Assembly language lab 4
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Assembly Language – Lab 4. Logic, Shift, and Rotate Instructions, Binary Representation. Logic Instructions. I nstructions to operate on bits are called logical instructions . The logical Instructions: AND , OR , XOR , NOT Syntax for AND , OR , XOR , and TEST instructions:

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Assembly Language – Lab 4

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Assembly language lab 4

Assembly Language – Lab 4

Logic, Shift, and Rotate Instructions, Binary Representation


Logic instructions

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

Clearing Bits: AND Instruction

  • A bit and operation compares two bits and sets the result to 0 if either of the bits is 0.

  • 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

1 and 0 returns 0

0 and 1 returns 0

0 and 0 returns 0

1 and 1 returns 1


Setting bits or i nstruction

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

Example 1

  • Assign value 2 in register AL, the print the value of that register.


Xor instruction

XOR instruction

  • The XOR operation compares two bits and sets the result to 1 if the bits are different.

  • 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

1 and 0 returns 1

0 and 1 returns 1

1 and 1 returns 0

0 and 0 returns 0


Example 2

Example 2

  • Converts uppercase letter to lowercase using OR instruction.


Shifting bits to the left

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

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

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

Example 3

  • Count the number of 1 bits in BX (BX=10111101B), without changing BX. Put the answer in AX.


Rotate instructions ror instruction

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

Example 4

  • Input a binary number then display it in a newline.


Example 4 cont d

Example 4 – Cont’d


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