Assembly Language is a low-level programming language. The instructions within Assembly Language are similar to machine code instructions.
Arithmetic instructions in Assembly Language are executed in the Arithmetic Logical Unit (ALU) of the computer’s processor.
For this shot, we will be discussing arithmetic instructions in the Assembly Language of the x86 machine architecture.
Assembly Language allows us to perform basic unsigned integer arithmetic operations. These operations are:
The syntax for writing addition or subtraction instructions is as follows:
operation destination, source
operation: the intended arithmetic operation, i.e.,
destination: the accumulator register or memory location where we will store our final result after adding or subtracting the value in source register or memory location.
source: the register or memory location that contains the value to be added or subtracted to/from the original content of the destination register.
The destination and source can be a combination of the following:
mov ax, 10 # move 10 to register 'ax' mov bx, 5 # move 5 to register 'bx' add ax, bx
mov ax, 10 # move 10 to register 'ax' add ax, [num1] # access value stored at location num1 and store sum # again in the register 'ax'
mov ax, 10 add [num1], ax
mov ax, 10 add ax, 5 # add 5 to the register 'ax' value and store the sum in 'ax'
add [num1], 5 # add 5 to the value stored at num1 and replace the value # there with the sum
We can also apply the above source/destination combinations to multiplication and division where applicable.
dec instructions can be used to
decrement the contents of their respective operands by one.
inc instruction has no effect on the carry flag.
dec instruction sets the zero flag if the result of the operation is
The syntax for writing multiplication or division instructions is as follows:
mul performs an unsigned multiplication of the source operand and the accumulator.
If the source operand is a byte (8 bits), we multiply it by the value stored in the register
AL. The result is returned in the registers
AL. The higher half of the 16-bit result is stored in
AH and the lower half in
AL. This means that if the result is small enough to be represented in 8 bits,
AH would contain
If the source operand is a word (16 bits), then we multiply it by the value stored in the register
AX and the result is returned in the registers
AX. The higher half of the 32-bit result is stored in
DX and the lower half in
div performs an integer division of the accumulator and the source operand. The result consists of an integer quotient and remainder.
If the source operand is a byte, the 16-bit number stored in
AX is divided by the operand. The 8-bit quotient is stored in
AL and the 8-bit remainder in
If the source operand is a word, the 32-bit number stored in
DX : AX is divided by the operand. The higher half of the number is stored in
DX and the lower in
AX. The 16-bit quotient is stored in
AX and the remainder in
The following code shows how we can implement arithmetic instructions in assembly language:
# a program to show arithmetic instructions in AL # addition mov ax, 5 # load number in ax mov bx, 2 # load number in bx add ax, bx # accumulate sum in ax # subtraction mov cx, 10 mov dx, 3 sub cx, dx # accumulate difference in cx # refreshing registers mov ax, 0 mov bx, 0 mov cx, 0 mov dx, 0 # multiplication - 8 bit source mov al, 5 mov bl, 10 mul bl # result in ax # refreshing registers mov ax, 0 mov bx, 0 # multiplication - 16 bit source mov ax, 5 mov bx, 10 mul bx # result in dx:ax # refreshing registers mov ax, 0 mov bx, 0 # divison - 8 bit source mov al, 23 mov bl, 4 div bl # quotient in al, remainder in ah # refreshing registers mov ax, 0 mov bx, 0 # divison - 16 bit source mov ax, 23 mov bx, 4 div bx # quotient in ax, remainder in dx mov ax, 0x4c00 # terminate program int 0x21
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