Assembly Language for Intel
Assembly Language for Intel - - Based Based Computers, 4
Computers, 4 th th Edition Edition
Chapter 6: Conditional Processing
Kip R. Irvine
Irvine, Kip R. Assembly Language for Intel-Based Computers, 2003.
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Chapter Overview Chapter Overview
• Boolean and Comparison Instructions
• Conditional Jumps
• Conditional Loop Instructions
• Conditional Structures
• Application: Finite-State Machines
• Using the .IF Directive
Boolean and Comparison Instructions Boolean and Comparison Instructions
• CPU Status Flags
• AND Instruction
• OR Instruction
• XOR Instruction
• NOT Instruction
• Applications
• TEST Instruction
• CMP Instruction
Irvine, Kip R. Assembly Language for Intel-Based Computers, 2003.
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Status Flags
Status Flags - - Review Review
• The Zero flag is set when the result of an operation equals zero.
• The Carry flag is set when an instruction generates a result that is too large (or too small) for the destination operand.
• The Sign flag is set if the destination operand is negative, and it is clear if the destination operand is positive.
• The Overflow flag is set when an instruction generates an invalid signed result.
• Less important:
• The Parity flag is set when an instruction generates an even number of 1 bits in the low byte of the destination operand.
• The Auxiliary Carry flag is set when an operation produces a carry
out from bit 3 to bit 4.
AND Instruction AND Instruction
• Performs a Boolean AND operation between each pair of matching bits in two operands
• Syntax:
AND destination, source
(same operand types as MOV)
0 0 1 1 1 0 1 1 0 0 0 0 1 1 1 1 0 0 0 0 1 0 1 1
AND
unchanged cleared
AND
Irvine, Kip R. Assembly Language for Intel-Based Computers, 2003.
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OR Instruction OR Instruction
• Performs a Boolean OR operation between each pair of matching bits in two operands
• Syntax:
OR destination, source
OR
0 0 1 1 1 0 1 1 0 0 0 0 1 1 1 1 0 0 1 1 1 1 1 1
OR
set
unchanged
XOR Instruction XOR Instruction
• Performs a Boolean exclusive-OR operation between each pair of matching bits in two operands
• Syntax:
XOR destination, source XOR
0 0 1 1 1 0 1 1 0 0 0 0 1 1 1 1 0 0 1 1 0 1 0 0
XOR
inverted unchanged
XOR is a useful way to toggle (invert) the bits in an operand.
Irvine, Kip R. Assembly Language for Intel-Based Computers, 2003.
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NOT Instruction NOT Instruction
• Performs a Boolean NOT operation on a single destination operand
• Syntax:
NOT destination NOT
0 0 1 1 1 0 1 1 1 1 0 0 0 1 0 0
NOT
inverted
Applications
Applications (1 of 5) (1 of 5)
mov al,'a' ; AL = 01100001b
and al,11011111b ; AL = 01000001b
• Task: Convert the character in AL to upper case.
• Solution: Use the AND instruction to clear bit 5.
Irvine, Kip R. Assembly Language for Intel-Based Computers, 2003.
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Applications
Applications (2 of 5) (2 of 5)
mov al,6 ; AL = 00000110b
or al,00110000b ; AL = 00110110b
• Task: Convert a binary decimal byte into its equivalent ASCII decimal digit.
• Solution: Use the OR instruction to set bits 4 and 5.
The ASCII digit '6' = 00110110b
Applications
Applications (3 of 5) (3 of 5)
mov ax,40h ; BIOS segment
mov ds,ax
mov bx,17h ; keyboard flag byte
or BYTE PTR [bx],01000000b ; CapsLock on
• Task: Turn on the keyboard CapsLock key
• Solution: Use the OR instruction to set bit 6 in the keyboard flag byte at 0040:0017h in the BIOS data area.
This code only runs in Real-address mode, and it does not
work under Windows NT, 2000, or XP.
Irvine, Kip R. Assembly Language for Intel-Based Computers, 2003.
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Applications
Applications (4 of 5) (4 of 5)
mov ax,wordVal
and ax,1 ; low bit set?
jz EvenValue ; jump if Zero flag set
• Task: Jump to a label if an integer is even.
• Solution: AND the lowest bit with a 1. If the result is Zero, the number was even.
JZ (jump if Zero) is covered in Section 6.3.
Your turn: Write code that jumps to a label if an integer is
negative.
Applications
Applications (5 of 5) (5 of 5)
or al,al
jnz IsNotZero ; jump if not zero
• Task: Jump to a label if the value in AL is not zero.
• Solution: OR the byte with itself, then use the JNZ (jump if not zero) instruction.
