1. Microprocessor & Assembly Language
Logical, shift, rotate, and bit Instructions

2. Basic Logic Instructions
Include AND, OR, Exclusive-OR, and NOT.
– also TEST, a special form of the AND instruction
– NEG, similar to the NOT instruction
Logic operations provide binary bit control in low-level software.
–Can manipulate bits, do logical operations, allow bits to be set, cleared, or complemented and do arithmetic operations
• Low-level software appears in machine language or assembly language form and often controls the I/O devices in a system.
Logic operations always clear the carry and overflow flags, while the other flags change to reflect the condition of the result
Except not instruction it does not affect any flags

3. AND Instruction The AND operation performs logical multiplication
Figures: (a) The truth table for the AND operation and (b) the logic symbol of an AND gate
The AND operation clears bits of a binary number
The task of clearing a bit in a binary number is called masking

4. AND Examples AND Form: AND dest, source ;dest := dest and source
AND instruction uses any addressing mode except memory-to-memory and segment register addressing
The two operand must be the same size
and reg, reg
and mem, reg
and reg, mem
and reg, immediate data
and mem, immediate data

5. OR Instruction
The OR operation performs logical addition and is often called the Inclusive-OR function
Figure: (a) The truth table for the OR operation and (b) the logic symbol of an OR gate
The OR operation clears bits of a binary number

6. OR Example
OR uses the same formats as AND

7. XOR Instruction Exclusive-OR:
If the inputs are both 0 or both 1, the output is 0
If the inputs are different, the output is 1
(a) truth table of XOR (b) Symbol of XOR gate
Exclusive-OR is sometimes called a comparator
XOR instruction allows part of a number to be inverted
A common use for the Exclusive-OR instruction is to clear a register to zero. For example, the XOR CH,CH instruction clears register CH to 00H

8. XOR Example
XOR uses the same formats as AND

9. TEST Instruction The TEST instruction performs the AND operation BUT
AND instruction changes the destination operand
TEST instruction does not changes the destination operand
TEST only affects the condition of the flag register, which indicates the result of the test
The zero flag (Z) is a logic 1 if the bit under test is a zero, and (indicating a nonzero result) if the bit under test is not zero.
The TEST instruction functions in the same manner as a CMP instruction, The difference is
TEST instruction normally tests a single bit (or occasionally multiple bits)
CMP instruction tests the entire byte, word, or doubleword.
The TEST instruction uses the same addressing modes as the AND instruction
The destination operand is normally tested against immediate data
The value of immediate data is 1 to test the rightmost bit position, 2 to test the next bit, 4 for the next,and so on.
Usually the TEST instruction is followed by either the JZ (jump if zero) or JNZ (jump if not zero) instruction

10. Test Example Example 1: Examples: TEST AL,1 ;test right bit
JNZ RIGHT ;if set
TEST AL,128 ;test left bit
JNZ LEFT ;if set
Examples:

11. Additional Test Instructions
The through the Pentium 4 processors contain additional test instructions that test single bit positions
Example:
BTS CX, 0 ; set bit 0

12. NOT and NEG Instructions
Logical inversion, or the one’s complement (NOT), and arithmetic sign inversion, or the two’s complement (NEG)
NOT and NEG can use any addressing mode except segment register addressing
Examples:

13. Shift Instructions
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)
division by powers of 2-n (right shift)
The microprocessor’s instruction
set contains four different shift
instructions as shown in figure

14. Shift Instructions Logical shift function with unsigned numbers
• Arithmetic shift function with signed numbers
• Logical shifts multiply or divide unsigned data;
arithmetic shifts multiply or divide signed data.
– a shift left always multiplies by 2 for each bit position shifted
– a shift right always divides by 2 for each position
– shifting a two places, multiplies or divides by 4
Shift form:
shl dest, count
shl dest, CL ; cl aontain count number
• Destination can be memory location or register; Segment shift not allowed.

15. Shift Examples
Two different forms of shifts:

16. Rotate
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
Rotation dressing modes are the same shifts
The four available rotate instructions appear:

17. Rotation Examples