Arithmetic

Arithmetic Instructions

ARM64 provides instructions for basic arithmetic operations. These are the workhorses of any program -- almost every computation boils down to adds, subtracts, and multiplies.

ADD and SUB

ADD adds two values, SUB subtracts:

ADD X0, X1, X2      // X0 = X1 + X2
ADD X0, X1, #10     // X0 = X1 + 10
SUB X0, X1, X2      // X0 = X1 - X2
SUB X0, X1, #5      // X0 = X1 - 5

The first operand is the destination, the second is the first source, and the third is either a register or an immediate. The destination can be the same as a source:

ADD X0, X0, #1      // X0 = X0 + 1 (increment)

MUL

MUL multiplies two registers:

MUL X0, X1, X2      // X0 = X1 * X2

Note: MUL only takes register operands, not immediates. To multiply by a constant, load it into a register first.

UDIV -- Unsigned Division

UDIV performs unsigned integer division (truncates toward zero):

UDIV X0, X1, X2     // X0 = X1 / X2 (integer division)

ARM64 has no modulo (remainder) instruction. To compute the remainder, use the pattern:

UDIV X2, X0, X1     // quotient = X0 / X1
MUL  X3, X2, X1     // temp = quotient * divisor
SUB  X4, X0, X3     // remainder = X0 - temp

For example, to get 58 % 10 = 8: divide 58/10 = 5, then 58 - 5*10 = 8.

Converting Numbers to ASCII Digits

To print a number, you need to convert it to ASCII characters. The ASCII code for the digit '0' is 48. So to convert a single digit (0-9) to its ASCII character, add 48:

MOV X0, #7          // the number 7
ADD X0, X0, #48     // now 55, which is ASCII '7'

For a two-digit number like 58, extract each digit with division and remainder:

MOV X1, #10
UDIV X2, X0, X1     // X2 = 58 / 10 = 5 (tens digit)
MUL X3, X2, X1      // X3 = 5 * 10 = 50
SUB X3, X0, X3      // X3 = 58 - 50 = 8 (ones digit)
ADD X2, X2, #48     // '5'
ADD X3, X3, #48     // '8'

Tip: This divide-multiply-subtract pattern for computing remainders is one of the most common patterns in assembly. You will use it in nearly every lesson that prints numbers.

As Spock would say: "Captain, the odds of survival are 2,467 to 1." Good thing ARM64 has the arithmetic instructions to verify that calculation.

Your Task

Compute 7 * 8 + 2, convert the result (58) to a two-digit ASCII string, and print it followed by a newline.

The result is 58, so you need to print the characters '5', '8', and '\n'.