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Assembly Language – Lab 4PowerPoint Presentation

Assembly Language – Lab 4

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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.
- 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

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.
- 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

- 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

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