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# Assembly Language – Lab 4 - PowerPoint PPT Presentation

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

Logic, Shift, and Rotate Instructions, Binary Representation

• 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.

• 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 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

• 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.

• 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

• Converts uppercase letter to lowercase using OR instruction.

• 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

• 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

• 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.

• 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.

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

Example 4 – Cont’d