Page 114 - ARM 64 Bit Assembly Language

100 Chapter 4

5 do
6 {} // loop body does nothing
7 while (getchar() != ’A’)
8 return 0;
9 }

An AArch64 translation of the C program uses the cmp instruction to test whether or not the
input character is ‘A’ and a conditional branch instruction to implement the do-while loop:

1 .text
2 .type main, %function
3 .global main
4 main:
5 stp x29, x30, [sp, #-32]!
6 str x19, [sp, #16]
7 mov x19, #’A’
8
9 loop: // beginning of do-while loop
10
11 // loop body does nothing
12
13 bl getchar // call getchar to get next character in x0
14 cmp x0, x19 // compare it to ’A’
15 bne loop // repeat loop if the character is not equal to ’A’
16
17 ldr x20, [sp, #16]
18 ldp x29, x30, [sp], #32
19 ret
20 .size main, (. - main)

4.2.10 Conditional operations

These conditional select operations set the destination register to the first operand, Rn if the
condition is true, and to the second operand optionally incremented, inverted, or negated if the
condition is false:

csel Conditional Select,
csinc Conditional Select Increment,
csinv Conditional Select Invert,
csneg Conditional Select Negate.

There are five aliases that are derived from the previous instructions:
cinc Conditional Increment,
cinv Conditional Invert,