Assembly Language – Lab 4
Logic, Shift, and Rotate Instructions, Binary Representation
Logic Instructions
• Instructions to operate on bits are called logical instructions.
• The logical Instructions: AND, OR, XOR, NOT
• Syntax for AND, OR, XOR, and TEST instructions:
op-code destination, source
• NOT syntax - NOT destination
Clearing Bits: AND Instruction
• A bit and operation compares two bits and sets the result to 0 if either of
the bits is 0.
1 and 0 returns 0
0 and 1 returns 0
0 and 0 returns 0
1 and 1 returns 1
• To clear bit 5 of a byte we and the byte with 1101 1111
MOV AL, 62H ; AL = 0110 0010
AND AL, 0DFH ; and it with 1101 1111
; AL is 42H 0100 0010
Setting Bits: OR Instruction
• A bit OR operation compares two bits and sets the result to 1 if either bit is
set to 1.
1 and 0 returns 1
0 and 1 returns 1
1 and 1 returns 1
0 and 0 returns 0
Example 1
• Assign value 2 in register AL, the print the value of that register.
XOR instruction
• The XOR operation compares two bits and sets the result to 1 if the bits are
different.
1 and 0 returns 1
0 and 1 returns 1
1 and 1 returns 0
0 and 0 returns 0
• A common use of XOR is to clear a register, i.e. set all bits to 0, for example,
we can clear register AX as follows
XOR AX, AX
Example 2
• Converts uppercase letter to lowercase using OR instruction.
Shifting Bits to the Left
• To shift 1 bit to the left we use:
SHL destination,1 or
SHL destination, CL ; CL = number of shifts
• Each destination bit is shifted one position to the left
• The lsb (least significant bit) is filled with 0 (in the rightmost).
• Leftmost bits is moved into CF (Carry flag).
• CF contains the last bit shifted out.
• Each left shift multiplies by 2 the operand for both signed and
unsigned interpretations.
MOV AL, 82H ; AL = 1000 0010B
SHL AL, 2 ; AL = 0000 1000B, CF=0
Shifting bits to the right
• To shift to the right use either:
SHR destination, CL ;value of CL = number of shifts
SHR destination, 1
• The msb (most significant bit) is filled with 0 (leftmost).
• rightmost bits is moved into CF (Carry flag).
• Each single-bit right shift divides the unsigned value by 2.
MOV BL,13 ;BL = 0000 1101B = 13
SHR BL,2 ;BL = 0000 0011B = 3 (DIV BY 4),CF=0
Rotate Instructions (ROL instruction)
• The ROL (rotate left) instruction shifts the bits in the destination to the left.
• The msb is shifted into the rightmost bit and also into the CF.
Example 3
• Count the number of 1 bits in BX (BX=10111101B), without changing BX. Put
the answer in AX.
Rotate Instructions (ROR instruction)
• The ROR (rotate right) instruction shifts the bits in the destination to the right.
• The rightmost is shifted into the msb bit and also into the CF.
Example 4
• Input a binary number then display it in a newline.
Example 4 – Cont’d