How To Clear Register In Assembly

Assembly - Logical Instructions

The processor education set provides the instructions AND, OR, XOR, Test, and Non Boolean logic, which tests, sets, and clears the $.25 according to the need of the program.

The format for these instructions −

Sr.No.	Didactics	Format
i	AND	AND operand1, operand2
2	OR	OR operand1, operand2
3	XOR	XOR operand1, operand2
iv	TEST	TEST operand1, operand2
5	Not	Not operand1

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

The AND Didactics

The AND didactics is used for supporting logical expressions by performing bitwise AND performance. The bitwise AND operation returns 1, if the matching $.25 from both the operands are 1, otherwise it returns 0. For example −

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

The AND operation can exist used for clearing one or more bits. For example, say the BL register contains 0011 1010. If you need to clear the high-society bits to zilch, you AND information technology with 0FH.

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

Permit's take up some other example. If you want to cheque whether a given number is odd or even, a simple exam would exist to check the to the lowest degree meaning bit of the number. If this is ane, the number is odd, else the number is even.

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

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

The following program illustrates this −

Example

section .text    global _start            ;must be declared for using gcc 	 _start:                     ;tell linker entry betoken    mov   ax,   8h           ;getting 8 in the ax     and   ax, 1              ;and ax with 1    jz    evnn    mov   eax, 4             ;organisation phone call number (sys_write)    mov   ebx, 1             ;file descriptor (stdout)    mov   ecx, odd_msg       ;bulletin to write    mov   edx, len2          ;length of message    int   0x80               ;call kernel    jmp   outprog  evnn:          mov   ah,  09h    mov   eax, 4             ;organisation call number (sys_write)    mov   ebx, 1             ;file descriptor (stdout)    mov   ecx, even_msg      ;bulletin to write    mov   edx, len1          ;length of message    int   0x80               ;call kernel  outprog:     mov   eax,i              ;arrangement phone call number (sys_exit)    int   0x80               ;call kernel  section   .data even_msg  db  'Even Number!' ;message showing fifty-fifty 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 post-obit consequence −

Even Number!

Modify 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 articulate the entire register you lot can AND it with 00H.

The OR Instruction

The OR instruction is used for supporting logical expression by performing bitwise OR performance. The bitwise OR operator returns 1, if the matching bits from either or both operands are ane. Information technology returns 0, if both the $.25 are zero.

For example,

          Operand1:     0101              Operand2:     0011 ---------------------------- Later on OR -> Operand1:    0111

The OR operation can exist used for setting one or more bits. For case, let us assume the AL register contains 0011 1010, you need to set the four low-lodge bits, you tin OR information technology 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 educational activity,

OR AL, BL

should store seven in the AL register −

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

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

The XOR Didactics

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

For case,

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

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

XOR     EAX, EAX

The Examination Teaching

The Examination instruction works aforementioned every bit the AND operation, but dissimilar AND instruction, information technology does not change the beginning operand. So, if we need to bank check whether a number in a register is fifty-fifty or odd, we tin can also practise this using the Exam instruction without changing the original number.

Examination    AL, 01H JZ      EVEN_NUMBER

The Non Instruction

The Non instruction implements the bitwise Non operation. Not operation reverses the bits in an operand. The operand could be either in a register or in the retention.

For example,

          Operand1:    0101 0011 After Non -> Operand1:    1010 1100

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