What do these instructions mean in assembly

Assembly - Logical Instructions


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The processor instruction set provides the instructions AND, OR, XOR, TEST, and NOT Boolean logic, which tests, sets, and clears the bits according to the need of the program.

The format for these instructions −

Sr.No.InstructionFormat
1ANDAND operand1, operand2
2OROR operand1, operand2
3XORXOR operand1, operand2
4TESTTEST operand1, operand2
5NOTNOT operand1

The first operand in all the cases could be either in register or in memory. The second operand could be either in register/memory or an immediate (constant) value. However, memory-to-memory operations are not possible. These instructions compare or match bits of the operands and set the CF, OF, PF, SF and ZF flags.

The AND Instruction

The AND instruction is used for supporting logical expressions by performing bitwise AND operation. The bitwise AND operation returns 1, if the matching bits from both the operands are 1, otherwise it returns 0. For example −

Operand1: 0101 Operand2: 0011 ---------------------------- After AND -> Operand1: 0001

The AND operation can be used for clearing one or more bits. For example, say the BL register contains 0011 1010. If you need to clear the high-order bits to zero, you AND it with 0FH.

AND BL, 0FH ; This sets BL to 0000 1010

Let's take up another example. If you want to check whether a given number is odd or even, a simple test would be to check the least significant bit of the number. If this is 1, the number is odd, else the number is even.

Assuming the number is in AL register, we can write −

AND AL, 01H ; ANDing with 0000 0001 JZ EVEN_NUMBER

The following program illustrates this −

Example

Live Demo

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_msg

When the above code is compiled and executed, it produces the following result −

Even Number!

Change the value in the ax register with an odd digit, like −

mov ax, 9h ; getting 9 in the ax

The program would display:

Odd Number!

Similarly to clear the entire register you can AND it with 00H.

The OR Instruction

The OR instruction is used for supporting logical expression by performing bitwise OR operation. The bitwise OR operator returns 1, if the matching bits from either or both operands are one. It returns 0, if both the bits are zero.

For example,

Operand1: 0101 Operand2: 0011 ---------------------------- After OR -> Operand1: 0111

The OR operation can be used for setting one or more bits. For example, let us assume the AL register contains 0011 1010, you need to set the four low-order bits, you can OR it with a value 0000 1111, i.e., FH.

OR BL, 0FH ; This sets BL to 0011 1111

Example

The following example demonstrates the OR instruction. Let us store the value 5 and 3 in the AL and the BL registers, respectively, then the instruction,

OR AL, BL

should store 7 in the AL register −

Live Demo

section .text global _start ;must be declared for using gcc _start: ;tell linker entry point mov al, 5 ;getting 5 in the al mov bl, 3 ;getting 3 in the bl or al, bl ;or al and bl registers, result should be 7 add al, byte '0' ;converting decimal to ascii mov [result], al mov eax, 4 mov ebx, 1 mov ecx, result mov edx, 1 int 0x80 outprog: mov eax,1 ;system call number (sys_exit) int 0x80 ;call kernel section .bss result resb 1

When the above code is compiled and executed, it produces the following result −

7

The XOR Instruction

The XOR instruction implements the bitwise XOR operation. The XOR operation sets the resultant bit to 1, if and only if the bits from the operands are different. If the bits from the operands are same (both 0 or both 1), the resultant bit is cleared to 0.

For example,

Operand1: 0101 Operand2: 0011 ---------------------------- After XOR -> Operand1: 0110

XORing an operand with itself changes the operand to 0. This is used to clear a register.

XOR EAX, EAX

The TEST Instruction

The TEST instruction works same as the AND operation, but unlike AND instruction, it does not change the first operand. So, if we need to check whether a number in a register is even or odd, we can also do this using the TEST instruction without changing the original number.

TEST AL, 01H JZ EVEN_NUMBER

The NOT Instruction

The NOT instruction implements the bitwise NOT operation. NOT operation reverses the bits in an operand. The operand could be either in a register or in the memory.

For example,

Operand1: 0101 0011 After NOT -> Operand1: 1010 1100