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Assembly Language for Intel

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Assembly Language for Intel

Assembly Language for Intel - - Based Based Computers, 4

Computers, 4

thth

Edition Edition

Chapter 9: Strings and Arrays

Kip R. Irvine

(2)

Chapter Overview Chapter Overview

• String Primitive Instructions

• Selected String Procedures

• Two-Dimensional Arrays

• Searching and Sorting Integer Arrays

(3)

String Primitive Instructions String Primitive Instructions

• MOVSB, MOVSW, and MOVSD

• CMPSB, CMPSW, and CMPSD

• SCASB, SCASW, and SCASD

• STOSB, STOSW, and STOSD

• LODSB, LODSW, and LODSD

(4)

MOVSB, MOVSW, and MOVSD

MOVSB, MOVSW, and MOVSD

(1 of 2)(1 of 2)

• The MOVSB, MOVSW, and MOVSD instructions copy data from the memory location pointed to by ESI to the memory location pointed to by EDI.

.data

source DWORD 0FFFFFFFFh target DWORD ?

.code

mov esi,OFFSET source mov edi,OFFSET target movsd

(5)

MOVSB, MOVSW, and MOVSD

MOVSB, MOVSW, and MOVSD

(2 of 2)(2 of 2)

• ESI and EDI are automatically incremented or decremented:

• MOVSB increments/decrements by 1

• MOVSW increments/decrements by 2

• MOVSD increments/decrements by 4

(6)

Direction Flag Direction Flag

• The Direction flag controls the incrementing or decrementing of ESI and EDI.

• DF = clear (0): increment ESI and EDI

• DF = set (1): decrement ESI and EDI

The Direction flag can be explicitly changed using the CLD and STD instructions:

CLD ; clear Direction flag STD ; set Direction flag

(7)

Using a Repeat Prefix Using a Repeat Prefix

• REP (a repeat prefix) can be inserted just before MOVSB, MOVSW, or MOVSD.

• ECX controls the number of repetitions

• Example: Copy 20 doublewords from source to target

.data

source DWORD 20 DUP(?) target DWORD 20 DUP(?) .code

cld ; direction = forward

mov ecx,LENGTHOF source ; set REP counter mov esi,OFFSET source

mov edi,OFFSET target rep movsd

(8)

Your turn . . . Your turn . . .

• Use MOVSD to delete the first element of the following doubleword array. All subsequent array values must be moved one position forward toward the beginning of the array:

array DWORD 1,1,2,3,4,5,6,7,8,9,10

.data

array DWORD 1,1,2,3,4,5,6,7,8,9,10 .code

cld

mov ecx,(LENGTHOF array) - 1 mov esi,OFFSET array+4

mov edi,OFFSET array rep movsd

(9)

CMPSB, CMPSW, and CMPSD CMPSB, CMPSW, and CMPSD

• The CMPSB, CMPSW, and CMPSD instructions

each compare a memory operand pointed to by ESI to a memory operand pointed to by EDI.

• CMPSB compares bytes

• CMPSW compares words

• CMPSD compares doublewords

• Repeat prefix (REP) is often used

(10)

Comparing a Pair of

Comparing a Pair of Doublewords Doublewords

.data

source DWORD 1234h target DWORD 5678h .code

mov esi,OFFSET source mov edi,OFFSET target

cmpsd ; compare doublewords

ja L1 ; jump if source > target jmp L2 ; jump if source <= target

If source > target, the code jumps to label L1; otherwise, it jumps to label L2

(11)

Comparing Arrays Comparing Arrays

.data

source DWORD COUNT DUP(?) target DWORD COUNT DUP(?) .code

mov ecx,COUNT ; repetition count mov esi,OFFSET source

mov edi,OFFSET target

cld ; direction = forward

repe cmpsd ; repeat while equal

Use a REPE (repeat while equal) prefix to compare corresponding elements of two arrays.

