Page 84 - ARM 64 Bit Assembly Language

Load/store and branch instructions 69

bottom of the stack), and stores the procedure link register (x30)at Mem[sp + 8],which
completely fills the 16 bytes that were allocated on the stack.

1 stp x29, x30, [sp, #-16]!

The opposite of the previous instruction can be used to restore the values of the frame pointer
and procedure link registers:

1 ldp x29, x30, [sp], #16

Here x29 is set to Mem[sp] and x30 is set to Mem[sp + 8]. Then, sp is incremented
by 16, essentially deleting the values of x29 and x30 from the stack. Although the bits
are still there, they may be overwritten by subsequent instructions that store data on the
stack.

The following assembly program demonstrates the two instructions used together to save and
restore a pair of registers.

1 .text
2 .type main, %function
3 .global main
4 main:
5 stp x29, x30, [sp, #-16]! // Store x29 and x30 on the stack
6 mov x29, #0x0 // change their contents
7 mov x30, #0x0
8 ldp x29, x30, [sp], #16 // Restore x29 and x30, and sp
9 ret
10 .size main, (. - main)

The following function demonstrates how to use the stp and ldp instructions to save, and
later restore, all of the non-volatile registers.

1 .text
2 .type exmpl, %function
3 .global exmpl
4 exmpl:
5 stp x19, x20, [sp, #-16]! // Store x19 and x20 on the stack
6 stp x21, x22, [sp, #-16]! // Store x21 and x22 on the stack
7 stp x23, x24, [sp, #-16]! // Store x23 and x24 on the stack
8 stp x25, x26, [sp, #-16]! // Store x25 and x26 on the stack
9 stp x27, x28, [sp, #-16]! // Store x27 and x28 on the stack
10 stp x29, x30, [sp, #-16]! // Store x29 and x30 on the stack
11
12 // The function can now use x19 through x30. If it calls another
13 // function, then it is guaranteed that these registers will not
14 // be modified by the function that was called.
15 // INSERT FUNCTION BODY HERE