Page 205 - ARM 64 Bit Assembly Language

Integer mathematics 193

5 .text
6 .type main %function
7 .global main
8 /* Multiplies x and y by adding a series of partial products.
9 Shifts x (multiplicand) left and y (multiplier) right. Only
10 adds if the least significant bit of multiplier is 1. */
11 main: stp x29, x30, [sp, #-16]!
12 mov x0, #0 // resultLow = 0
13 mov x1, #0 // resultHigh = 0
14 adr x2, x // x2 = &x
15 ldr x2, [x2] // x2 = multiplicandLow
16 mov x3, #0 // x3 = multiplicandHigh
17 adr x4, y // x4 = &y
18 ldr x4, [x4] // x4 = multiplier
19 loop: tst x4, #1 // Only add if lowest bit of
20 beq endIf // multiplier is 1:
21 adds x0, x0, x2 // resultLow += multiplicandLow.
22 adc x1, x1, x3 // resultHigh +=
23 // multiplicandHigh + Carry
24 endIf: // shift 128-bit multiplicand left. Carry bit from lo to hi.
25 ands xzr, x2, 0x8000000000000000
26 lsl x2, x2, #1
27 lsl x3, x3, #1
28 cinc x3, x3, ne
29 // shift multiplier right
30 lsr x4, x4, #1
31 // if (y != 0) repeat loop
32 cmp x4, xzr
33 bne loop
34 // print results
35 mov x4, x0
36 mov x3, x1
37 adr x2, y
38 ldr x2, [x2]
39 adr x1, x
40 ldr x1, [x1]
41 adr x0, msg
42 bl printf
43 mov w0, #0 // return 0
44 ldp x29, x30, [sp], #16
45 ret
46 .size main,(. - main)

Listing 7.3 AArch64 assembly code for multiplication with a 64 bit result without using
mul.

1 .section .rodata
2 x: .8byte 0x57