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Lecture 6 Conditional Processing. Assembly Language for Intel-Based Computers , 4th edition Kip R. Irvine. Outline. Boolean and Comparison Instructions Conditional Jumps Conditional Loops High-Level Logic Structures. Logic Instructions. Syntax for AND, OR, XOR, and TEST :

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lecture 6 conditional processing

Lecture 6Conditional Processing

Assembly Language for

Intel-Based Computers,

4th edition

Kip R. Irvine

outline
Outline
  • Boolean and Comparison Instructions
  • Conditional Jumps
  • Conditional Loops
  • High-Level Logic Structures
logic instructions
Logic Instructions
  • Syntax for AND, OR, XOR, and TEST:
    • op-code destination, source
  • AND, OR and XOR perform the Boolean bitwise operation and store the result into destination.
  • TEST is just an AND but the result is not stored
  • Both operands must be of the same type
    • either byte, word or doubleword
  • Both operands cannot be memory at same time
    • again: memory to memory operations are forbidden
  • They modify OF, CF, SF, ZF, PF according to the result of the operation (as usual)
logic instructions cont
Logic Instructions (cont.)
  • The source is often an immediate operand called a bit mask: used to fix certain bits to 0 or 1
  • To clear a bit we use an AND since:
    • 0 AND b = 0 (b is cleared)
    • 1 AND b = b (b is conserved)
  • Ex: to clear the sign bit of AL without affecting the others, we do:
    • AND al,7Fh ;msb of AL is clearedsince 7Fh = 0111 1111b
logic instructions cont5
Logic Instructions (cont.)
  • To set (ie: set to 1) certain bits, we use OR:
    • 1 OR b = 1 (b is set)
    • 0 OR b = b (b is conserved)
  • To set the sign bit of AH, we do:
    • OR ah,80h
  • To test if ECX=0 we can do:
    • OR ecx,ecx
    • since this does not change the number in ECX and set ZF=1 if and only if ecx=0
logic instructions cont6
Logic Instructions (cont.)
  • XOR can be used to complement, conserve, or clear certain bits because:
    • b XOR 1 = NOT(b) (b is complemented)
    • b XOR 0 = b (b is conserved)
    • b XOR b = 0 (b is cleared)
  • Ex: to initialize a register to 0 we can use a two-byte instruction:
    • XOR ax,ax
    • instead of the three-byte instruction:
    • MOV ax,0This was faster on 8088\'s
logic instructions cont7
Logic Instructions (cont.)
  • Using OR and XOR in strange ways:
  • XOR ax, ax MOV ax, 0 OR ax, ax CMP ax, 0bytes 2 386/88 3 cycles 4 cycles286 2 cycles 2 cycles386 2 cycles 2 cycles486 1 cycle 1 cycle
  • Machine code33 C0 B8 0000 0B C0 83 F8 00
logic instructions cont8
Logic Instructions (cont.)
  • To convert from upper case to lower case we can use the usual method:
    • ADD dl,20h
  • But "A" = 0100 0001b "a" = 0110 0001b
    • OR dl,20h;converts from upper to lower case
    • AND dl,0DFh;converts from lower to upper casesince DFh = 1101 1111b
  • OR dl,20h and AND dh, 20hleaves lower case characters in dl unchanged and upper case characters in dh unchanged

Bit 5

logic instructions cont9
Logic Instructions (cont.)
  • To invert all the bits (ones complement):
    • NOT destination
    • does not affect any flag and destination cannot be an immediate operand
  • Reflection
    • What are the easiest ways to clearing or setting individual CPU flags?
the cmp instruction
The CMP instruction
    • CMP destination,source
  • Performs: destination - source but does not store the result of this subtraction into destination
  • But the flags are affected like SUB
  • Same restrictions on operands as SUB
  • Very often used just before performing a conditional jump
  • CMP is similar to TEST except TEST does AND operation
conditional jumps
Conditional Jumps
  • Jumps Based On . . .
    • Specific flags
    • Equality
    • Unsigned comparisons
    • Signed Comparisons
  • Applications
  • Encrypting a String
  • Bit Test (BT) Instruction
j cond instruction
Jcond Instruction
  • A conditional jump instruction branches to a label when specific register or flag conditions are met
  • Examples:
    • JB, JC jump to a label if the Carry flag is set
    • JE, JZ jump to a label if the Zero flag is set
    • JS jumps to a label if the Sign flag is set
    • JNE, JNZ jump to a label if the Zero flag is clear
    • JECXZ jumps to a label if ECX equals 0
applications 1 of 5

