Logic Instructions - WordPress.com · Basic Logic Instructions (AND, OR, XOR, TEST, NOT, NEG) Shift and Rotate instructions (SHL, SAL, SHR, SAR) Logic Instructions Course Instructor

Basic Logic Instructions (AND, OR, XOR, TEST, NOT, NEG)

Shift and Rotate instructions (SHL, SAL, SHR, SAR)

Logic Instructions

Course Instructor

Mohammed Abdul kader

Lecturer, EEE, IIUC

Segment 4A

Basic Logic Instructions

• AND

• OR

• Exclusive OR (XOR)

• NOT

• TEST

• NEG

AND Instruction

• AND operation performs logical Multiplication.

• The AND operation clears bits of a binary number. The task of clearing a bit in a binary

number is called masking.

• AND instruction uses any addressing mode except memory to memory and segment

register addressing.

Example: AND AL, BL

AND AX, [DI]

Lecture materials on "Logic Instructions" By- Mohammed abdul kader, Lecturer, EEE, IIUC 2

OR Instruction

• OR operation performs logical addition and is often called the Inclusive-OR

function

• OR instruction is used to set bit of a unknown binary number.

Example:

OR AH, BL ; AH= AH or BL

OR DX, [BX] ; DX is Ored with the word contents of data

segment memory location addressed by BX

Basic Logic Instructions (Continued)


Exclusive-OR Instruction

• The exclusive-OR instruction produces 1 when input logics are different otherwise it

produces 0. Because of this Exclusive-OR is sometimes called a comparator.

• The exclusive-OR instruction is useful if some bits of a register or memory location must be

inverted without changing the other bits.

• Example: XOR CH, DL ; CH= CH xor DL

XOR DX, [SI] ; DX is exclusive-Ored with the word

content of the data segment memory location addressed by SI.




Problem:

Develop a short sequence of instructions that sets the rightmost five bits, invert leftmost three

bits and clear middle four bits of DI without changing the remaining three bits of DI. Save the

result in SI.

invert clear set

b15 b14 b13 b12 b11 b10 b9 b8 b7 b6 b5 b4 b3 b2 b2 B0

1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 XOR with E000H

to invert

1 1 1 1 1 1 0 0 0 0 1 1 1 1 1 1 AND with FC3FH

to clear

0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 OR with 001FH to

set

Solution:

XOR DI, E000H ; invert leftmost three bits

AND DI, FC3FH ; clear middle three bits

OR DI, 001FH ; set rightmost five bits

MOV SI, DI ; save result in SI


TEST Instruction

• TEST instruction performs the AND operation. The difference is that the AND

instruction changes the destination operand, whereas the TEST instruction does not.

• The TEST instruction functions in the manner as a CMP instruction. The difference is

that the TEST instruction normally a single bit, whereas the CMP instruction tests the

entire byte or, word.

• It only changes zero flag (Z) bit. The zero flag (Z) is a logic 1 (indicating a zero result)

if the bit under test is a zero and Z=0 (indicating a nonzero result) if the bit under test

is not zero.

• Usually the TEST instruction is followed by either the JZ (jump if result zero, Z=1) or,

JNZ (jump if result not zero, Z=0) instruction.

Example: TEST DL, DH ; DL is ANDed with DH.

TEST AH, 4 ; AH is ANDed with 4.

Problem: (a) Write an instruction set to test the rightmost and leftmost bit position of AL

register. The program should jumps to the operand address RIGHT if right most bit of AL is

set and jumps to the address LEFT if leftmost bit of AL is set.



NOT

NOT instruction performs logical inversion (invert all bits of a byte or word) or find the

one’s complement.

NEG

NEG instruction performs arithmetic sign inversion or, the two’s complement. The

arithmetic sign of a signed number changes from positive to negative or from negative to

positive by NEG instruction.

Example: NOT CH ; CH is one’s complemented

NOT BYTE PTR[BX] ; The byte contents of data segment memory

location addressed by BX are one’s complemented.

