Boolean and Comparison Instructions

1
Boolean and Comparison Instructions
Operation Description
AND AND Destination, Source
OR OR Destination, Source
XOR XOR Destination, Source
NOT NOT Destination
TEST Implied AND Destination, Source, Only FLAGS changed
BT,BTC,BTR,BTS  
2
AND Instruction

• Bitwise AND (result placed in destination)
• Mov al, 00001111b
• And al, 00111011b al ?
• al 00001011b (0Bh)
• Mov al, 6Dh
• And 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

3
ApplicationConverting Characters to Uppercase

• 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, 61h
• AND al, 11011111b (DFh)
• al 41h

4
OR Instruction

• Bitwise OR (result placed in destination)
• Mov al, 0Fh
• Or al, 61h al ?
• al 6Fh
• Mov al, 3Dh
• Or 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

5
Application Converting a Decimal Digit (byte) to
ASCII

• 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 5
• Mov al, 9
• OR al, 30h
• al 00111001b (39h)

6
ApplicationDetermining the sign of a value by
ORing register with itself

• OR 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
7
XOR Instruction

• Bitwise XOR (result stored in destination)
• Mov al, 94h
• XOR al, 37h al ?
• al 10100011b
• Mov al, 72h
• Xor 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

8
Parity Flag

• 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, 10110101b
• XOR al, 0
• al unchanged parity flag clear (PO)
• Mov al, 11001100b
• XOR al, 0
• al unchanged parity flag set (PE)

9
ApplicationHow to check the parity of 16-bit
registers

• Perform an XOR between upper-and lower bytes
• Mov ax, 64C1h 0110 0100 1100 0001
• XOR ah, al
• ah 10100101
• Parity bit set (PE)

10
Application How to check the parity of 32-bit
registers

• Perform an XOR between bytes
• Mov eax, 56D764C1h
• 0101 0110 1101 0111 0110 0100 1100 0001
• XOR ah, al ah 1010 0101
• Shr eax, 8
• XOR ah, al ah 0111 0010
• Shr eax, 8
• XOR ah, al ah 0010 0100
• Parity bit set (PE)

11
NOT Instruction

• Toggles all bits in an operand
• No flags are affected

12
TEST 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

13
CMP Instruction

• Implied SUB (no registers are changed)
• (flags may be modified)
• May modify Overflow, Sign, Zero, Aux. Carry, and
  Parity flags

14
CMP Instruction Results
Zero Flag Carry Flag CMP Results
0 1 destination lt source CMP 5, 10
0 0 destination gt source CMP 105, 10
1 0 destination source CMP 10, 10
15
CMP of Signed Integers
Flags CMP Results
SF ? OF destination lt source
SF OF destination gt source
ZF 1 destination source
16
In Class Problems

• Write a single instruction that clears the high 8
  bits of AX and does not change the low 8 bits.
• AND AX, 00FFh

17
In Class Problems

• Write a single instruction that sets the high 8
  bits of AX and does not change the low 8 bits.
• OR AX, FF00h

18
In Class Problems

• Write a single instruction that complements all
  the bits in EAX (do not use the NOT instruction)
• XOR EAX, FFFFFFFFh

19
In Class Problems

• 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