Transcript Logical Instructions

Microprocessor & Assembly
Language
Logical, shift, rotate,
and bit Instructions
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
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
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
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
OR Example
• OR uses the same formats as AND
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
XOR Example
• XOR uses the same formats as AND
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
Test Example
• Example 1:
TEST AL,1 ;test right bit
JNZ RIGHT ;if set
TEST AL,128 ;test left bit
JNZ LEFT ;if set
• Examples:
Additional Test Instructions
• The 80386 through the Pentium 4 processors contain
additional test instructions that test single bit positions
• Example:
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:
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
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.
Shift Examples
•Two different forms of shifts:
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:
Rotation Examples