec
.IF eax > 100 || eax < 0
mov edx, offset promptE
.ELSEIF eax >= 90
mov edx, offset promptA
.ELSEIF eax >= 80
mov edx, offset promptB
.ELSEIF eax >= 70
mov edx, offset promptC
.ELSEIF eax >= 60
mov edx, offset promptD
.ELSE
mov edx, offset promptF
.ENDIF
mov edx, offset stragain
call writestring
call readchar ;input char stored in AL register
.UNTIL al == 'N' || al == 'n‘

36. Reverse a String
Write an Assembly program that reverses a string

37. .data
original byte "I love Assembly :D", 0
reversed byte SIZEOF original dup(?)

38. .code
main PROC
mov ecx, SIZEOF original
mov esi, 0 ;index for original string
mov edi, ecx ;index for reversed string
dec edi ;because array is zero indexed
dec edi ;skip the null termination char
L:
mov al, original[edi]
mov reversed[esi], al
inc esi ;slide to next character
dec edi
loop L
mov reversed[esi], 0 ;null-terminate the
reversed string
mov edx, offset reversed
call writestring
call crlf

39. Find the Non Zero …
Write an Assembly program that looks for the first
nonzero value in an array of 16-bit integers.
If it finds one, it displays the value; otherwise, it displays
a message stating that a nonzero value was not found.
0, 0, 0, 0, -1, 20, 35, -12, 66, 4, 0
Test
Case

40. intArray SWORD 0, 0, 0, 0, -1, 20, 35, -12, 66,
4, 0
noneMsg BYTE "A non-zero value was not found", 0

41. mov ebx, OFFSET intArray ;point to the array
mov ecx, LENGTHOF intArray ;loop counter
L1:
cmp WORD PTR [ebx], 0 ;compare value to zero
jnz found ;found a value
add ebx, TYPE intArray ;point to next
loop L1 ;continue the loop
jmp notFound ;none found
found: ;display the value
movsx eax, WORD PTR[ebx] ;sign-extend into EAX
call WriteInt
jmp quit
notFound: ;display "not found" message
mov edx, OFFSET noneMsg
call WriteString
quit:
call Crlf
exit

42. mov ebx, OFFSET intArray ;point to the array
mov ecx, LENGTHOF intArray ;loop counter
L1:
cmp WORD PTR [ebx], 0 ;compare value to zero
jnz found ;found a value
add ebx, TYPE intArray ;point to next
loop L1 ;continue the loop
jmp notFound ;none found
found: ;display the value
movsx eax, WORD PTR[ebx] ;sign-extend into EAX
call WriteInt
jmp quit
notFound: ;display "not found" message
mov edx, OFFSET noneMsg
call WriteString
quit:
call Crlf
exit

43. MOVSX VS.MOVZX Real Example
The output will be -1
movsx eax, WORD PTR[ebx] ;sign-extend into EAX
call WriteInt
movzx eax, WORD PTR[ebx] ;zero-extend into EAX
call WriteInt
The output will be +65535 EAX = 0000FFFF
EAX = FFFFFFFF

44. MOVSX VS.MOVZX Real Example
The output will be 65535
movsx eax, WORD PTR[ebx] ;sign-extend into EAX
call WriteDec
movzx eax, WORD PTR[ebx] ;zero-extend into EAX
call WriteDec
The output will be 4294967295
EAX = 0000FFFF
EAX = FFFFFFFF

45. Assignment…

46. Triangle of *
Write an assembly program that reads the size of a right
triangle and then prints it by asterisks. For example, if
your program reads a size of 5, it should print:
*
**
***
****
*****
Hint: WriteChar is an Irvine function that prints a single
character. This character must be stored in AL register.

47. Two Largest…
Write an assembly program that takes 10 integers and
finds the two largest values among input integers

48. Questions?!

49. Thanks!