ORing any number with itself does not change its value.
Irvine, Kip R. Assembly Language for Intel-Based Computers, 2003.
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TEST Instruction TEST Instruction
• Performs a nondestructive AND operation between each pair of matching bits in two operands
• No operands are modified, but the Zero flag is affected.
• Example: jump to a label if either bit 0 or bit 1 in AL is set.
test al,00000011b jnz ValueFound
• Example: jump to a label if neither bit 0 nor bit 1 in AL is set.
test al,00000011b
jz ValueNotFound
CMP Instruction
CMP Instruction (1 of 3) (1 of 3)
• Compares the destination operand to the source operand
• Nondestructive subtraction of source from destination (destination operand is not changed)
• Syntax: CMP destination, source
• Example: destination == source
mov al,5
cmp al,5 ; Zero flag set
• Example: destination < source mov al,4
cmp al,5 ; Carry flag set
Irvine, Kip R. Assembly Language for Intel-Based Computers, 2003.
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CMP Instruction
CMP Instruction (2 of 3) (2 of 3)
• Example: destination > source
mov al,6
cmp al,5 ; ZF = 0, CF = 0
(both the Zero and Carry flags are clear)
The comparisons shown so far were unsigned.
CMP Instruction
CMP Instruction (3 of 3) (3 of 3)
• Example: destination > source
mov al,5
cmp al,-2 ; Sign flag == Overflow flag
The comparisons shown here are performed with signed integers.
• Example: destination < source mov al,-1
cmp al,5 ; Sign flag != Overflow flag
Irvine, Kip R. Assembly Language for Intel-Based Computers, 2003.
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Conditional Jumps Conditional Jumps
• Jumps Based On . . .
• Specific flags
• Equality
• Unsigned comparisons
• Signed Comparisons
• Applications
• Encrypting a String
• Bit Test (BT) Instruction
J J cond cond Instruction 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
Irvine, Kip R. Assembly Language for Intel-Based Computers, 2003.
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Jumps Based on Specific Flags
Jumps Based on Specific Flags
Jumps Based on Equality
Jumps Based on Equality
Irvine, Kip R. Assembly Language for Intel-Based Computers, 2003.
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Jumps Based on Unsigned Comparisons
Jumps Based on Unsigned Comparisons
Jumps Based on Signed Comparisons
Jumps Based on Signed Comparisons
Irvine, Kip R. Assembly Language for Intel-Based Computers, 2003.
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Applications
Applications (1 of 5) (1 of 5)
cmp eax,ebx ja Larger
• Task: Jump to a label if unsigned EAX is greater than EBX
• Solution: Use CMP, followed by JA
cmp eax,ebx jg Greater
• Task: Jump to a label if signed EAX is greater than EBX
• Solution: Use CMP, followed by JG
Applications
Applications (2 of 5) (2 of 5)
cmp eax,Val1
jbe L1 ; below or equal
• Jump to label L1 if unsigned EAX is less than or equal to Val1
cmp eax,Val1 jle L1
• Jump to label L1 if signed EAX is less than or equal to Val1
Irvine, Kip R. Assembly Language for Intel-Based Computers, 2003.
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Applications
Applications (3 of 5) (3 of 5)
mov Large,bx cmp ax,bx jna Next
mov Large,ax Next:
• Compare unsigned AX to BX, and copy the larger of the two into a variable named Large
mov Small,ax cmp bx,ax jnl Next
mov Small,bx Next:
• Compare signed AX to BX, and copy the smaller of the two
into a variable named Small
Applications
Applications (4 of 5) (4 of 5)
cmp WORD PTR [esi],0 je L1
• Jump to label L1 if the memory word pointed to by ESI equals Zero
test DWORD PTR [edi],1 jz L2
• Jump to label L2 if the doubleword in memory pointed to by
EDI is even
Irvine, Kip R. Assembly Language for Intel-Based Computers, 2003.
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Applications
Applications (5 of 5) (5 of 5)
and al,00001011b ; clear unwanted bits cmp al,00001011b ; check remaining bits
je L1 ; all set? jump to L1
• 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.
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 L2 if AL has even parity.
• Write code that jumps to label L3 if EAX is negative.
• Write code that jumps to label L4 if the expression
(EBX – ECX) is greater than zero.
Irvine, Kip R. Assembly Language for Intel-Based Computers, 2003.
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Encrypting a String Encrypting a String
KEY = 239 .data
buffer BYTE BUFMAX DUP(0) bufSize DWORD ?
.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
The following loop uses the XOR instruction to transform every
character in a string into a new value.
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
Irvine, Kip R. Assembly Language for Intel-Based Computers, 2003.