Example: NEG CH ; CH is two’s complemented.

NEG AX; AX is two’s complemented.



SHIFT and ROTATE instructions

SHIFT and ROTATE instructions are used to shift or rotate any memory data or register. The

instructions mostly use to control I/O devices.

SHIFT Instruction

• Shift instructions position or move numbers to the left or right within a register or memory

location.

• They also perform simple arithmetic such as multiplication by powers of 2+𝑛 (𝑙𝑒𝑓𝑡 𝑠ℎ𝑖𝑓𝑡) and

division by powers of 2−𝑛 𝑟𝑖𝑔ℎ𝑡 𝑠ℎ𝑖𝑓𝑡 . • The microprocessor instruction set contains four different shift instruction.

Two logical shift : (a) Shift operand bits left (SHL)

(b) Shift operand bits right (SHR)

Two arithmetic shift: (c) Shift arithmetic Left (SAL)

(d) Shift arithmetic Right (SAR)

• Logical shifts (SHL and SHR): The logical shifts move a zero into the rightmost bit position

for a logical left shift and a 0 into the leftmost bit position for a logical right shift. MSB is shifted

to CF in case of SHL and LSB shifted to CF in case of SHR.


SHIFT and ROTATE instructions (continued)

• Arithmetic shifts (SAL and SAR): The arithmetic left shift and logical left shift are

identical. The arithmetic right shift and logical right shift are different because the arithmetic

right shift copies the sign bit through the numbers, where as logical right shift copies a 0

through the numbers.

• Logical VS arithmetic shift: Logical shift operations function with unsigned numbers

and arithmetic shifts function with signed numbers. Logical shifts multiply or divide

unsigned data, and arithmetic shifts multiply or divide signed data.



• Two different modes used in shift counting: One modes uses an immediate shift count

and other uses register CL to hold the shift count. Note that CL must hold the shift

count.

• Example: SHL AX, 1 ; AX is logically shifted left 1 place.

SHR BX, 12 ; BX is logically shifted right 12 places (12 times)

SAR SI, 2 ; SI is arithmetically shifted right 2 places.

SAL DATA, CL ; The contents of data segment memory location

DATA are arithmetically shifted left the number

of spaces specified by CL.

• Division by SHIFT instructions: A shift right always divides by 2 for each bit

position shifted. Two times right shifting divides by 4. For n times right shift the number

is divided by 2−𝑛.

• Multiplication by SHIFT instructions: A shift left always multiplies by 2 for each

bit position shifted. Two times left shifting multiplies by 4. For n times left shift the

number is multiplied by 2𝑛.



ROTATE instruction

• Rotate instructions position binary data by rotating the information in a register or memory

location, either from one end to another or through the carry flag.

• There are four available rotate instructions:

Rotate through carry left (RCL)

Rotate out of carry left (ROL)

Rotate through carry right (RCR)

Rotate out of carry right (ROR)



ROTATE instruction (Continued)

• A rotate count can be immediate or located in register CL.

• Example: ROL SI, 14 ; SI rotates left (out of carry) 14 places.

RCL BL, 6 ; BL rotates left through carry 6 places.

RCR AH, CL ; AH rotates right through carry the number of places

specified by CL.

ROR WORD PTR[BP], 2 ; The word contents of the stack segment memory

location addressed by BP rotate right (out of carry) 2 places.

Problem:

(a) AX=0F07H and BX=6644H, find the value of AX and BX after the execution following

instructions-

MOV CL, 4;

RCL AX, CL;

ROR BX, CL

(b) AX=0F07H and BX=6644H, find the value of AX and BX after the execution following

instructions-

SHL AX, 4

SAR BX, 3


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Logic Instructions - WordPress.com · Basic Logic Instructions (AND, OR, XOR, TEST, NOT, NEG) Shift and Rotate instructions (SHL, SAL, SHR, SAR) Logic Instructions Course Instructor