Task: Jump to a label if unsigned EAX is greater than EBX

  • Solution: Use CMP, followed by JA

cmp eax,ebx

ja Larger

  • Task: Jump to a label if signed EAX is greater than EBX
  • Solution: Use CMP, followed by JG

cmp eax,ebx

jg Greater

Applications (1 of 5)
applications 2 of 5

Jump to label L1 if unsigned EAX is less than or equal to Val1

cmp eax,Val1

jbe L1 ; below or equal

  • Jump to label L1 if signed EAX is less than or equal to Val1

cmp eax,Val1

jle L1

Applications (2 of 5)
applications 3 of 5

Compare unsigned AX to BX, and copy the larger of the two into a variable named Large

    • mov Large,bx
    • cmp ax,bx
    • jna Next
    • mov Large,ax
  • Next:
  • Compare signed AX to BX, and copy the smaller of the two into a variable named Small
    • mov Small,ax
    • cmp bx,ax
    • jnl Next
    • mov Small,bx
  • Next:
Applications (3 of 5)
applications 4 of 5
Applications (4 of 5)
  • Jump to label L1 if the memory word pointed to by ESI equals Zero
  • cmp WORD PTR [esi],0
  • je L1
  • Jump to label L2 if the doubleword in memory pointed to by EDI is even

test DWORD PTR [edi],1

jz L2

applications 5 of 5
Applications (5 of 5)
  • Task: Jump to label L1 if bits 0, 1, and 3 in AL are all set.
  • Solution: Clear all bits except bits 0, 1,and 3. Then compare the result with 00001011 binary.

and al,00001011b ; clear unwanted bits

cmp al,00001011b ; check remaining bits

je L1 ; all set? jump to L1

your turn
Your turn . . .
  • Write code that jumps to label L1 if either bit 4, 5, or 6 is set in the BL register.
  • Write code that jumps to label L1 if bits 4, 5, and 6 are all set in the BL register.
  • Write code that jumps to label L3 if EAX is negative.
encrypting a string
Encrypting a String

The following loop uses the XOR instruction to transform every character in a string into a new value.

  • KEY = 239 ; can be any byte value
  • BUFMAX = 128
  • .data
  • buffer BYTE BUFMAX+1 DUP(0)
  • bufSize DWORD BUFMAX
  • .code
    • mov ecx,bufSize ; loop counter
    • mov esi,0 ; index 0 in buffer
  • L1:
    • xor buffer[esi],KEY ; translate a byte
    • inc esi ; point to next byte
    • loop L1
string encryption program
String Encryption Program
  • Tasks:
    • Input a message (string) from the user
    • Encrypt the message
    • Display the encrypted message
    • Decrypt the message
    • Display the decrypted message

View the Encrypt.asm program\'s source code. Sample output:

Enter the plain text: Attack at dawn.

Cipher text: «¢¢Äîä-Ä¢-ïÄÿü-Gs

Decrypted: Attack at dawn.

conditional loop instructions
Conditional Loop Instructions
  • LOOPZ and LOOPE
  • LOOPNZ and LOOPNE
loopz and loope
LOOPZ and LOOPE
  • Syntax:

LOOPE destination

LOOPZ destination

  • Logic:
    • ECX  ECX – 1
    • if ECX > 0 and ZF=1, jump to destination
  • Useful when scanning an array for the first element that does not match a given value.
loopnz and loopne
LOOPNZ and LOOPNE
  • LOOPNZ (LOOPNE) is a conditional loop instruction
  • Syntax:

