boolean and comparison_instructions

Boolean and Comparison Instructions

Operation DescriptionAND AND Destination, Source

OR OR Destination, Source

XOR XOR Destination, Source

NOT NOT Destination

TESTImplied AND Destination, Source, Only FLAGS changed

BT,BTC,BTR,BTS 05/03/23 SHOUA IQBAL 1

Bitwise AND (result placed in destination)

Mov al, 00001111bAnd al, 00111011b al = ?

• al = 00001011b (0Bh)Mov al, 6DhAnd al, 4Ah al = ?

• al = 48h

• AND with a 0 to clear bits • AND with a 1 to preserve bits (bit

extraction) • Always clears the Overflow and Carry flags. • May modify the Sign, Zero, and Parity flag

AND Instruction

05/03/23 SHOUA IQBAL 2

Lowercase A (‘a’) is 61 0 1 1 0 0 0 0 1 Uppercase A (‘A’) is 41 0 1 0 0 0 0 0 1

Need to clear bit 5

Mov al, 61hAND al, 11011111b ;(DFh)

;al = 41h

Application:Converting Characters to Uppercase


Bitwise OR (result placed in destination)

Mov al, 0Fh Or al, 61h al = ?

• al = 6FhMov al, 3DhOr al, 74h al = ?

• al = 7Dh OR with 1 to set selected bits OR with 0 to preserve bits Always clears the Overflow and Carry Flags May modify the Sign, Zero, and Parity Flags

OR Instruction


Binary representation for 9 0 0 0 0 1 0 0 1 Ascii 9 is 39h 0 0 1 1 1 0 0 1

Need to set bits 4 and 5Mov al, 9OR al, 30h

;al = 00111001b (39h)

Application: Converting a Decimal Digit (byte) to ASCII


Application:Determining the sign of a value by ORing register with

itselfOR al, al

Zero Flag Sign Flag Value in AL is …

clear clear greater than 0

set clear equal to 0

clear set less than 0


Bitwise XOR (result stored in destination)

Mov al, 94hXOR al, 37h al = ?

al = 10100011bMov al, 72hXor al, 0DCh al = ?

al = 10101110b• XOR reverses itself when applied twice to the

same operand (data encryption)• Clears the Overflow and Carry flags. • May modify the sign, zero, and parity flags

XOR Instruction


Indicates if the LOWEST BYTE of the result of a bitwise

or arithmetic operation has an even or odd number of 1 bits.

Mov al, 10110101bXOR al, 0

; al unchanged parity flag clear (PO)Mov al, 11001100bXOR al, 0

; al unchanged parity flag set (PE)

Parity Flag


Perform an XOR between upper-and lower bytes

Mov ax, 64C1h ;0110 0100 1100 0001XOR ah, al

;ah = 10100101 Parity bit set (PE)

Application:How to check the parity of 16-bit registers


Perform an XOR between bytes

Mov eax, 56D764C1h;0101 0110 1101 0111 0110 0100 1100 0001

XOR ah, al ;ah = 1010 0101Shr eax, 8XOR ah, al ;ah = 0111 0010Shr eax, 8XOR ah, al ;ah = 0010 0100

Parity bit set (PE)

Application: How to check the parity of 32-bit registers


Toggles all bits in an operand No flags are affected

NOT Instruction


Implied AND (no registers are changed)

(flags may be modified)

• Valuable for determining if individual bits are set.

• Always clears the Overflow and Carry flags• May modify the sign, zero, and parity flags

TEST Instruction


Implied SUB (no registers are changed)

(flags may be modified)

• May modify Overflow, Sign, Zero, Aux. Carry, and Parity flags

CMP Instruction


CMP Instruction Results

Zero Flag Carry Flag CMP Results

0 1 destination < sourceCMP 5, 10

0 0 destination > sourceCMP 105, 10

1 0 destination = sourceCMP 10, 10


CMP of Signed IntegersFlags CMP Results

SF ≠ OF destination < source

SF = OF destination > source

ZF = 1 destination = source


1. Write a single instruction that clears the high

8 bits of AX and does not change the low 8 bits.

AND AX, 00FFh

In Class Problems


2. Write a single instruction that sets the high 8

bits of AX and does not change the low 8 bits.

OR AX, FF00h

In Class Problems


3. Write a single instruction that complements

all the bits in EAX (do not use the NOT instruction)

XOR EAX, FFFFFFFFh

In Class Problems


4. Write instructions that set the Zero flag if the

32-bit value in EAX is even, and clear the Zero flag if EAX is odd.

TEST EAX, 0000 0000 0000 0000 0000 0000 0000 0001b

In Class Problems


boolean and comparison_instructions

Education