(12)

Example: Comparing Two Strings

Example: Comparing Two Strings

(1 of 3)(1 of 3)

.data

source BYTE "MARTIN "

dest BYTE "MARTINEZ"

str1 BYTE "Source is smaller",0dh,0ah,0

str2 BYTE "Source is not smaller",0dh,0ah,0

This program compares two strings (source and

destination). It displays a message indicating whether the lexical value of the source string is less than the destination string.

Source is smaller

Screen output:

(13)

Example: Comparing Two Strings

Example: Comparing Two Strings

(2 of 3)(2 of 3)

.code

main PROC

cld ; direction = forward

mov esi,OFFSET source mov edi,OFFSET dest mov cx,LENGTHOF source repe cmpsb

jb source_smaller

mov edx,OFFSET str2 ; "source is not smaller"

jmp done

source_smaller:

mov edx,OFFSET str1 ; "source is smaller"

done:

call WriteString exit

main ENDP END main

(14)

Example: Comparing Two Strings

Example: Comparing Two Strings

(3 of 3)(3 of 3)

• The following diagram shows the final values of ESI and EDI after comparing the strings:

M

M

A R T I N

A R T I N E Z

M

M

A R T I N

A R T I N E Z

After Before

Source:

Dest:

SI

DI

SI

DI

Before After

(15)

SCASB, SCASW, and SCASD SCASB, SCASW, and SCASD

• The SCASB, SCASW, and SCASD instructions compare a value in AL/AX/EAX to a byte, word, or doubleword, respectively, addressed by EDI.

• Useful types of searches:

• Search for a specific element in a long string or array.

• Search for the first element that does not match a given value.

(16)

SCASB Example SCASB Example

.data

alpha BYTE "ABCDEFGH",0 .code

mov edi,OFFSET alpha

mov al,'F' ; search for 'F'

mov ecx,LENGTHOF alpha cld

repne scasb ; repeat while not equal jnz quit

dec edi ; EDI points to 'F'

Search for the letter 'F' in a string named alpha:

What is the purpose of the JNZ instruction?

(17)

STOSB, STOSW, and STOSD STOSB, STOSW, and STOSD

• The STOSB, STOSW, and STOSD instructions store the contents of AL/AX/EAX, respectively, in memory at the offset pointed to by EDI.

• Example: fill an array with 0FFh

.data

Count = 100

string1 BYTE Count DUP(?) .code

mov al,0FFh ; value to be stored

mov edi,OFFSET string1 ; ES:DI points to target mov ecx,Count ; character count

cld ; direction = forward

rep stosb ; fill with contents of AL

(18)

LODSB, LODSW, and LODSD LODSB, LODSW, and LODSD

• The LODSB, LODSW, and LODSD instructions load a byte or word from memory at ESI into AL/AX/EAX, respectively.

.data

array 1,2,3,4,5,6,7,8,9 dest 9 DUP(?)

.code

mov esi,OFFSET array mov edi,OFFSET dest mov ecx,LENGTHOF array cld

L1: lodsb

or al,30h stosb

loop L1

(19)

Array Multiplication Example Array Multiplication Example

.data

array DWORD 1,2,3,4,5,6,7,8,9,10 multiplier DWORD 10

.code

cld ; direction = up

mov esi,OFFSET array ; source index

mov edi,esi ; destination index mov ecx,LENGTHOF array ; loop counter

L1: lodsd ; copy [ESI] into EAX

mul multiplier ; multiply by a value

stosd ; store EAX at [EDI]

loop L1

Multiply each element of a doubleword array by a constant value.

(20)

Your turn . . . Your turn . . .

• Write a program that converts each unpacked binary- coded decimal byte belonging to an array into an

ASCII decimal byte and copies it to a new array.

mov esi,OFFSET array mov edi,OFFSET dest mov ecx,LENGTHOF array cld

L1: lodsb ; load into AL

or al,30h ; convert to ASCII

stosb ; store into memory

loop L1 .data

array BYTE 1,2,3,4,5,6,7,8,9

dest BYTE (LENGTHOF array) DUP(?)