LOOPNZ destination

LOOPNE destination

  • Logic:
    • ECX  ECX – 1;
    • if ECX > 0 and ZF=0, jump to destination
  • Useful when scanning an array for the first element that matches a given value.
loopnz example
LOOPNZ Example

The following code finds the first positive value in an array:

  • .data
  • array SWORD -3,-6,-1,-10,10,30,40,4
  • sentinel SWORD 0
  • .code
    • mov esi,OFFSET array
    • mov ecx,LENGTHOF array
  • next:
    • test WORD PTR [esi],8000h ; test sign bit
    • pushfd ; push flags on stack
    • add esi,TYPE array
    • popfd ; pop flags from stack
    • loopnz next ; continue loop
    • jnz quit ; none found
    • sub esi,TYPE array ; ESI points to value
  • quit:
your turn29
Your turn . . .

Locate the first nonzero value in the array. If none is found, let ESI point to the sentinel value:

  • .data
  • array SWORD 50 DUP(?)
  • sentinel SWORD 0FFFFh
  • .code
    • mov esi,OFFSET array
    • mov ecx,LENGTHOF array
  • L1: cmp WORD PTR [esi],0 ; check for zero
  • (fill in your code here)
  • quit:
solution
. . . (solution)
  • .data
  • array SWORD 50 DUP(?)
  • sentinel SWORD 0FFFFh
  • .code
    • mov esi,OFFSET array
    • mov ecx,LENGTHOF array
  • L1: cmp WORD PTR [esi],0 ; check for zero
    • pushfd ; push flags on stack
    • add esi,TYPE array
    • popfd ; pop flags from stack
    • loope L1 ; continue loop
    • jz quit ; none found
    • sub esi,TYPE array ; ESI points to value
  • quit:
conditional structures
Conditional Structures
  • Block-Structured IF Statements
  • Compound Expressions with AND
  • Compound Expressions with OR
  • WHILE Loops
  • Table-Driven Selection
block structured if statements
Block-Structured IF Statements

Assembly language programmers can easily translate logical statements written in C++/Java into assembly language. For example:

if( op1 == op2 )

X = 1;

else

X = 2;

    • mov eax,op1
    • cmp eax,op2
    • jne L1
    • mov X,1
    • jmp L2
  • L1: mov X,2
  • L2:
your turn33
Your turn . . .

Implement the following pseudocode in assembly language. All values are unsigned:

if( ebx <= ecx )

{

eax = 5;

edx = 6;

}

    • cmp ebx,ecx
    • ja next
    • mov eax,5
    • mov edx,6
  • next:

(There are multiple correct solutions to this problem.)

your turn34
Your turn . . .

Implement the following pseudocode in assembly language. All values are 32-bit signed integers:

if( var1 <= var2 )

var3 = 10;

else

{

var3 = 6;

var4 = 7;

}

    • mov eax,var1
    • cmp eax,var2
    • jle L1
    • mov var3,6
    • mov var4,7
    • jmp L2
  • L1: mov var3,10
  • L2:

(There are multiple correct solutions to this problem.)

compound expression with and 1 of 3
Compound Expression with AND (1 of 3)
  • When implementing the logical AND operator, consider that HLLs use short-circuit evaluation
  • In the following example, if the first expression is false, the second expression is skipped:

if (al > bl) AND (bl > cl)

X = 1;

compound expression with and 2 of 3
Compound Expression with AND (2 of 3)

if (al > bl) AND (bl > cl)

X = 1;

This is one possible implementation . . .

cmp al,bl ; first expression...

ja L1

jmp next

L1:

cmp bl,cl ; second expression...

ja L2

jmp next

L2: ; both are true

mov X,1 ; set X to 1

next:

compound expression with and 3 of 3
Compound Expression with AND (3 of 3)

if (al > bl) AND (bl > cl)

X = 1;

But the following implementation uses 29% less code by reversing the first relational operator. We allow the program to "fall through" to the second expression:

cmp al,bl ; first expression...

jbe next ; quit if false

cmp bl,cl ; second expression...

jbe next ; quit if false

mov X,1 ; both are true

next:

your turn38
Your turn . . .

