Page 109 - ARM 64 Bit Assembly Language

Data processing and other instructions 95

smulh Signed multiply high and
umulh Unsigned multiply high.

4.2.7.1 Syntax

<op> Xd, Xn, Xm

• The <op> is either smulh or umulh.

4.2.7.2 Operations

Name Effect Description
smulh Xd ← (signExtend(Xn) × Signed multiply high.
signExtend(Xm))[127 : 64]
umulh Xd ← (Xn × Xm)[127 : 64] Unsigned multiply high.

4.2.7.3 Examples

1 mul x0, x1, x2 // x0 = x1 * x2
2 smulh x1, x1, x2 // x1 = (signExtend(x1) * signExtend(x2))[127:64]

The following example shows how a C program and an AArch64 program could implement
a simple command-line tool for multiplying 64-bit unsigned numbers, without rounding or
truncating the result. To change the program to work with signed numbers, the only change
needed for the AArch64 version would be to replace umulh with smulh.

1 #include <stdio.h>
2 #include <stdint.h>
3
4 /* longMul takes two unsigned longs as input x and y and returns a
5 128-bit product. */
6 __uint128_t longMul(uint64_t x, uint64_t y)
7 {
8 return (__uint128_t)x*(__uint128_t) y;
9 }
10
11 /* Reads two unsigned longs from stdin and prints the resulting
12 product on stdout. */
13 int main(void)
14 {
15 long x, y, lo, hi;
16 printf("Enter a number in hex: ");
17 scanf("%lx", &x);
18 printf("Enter a number in hex: ");