(21)

Selected String Procedures Selected String Procedures

• Str_compare Procedure

• Str_length Procedure

• Str_copy Procedure

• Str_trim Procedure

• Str_ucase Procedure

The following string procedures may be found in the Irvine32 and Irvine16 libraries:

(22)

Str_compare

Str_compare Procedure Procedure

• Compares string1 to string2, setting the Carry and Zero flags accordingly

• Prototype:

For example, if string1 > string2, CF=0, ZF=0 Or, if string1 < string2, CF=1, ZF=0

Str_compare PROTO,

string1:PTR BYTE, ; pointer to string string2:PTR BYTE ; pointer to string

(23)

Str_compare

Str_compare Source Code Source Code

Str_compare PROC USES eax edx esi edi, string1:PTR BYTE, string2:PTR BYTE mov esi,string1

mov edi,string2 L1: mov al,[esi]

mov dl,[edi]

cmp al,0 ; end of string1?

jne L2 ; no

cmp dl,0 ; yes: end of string2?

jne L2 ; no

jmp L3 ; yes, exit with ZF = 1 L2: inc esi ; point to next

inc edi

cmp al,dl ; chars equal?

je L1 ; yes: continue loop

(24)

Str_length

Str_length Procedure Procedure

• Calculates the length of a null-terminated string and returns the length in the EAX register.

• Prototype:

.data

myString BYTE "abcdefg",0 .code

INVOKE Str_length, ADDR myString

; EAX = 7

Example:

Str_length PROTO,

pString:PTR BYTE ; pointer to string

(25)

Str_length

Str_length Source Code Source Code

Str_length PROC USES edi,

pString:PTR BYTE ; pointer to string mov edi,pString

mov eax,0 ; character count L1:

cmp byte ptr [edi],0 ; end of string?

je L2 ; yes: quit

inc edi ; no: point to next

inc eax ; add 1 to count

jmp L1 L2: ret

Str_length ENDP

(26)

Str_copy

Str_copy Procedure Procedure

• Copies a null-terminated string from a source location to a target location.

• Prototype:

Str_copy PROTO,

source:PTR BYTE, ; pointer to string target:PTR BYTE ; pointer to string

(27)

Str_copy

Str_copy Source Code Source Code

Str_copy PROC USES eax ecx esi edi,

source:PTR BYTE, ; source string target:PTR BYTE ; target string

INVOKE Str_length,source ; EAX = length source

mov ecx,eax ; REP count

inc ecx ; add 1 for null byte

mov esi,source mov edi,target

cld ; direction = up

rep movsb ; copy the string

ret

Str_copy ENDP

(28)

Str_trim

Str_trim Procedure Procedure

• The Str_trim procedure removes all occurrences of a selected trailing character from a null-terminated string.

• Prototype:

Str_trim PROTO,

pString:PTR BYTE, ; points to string char:BYTE ; char to remove

.data

myString BYTE "Hello###",0 .code

INVOKE Str_trim, ADDR myString,”#”

myString = "Hello"

Example:

(29)

Str_trim

Str_trim Procedure Procedure

• Str_trim checks a number of possible cases (shown here with # as the trailing character):

• The string is empty.

• The string contains other characters followed by one or more trailing characters, as in "Hello##".

• The string contains only one character, the trailing character, as in "#"

• The string contains no trailing character, as in "Hello" or

"H".

• The string contains one or more trailing characters followed by one or more nontrailing characters, as in

"#H" or "###Hello".