Implement the following pseudocode in assembly language. All values are unsigned:

if( ebx <= ecx

&& ecx > edx )

{

eax = 5;

edx = 6;

}

    • cmp ebx,ecx
    • ja next
    • cmp ecx,edx
    • jbe next
    • mov eax,5
    • mov edx,6
  • next:

(There are multiple correct solutions to this problem.)

compound expression with or 1 of 2
Compound Expression with OR (1 of 2)
  • When implementing the logical OR operator, consider that HLLs use short-circuit evaluation
  • In the following example, if the first expression is true, the second expression is skipped:

if (al > bl) OR (bl > cl)

X = 1;

compound expression with or 1 of 240
Compound Expression with OR (1 of 2)

if (al > bl) OR (bl > cl)

X = 1;

We can use "fall-through" logic to keep the code as short as possible:

cmp al,bl ; is AL > BL?

ja L1 ; yes

cmp bl,cl ; no: is BL > CL?

jbe next ; no: skip next statement

L1: mov X,1 ; set X to 1

next:

while loops

This is a possible implementation:

top: cmp eax,ebx ; check loop condition

jae next ; false? exit loop

inc eax ; body of loop

jmp top ; repeat the loop

next:

WHILE Loops

A WHILE loop is really an IF statement followed by the body of the loop, followed by an unconditional jump to the top of the loop. Consider the following example:

while( eax < ebx)

eax = eax + 1;

your turn42
Your turn . . .

Implement the following loop, using unsigned 32-bit integers:

while( ebx <= val1)

{

ebx = ebx + 5;

val1 = val1 - 1

}

top: cmp ebx,val1 ; check loop condition

ja next ; false? exit loop

add ebx,5 ; body of loop

dec val1

jmp top ; repeat the loop

next:

using the if directive
Using the .IF Directive
  • Runtime Expressions
  • Relational and Logical Operators
  • MASM-Generated Code
  • .REPEAT Directive
  • .WHILE Directive
runtime expressions
Runtime Expressions
  • .IF, .ELSE, .ELSEIF, and .ENDIF can be used to evaluate runtime expressions and create block-structured IF statements.
  • Examples:

.IF eax > ebx

mov edx,1

.ELSE

mov edx,2

.ENDIF

.IF eax > ebx && eax > ecx

mov edx,1

.ELSE

mov edx,2

.ENDIF

  • MASM generates "hidden" code for you, consisting of code labels, CMP and conditional jump instructions.
masm generated code
MASM-Generated Code

.data

val1 DWORD 5

result DWORD ?

.code

mov eax,6

.IF eax > val1

mov result,1

.ENDIF

Generated code:

mov eax,6

cmp eax,val1

jbe @C0001

mov result,1

@C0001:

MASM automatically generates an unsigned jump (JBE) because val1 is unsigned.

masm generated code47
MASM-Generated Code

.data

val1 SDWORD 5

result SDWORD ?

.code

mov eax,6

.IF eax > val1

mov result,1

.ENDIF

Generated code:

mov eax,6

cmp eax,val1

jle @C0001

mov result,1

@C0001:

MASM automatically generates a signed jump (JLE) because val1 is signed.

masm generated code48
MASM-Generated Code

.data

result DWORD ?

.code

mov ebx,5

mov eax,6

.IF eax > ebx

mov result,1

.ENDIF

Generated code:

mov ebx,5

mov eax,6

cmp eax,ebx

jbe @C0001

mov result,1

@C0001:

MASM automatically generates an unsigned jump (JBE) when both operands are registers . . .

masm generated code49
MASM-Generated Code

.data

result SDWORD ?

.code

mov ebx,5

mov eax,6

.IF SDWORD PTR eax > ebx

mov result,1

.ENDIF

Generated code:

mov ebx,5

mov eax,6

cmp eax,ebx

jle @C0001

mov result,1

@C0001:

. . . unless you prefix one of the register operands with the SDWORD PTR operator. Then a signed jump is generated.

repeat directive
.REPEAT Directive

Executes the loop body before testing the loop condition associated with the .UNTIL directive.

