Page 110 - ARM 64 Bit Assembly Language

96 Chapter 4

19 scanf("%lx", &y);
20 __uint128_t prod = longMul(x, y);
21 lo = (uint64_t) prod;
22 hi = (uint64_t) (prod >> 64);
23 printf("%016lx * %016lx = %016lx%016lx\n", x, y, hi, lo);
24 return 0;
25 }

The AArch64 implementation executes the multiplication step with just two instructions; most
of the code is devoted to calling printf and scanf statements, and moving parameters into
position:

1 .text
2 /* longMul takes two unsigned longs as input on x0 and x1 and
3 returns a 128-bit product with the lower half "lo" in x0
4 and the higher half "hi" in x1. */
5 .type longMul, %function
6 longMul:
7 mul x9, x0, x1
8 umulh x1, x0, x1
9 mov x0, x9
10 ret
11 .type main, %function
12 .global main
13 /* Reads two unsigned longs from stdin and prints the resulting
14 product on stdout. */
15 main:
16 stp x29, x30, [sp, #-32]!
17 stp x19, x20, [sp, #16] // Callee-saved
18
19 // printf("Enter a number in hex: ")
20 adr x0, prompt
21 bl printf
22
23 // scanf("%ld", &x)
24 adr x0, fmtScan
25 adr x1, dataIn
26 bl scanf
27 adr x1, dataIn
28 ldr x19, [x1] // x
29
30 // printf("Enter a number in hex: ")
31 adr x0, prompt
32 bl printf
33
34 // scanf("%ld", &y)
35 adr x0, fmtScan
36 adr x1, dataIn