(30)

Str_trim

Str_trim Source Code Source Code

Str_trim PROC USES eax ecx edi,

pString:PTR BYTE, ; points to string

char:BYTE ; char to remove

mov edi,pString

INVOKE Str_length,edi ; returns length in EAX

cmp eax,0 ; zero-length string?

je L2 ; yes: exit

mov ecx,eax ; no: counter = string length dec eax

add edi,eax ; EDI points to last char

mov al,char ; char to trim

std ; direction = reverse

repe scasb ; skip past trim character

jne L1 ; removed first character?

dec edi ; adjust EDI: ZF=1 && ECX=0 L1: mov BYTE PTR [edi+2],0 ; insert null byte

L2: ret

Str_trim ENDP

(31)

Str_ucase

Str_ucase Procedure Procedure

• The Str_ucase procedure converts a string to all uppercase characters. It returns no value.

• Prototype:

Str_ucase PROTO,

pString:PTR BYTE ; pointer to string

.data

myString BYTE "Hello",0 .code

INVOKE Str_ucase,

Example:

(32)

Str_ucase

Str_ucase Source Code Source Code

Str_ucase PROC USES eax esi, pString:PTR BYTE

mov esi,pString

L1: mov al,[esi] ; get char

cmp al,0 ; end of string?

je L3 ; yes: quit

cmp al,'a' ; below "a"?

jb L2

cmp al,'z' ; above "z"?

ja L2

and BYTE PTR [esi],11011111b ; convert the char

L2: inc esi ; next char

jmp L1 L3: ret

(33)

Two Two - - Dimensional Arrays Dimensional Arrays

• Base-Index Operands

• Base-Index Displacement

(34)

Base Base - - Index Operand Index Operand

• A base-index operand adds the values of two registers (called base and index), producing an effective

address. Any two 32-bit general-purpose registers may be used.

• Base-index operands are great for accessing arrays of structures. (A structure groups together data under a single name. )

(35)

Structure Application Structure Application

A common application of base-index addressing has to do with addressing arrays of structures (Chapter 10).

The following definds a structure named COORD containing X and Y screen coordinates:

COORD STRUCT

X WORD ? ; offset 00 Y WORD ? ; offset 02 COORD ENDS

.data

setOfCoordinates COORD 10 DUP(<>)

Then we can define an array of COORD objects:

(36)

Structure Application Structure Application

The following code loops through the array and displays each Y-coordinate:

mov ebx,OFFSET setOfCoordinates

mov esi,2 ; offset of Y value mov eax,0

L1: mov ax,[ebx+esi]

call WriteDec

add ebx,SIZEOF COORD loop L1

(37)

Base Base - - Index Index - - Displacement Operand Displacement Operand

• A base-index-displacement operand adds base and index registers to a constant, producing an effective

address. Any two 32-bit general-purpose registers may be used.

• Common formats:

[ base + index + displacement ] displacement [ base + index ]

(38)

Two Two - - Dimensional Table Example Dimensional Table Example

Imagine a table with three rows and five columns.

table BYTE 10h, 20h, 30h, 40h, 50h BYTE 60h, 70h, 80h, 90h, 0A0h BYTE 0B0h, 0C0h, 0D0h, 0E0h, 0F0h NumCols = 5

10 20 30 40 50 60 70 80 90 A0 B0 C0 D0 E0 F0

(39)

Two Two - - Dimensional Table Example Dimensional Table Example

The following code loads the table element stored in row 1, column 2:

RowNumber = 1

ColumnNumber = 2

mov ebx,NumCols * RowNumber mov esi,ColumnNumber

mov al,table[ebx + esi]

10 20 30 40 50 60 70 80 90 A0

150 155

table[ebx]

B0 C0 D0 E0 F0

table[ebx + esi]

157

table

(40)

Searching and Sorting Integer Arrays Searching and Sorting Integer Arrays

• Bubble Sort

• A simple sorting algorithm that works well for small arrays

• Binary Search

• A simple searching algorithm that works well for large arrays of values that have been placed in either ascending or descending order

(41)

Bubble Sort Bubble Sort

3 1 7 5 2 9 4 3

1 3 7 5 2 9 4 3

1 3 7 5 2 9 4 3

1 3 5 7 2 9 4 3

1 3 5 2 7 9 4 3

1 3 5 2 7 4 9 3

1 3 5 7 2 4 3 9 One Pass (Bubble Sort)

