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# arm64
keywords: arm64, aarch64, abi
- 64bit synonyms: `arm64`, `aarch64`
- ISA type: `RISC`
- Endianness: `little`, `big`
## ASM skeleton
Small assembler skeleton, ready to use with following properties:
- use raw Linux syscalls (`man 2 syscall` for ABI)
- no `C runtime (crt)`
- gnu assembler [`gas`][gas_doc]
```armasm
// file: greet.S
#include <asm/unistd.h> // syscall NRs
.arch armv8-a
.section .text, "ax", @progbits
.align 2
.global _start
_start:
mov x0, 2 // fd
ldr x1, =greeting // buf
ldr x2, =greeting_len // &len
ldr x2, [x2] // len
mov w8, __NR_write // write(2) syscall
svc 0
mov x0, 0 // exit code
mov w8, __NR_exit // exit(2) syscall
svc 0
.align 3
.section .rodata, "a", @progbits
greeting:
.asciz "Hi ASM-World!\n"
greeting_len:
.int .-greeting
```
> man gcc: `file.S` assembler code that must be preprocessed.
To cross-compile and run:
```bash
> aarch64-linux-gnu-g++ -o greet greet.S -nostartfiles -nostdlib \
-Wl,--dynamic-linker=/usr/aarch64-linux-gnu/lib/ld-linux-aarch64.so.1 \
&& qemu-aarch64 ./greet
Hi ASM-World!
```
> Cross-compiling on `Ubuntu 20.04`, paths might differ on other distributions.
> Compiling natively on arm64, specifying the dynamic linker should not be required.
## References
- [Procedure Call Standard ARM64][aapcs64]
- [ARMv8 Programmer's Guide][armv8a_prog_guide]
- [GNU Assembler][gas_doc]
- [GNU Assembler Directives][gas_directives]
[aapcs64]: https://github.com/ARM-software/abi-aa/blob/master/aapcs64/aapcs64.rst
[armv8a_prog_guide]: https://developer.arm.com/documentation/den0024/a
[gas_doc]: https://sourceware.org/binutils/docs/as
[gas_directives]: https://sourceware.org/binutils/docs/as/Pseudo-Ops.html#Pseudo-Ops
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