Logical operators evaluate Boolean expressions and determine whether the entire expression is true or false based on the operator used. Logical instructions are also used with condition instructions in assembly language. And let's decide. The next code block to run if the expression is true.
Logical Operators in Assembly - AND, OR, XOR, TEST and NOT
Operators - AND, OR, XOR, TEST and NOT
The NASM assembly language processor instruction set provides AND, OR, XOR, TEST and NOT boolean logic, which tests, sets and clears bits as required by the program.
The format for these instructions −
| Sr.No. | Instruction | Format |
|---|---|---|
| 1 | AND | AND operand1, operand2 |
| 2 | OR | OR operand1, operand2 |
| 3 | XOR | XOR operand1, operand2 |
| 4 | TEST | TEST operand1, operand2 |
| 5 | NOT | NOT operand1 |
In all cases the first operand can be either in a register or in memory. The second operand can be either in register/memory or an immediate (constant) value. However, memory-to-memory operations are not possible. These instructions compare or match the bits of the operands and set the CF, OF, PF, SF, and ZF flags.
Operand1: Operand1: register or memory
Operand2: Operand2: register/memory or immediate (constant)
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After AND -> Operand: register/memoryAND instruction
AND instruction is used to support logical expressions by performing bitwise AND operation. The bitwise AND operation returns 1 if the matching bits of both operands are 1, otherwise it returns 0.
Syntax AND Register or memory, register/memory or immediate (constant)For example
AND AL, 01HTHE OR operator
OR instruction is used to support logical expression by performing bitwise OR operation. If the matching bits of either or both operands are one. So the bitwise OR operator returns 1, if both bits are zero, it returns 0.
The OR operation can also be used to set one or more bits.
Syntax OR Register or memory, register/memory or immediate (constant)For example
OR AL, BLXOR instruction
XOR instruction implements the bitwise XOR operation. If the bits from both the operands are different. The XOR operation sets the resulting bit to 1, if the bits of the operands are identical (both 0 or both 1), the resulting bit is set to 0.
XORing an operand with itself turns the operand into 0. It is used to clear a register.
Syntax XOR Register or memory, register/memory or immediate (constant)For example
XOR EAX, EAXTEST instructions
The TEST instruction works in the same way as the AND operation, but unlike the AND instruction, it does not change the first operand.
Syntax TEST Register or memory, register/memory or immediate (constant)For example
TEST AL, 01HNOT Instruction
The NOT instruction implements the bitwise NOT operation. NOT operation reverses the bits in an operand. The operand can be either in the register or in the memory. Returns 0 if the result is 1. Returns 1 if it is 0.
Syntax NOT Register or memory NOTE - Logical Instructions/Operators are used to compare more than one condition. Like true and false in other languages, 1 and 0 are used in assembly.
Exampal
Let us take an example. If you want to check whether the given number is odd or even.
section .text
global _start ;must be declared for using gcc
_start: ;tell linker entry point
mov ax, 8h ;getting 8 in the ax
and ax, 1 ;and ax with 1
jz evnn
mov eax, 4 ;system call number (sys_write)
mov ebx, 1 ;file descriptor (stdout)
mov ecx, odd_msg ;message to write
mov edx, len2 ;length of message
int 0x80 ;call kernel
jmp outprog
evnn:
mov ah, 09h
mov eax, 4 ;system call number (sys_write)
mov ebx, 1 ;file descriptor (stdout)
mov ecx, even_msg ;message to write
mov edx, len1 ;length of message
int 0x80 ;call kernel
outprog:
mov eax,1 ;system call number (sys_exit)
int 0x80 ;call kernel
section .data
even_msg db 'Even Number!' ;message showing even number
len1 equ $ - even_msg
odd_msg db 'Odd Number!' ;message showing odd number
len2 equ $ - odd_msgWhen the above code is compiled and executed, it produces the following result −
Even Number!
