Page 141 - ARM 64 Bit Assembly Language

Structured programming 127

Listing 5.20 Calling scanf and printf in AArch64 assembly.

1 .data
2 str1: .asciz "%d"
3 str2: .asciz "You entered %d\n"
4 n: .word 0
5 .text
6 .type main, %function
7 .global main
8 main:
9 stp x29, x30, [sp, #-16]! // push FP, LR onto stack
10 adr x0, str1 // x0 = &str1
11 adr x1, n // x1 = &n
12 bl scanf // scanf("%d",&n)
13 adr x0, str2 // x0 = &str2
14 adr x1, n // x1 = &n
15 ldr w1, [x1] // w1 = n
16 bl printf // print message
17 mov w0, #0 // load return value
18 ldp x29, x30, [sp], #16 // pop FP, LR from stack
19 ret // return from main
20 .size main, (. - main)

• The first eight parameters go in registers x0-x7.
• Any remaining parameters are pushed to the stack (in reverse order).

If the subroutine returns a value, then the value is stored in x0 before the function returns to
its caller. Calling a subroutine in AArch64 assembly usually requires several lines of code.
The number of lines required depends on how many arguments the subroutine requires, and
where the data for those arguments are stored. Some variables may already be in the correct
register. Others may need to be moved from one register to another. Still others may need to
be loaded from memory into a register or loaded and then stored to the stack. Careful pro-
gramming is required to minimize the amount of work that must be done just to move the
subroutine arguments into their required locations.

The AArch64 register set was introduced in Chapter 3. Some registers have special purposes
that are dictated by the hardware design. Others have special purposes that are dictated by
programming conventions. Programmers follow these conventions so that their subroutines
are compatible with each other. These conventions are simply a set of rules for how regis-
ters should be used. In AArch64 assembly, all of the registers have alternate names that can
be used to help remember the rules for using them. Fig. 5.1 shows an expanded view of the
AArch64 registers, including their alternate names and conventional use.