1 3 5 2 7 9 4 3

(shaded values have been exchanged)

Each pair of adjacent values is compared, and exchanged if the values are not ordered correctly:

(42)

Bubble Sort

Bubble Sort Pseudocode Pseudocode

N = array size, cx1 = outer loop counter, cx2 = inner loop counter:

cx1 = N - 1

while( cx1 > 0 ) {

esi = addr(array) cx2 = cx1

while( cx2 > 0 ) {

if( array[esi] < array[esi+4] )

exchange( array[esi], array[esi+4] ) add esi,4

dec cx2 }

dec cx1 }

(43)

Bubble Sort Implementation Bubble Sort Implementation

BubbleSort PROC USES eax ecx esi, pArray:PTR DWORD,Count:DWORD mov ecx,Count

dec ecx ; decrement count by 1 L1: push ecx ; save outer loop count

mov esi,pArray ; point to first value L2: mov eax,[esi] ; get array value

cmp [esi+4],eax ; compare a pair of values jge L3 ; if [esi] <= [edi], skip xchg eax,[esi+4] ; else exchange the pair mov [esi],eax

L3: add esi,4 ; move both pointers forward loop L2 ; inner loop

pop ecx ; retrieve outer loop count loop L1 ; else repeat outer loop

L4: ret

(44)

Binary Search Binary Search

• Searching algorithm, well-suited to large ordered data sets

• Divide and conquer strategy

• Each "guess" divides the list in half

• Classified as an O(log n) algorithm:

• As the number of array elements increases by a factor of n, the average search time increases by a factor of log n.

(45)

Binary Search Estimates

Binary Search Estimates

(46)

Binary Search

Binary Search Pseudocode Pseudocode

int BinSearch(int values[],

const int searchVal, int count) {

int first = 0;

int last = count - 1;

while( first <= last ) {

int mid = (last + first) / 2;

if( values[mid] < searchVal ) first = mid + 1;

else if( values[mid] > searchVal ) last = mid - 1;

else

return mid; // success }

return -1; // not found }

(47)

Binary Search Implementation

Binary Search Implementation

(1 of 3)(1 of 3)

BinarySearch PROC uses ebx edx esi edi,

pArray:PTR DWORD, ; pointer to array Count:DWORD, ; array size

searchVal:DWORD ; search value LOCAL first:DWORD, ; first position

last:DWORD, ; last position

mid:DWORD ; midpoint

mov first,0 ; first = 0

mov eax,Count ; last = (count - 1) dec eax

mov last,eax

mov edi,searchVal ; EDI = searchVal

mov ebx,pArray ; EBX points to the array

L1: ; while first <= last

mov eax,first cmp eax,last

(48)

Binary Search Implementation

Binary Search Implementation

(2 of 3)(2 of 3)

; mid = (last + first) / 2 mov eax,last

add eax,first shr eax,1

mov mid,eax

; EDX = values[mid]

mov esi,mid

shl esi,2 ; scale mid value by 4 mov edx,[ebx+esi] ; EDX = values[mid]

; if ( EDX < searchval(EDI) )

; first = mid + 1;

cmp edx,edi jge L2

mov eax,mid ; first = mid + 1 inc eax

mov first,eax

base-index addressing

(49)

Binary Search Implementation

Binary Search Implementation

(3 of 3)(3 of 3)

; else if( EDX > searchVal(EDI) )

; last = mid - 1;

L2: cmp edx,edi ; (could be removed) jle L3

mov eax,mid ; last = mid - 1 dec eax

mov last,eax

jmp L4 ; continue the loop

; else return mid

L3: mov eax,mid ; value found

jmp L9 ; return (mid)

L4: jmp L1 ; continue the loop L5: mov eax,-1 ; search failed

L9: ret

BinarySearch ENDP

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