Example:

; Display integers 1 – 10:

mov eax,0

.REPEAT

inc eax

call WriteDec

call Crlf

.UNTIL eax == 10

while directive
.WHILE Directive

Tests the loop condition before executing the loop body The .ENDW directive marks the end of the loop.

Example:

; Display integers 1 – 10:

mov eax,0

.WHILE eax < 10

inc eax

call WriteDec

call Crlf

.ENDW

macros
Macros
  • Introducing Macros
  • Defining Macros
  • Invoking Macros
  • Macro Examples
  • Nested Macros
  • Example Program: Wrappers
introducing macros
Introducing Macros
  • A macro1 is a named block of assembly language statements.
  • Once defined, it can be invoked (called) one or more times.
  • During the assembler\'s preprocessing step, each macro call is expanded into a copy of the macro.
  • The expanded code is passed to the assembly step, where it is checked for correctness.

1Also called a macro procedure.

defining macros
Defining Macros
  • A macro must be defined before it can be used.
  • Parameters are optional.
  • Each parameter follows the rules for identifiers. It is a string that is assigned a value when the macro is invoked.
  • Syntax:

macroname MACRO [parameter-1, parameter-2,...]

statement-list

ENDM

mnewline macro example
mNewLine Macro Example

This is how you define and invoke a simple macro.

mNewLine MACRO ; define the macro

call Crlf

ENDM

.data

.code

mNewLine ; invoke the macro

The assembler will substitute "call crlf" for "mNewLine".

mputchar macro
mPutChar Macro

Writes a single character to standard output.

mPutchar MACRO char

push eax

mov al,char

call WriteChar

pop eax

ENDM

Definition:

.code

mPutchar \'A\'

Invocation:

1 push eax

1 mov al,\'A\'

1 call WriteChar

1 pop eax

viewed in the listing file

Expansion:

invoking macros 1 of 2
Invoking Macros (1 of 2)
  • When you invoke a macro, each argument you pass matches a declared parameter.
  • Each parameter is replaced by its corresponding argument when the macro is expanded.
  • When a macro expands, it generates assembly language source code.
  • Arguments are treated as simple text by the preprocessor.
invoking macros 2 of 2
Invoking Macros (2 of 2)

Relationships between macros, arguments, and parameters:

mwritestr macro 1 of 2
mWriteStr Macro (1 of 2)

Provides a convenient way to display a string, by passing the string name as an argument.

mWriteStr MACRO buffer

push edx

mov edx,OFFSET buffer

call WriteString

pop edx

ENDM

.data

str1 BYTE "Welcome!",0

.code

mWriteStr str1

mwritestr macro 2 of 2
mWriteStr Macro (2 of 2)

The expanded code shows how the str1 argument replaced the parameter named buffer:

mWriteStr MACRO buffer

push edx

mov edx,OFFSET buffer

call WriteString

pop edx

ENDM

1 push edx

1 mov edx,OFFSET str1

1 call WriteString

1 pop edx

invalid argument
Invalid Argument
  • If you pass an invalid argument, the error is caught when the expanded code is assembled.
  • Example:

mPutchar MACRO char

push eax

mov al,char

call WriteChar

pop eax

ENDM

1 push eax

1 mov al,1234h ; error!

1 call WriteChar

1 pop eax

.code

mPutchar 1234h

blank argument
Blank Argument
  • If you pass a blank argument, the error is also caught when the expanded code is assembled.
  • Example:

mPutchar MACRO char

push eax

mov al,char

call WriteChar

pop eax

ENDM

.code

mPutchar

1 push eax

1 mov al,

1 call WriteChar

1 pop eax

your turn63
Your turn . . .
  • Write a macro named mMove32 that receives two 32-bit memory operands. The macro should move the source operand to the destination operand.

. . . Solution

mMove32 MACRO destination,source

push eax

mov eax,source

mov destination,eax

pop eax

ENDM

ad