Hello World in D using Assembly

[Renato Athaydes]

Hi, I just wanted to share [this 
gist](https://gist.github.com/renatoathaydes/950cd1e0d032bc65a693bcc469cd3afc) which shows how to write hello-world in ASM, in D.

D is very convenient for writing ASM! Here's what I got:

```d
extern(C) int main()
{
   auto hip = "hello D\n".ptr;
   size_t len = 8;

   //   write(1, message, length)
   asm {
     mov RDX, len;              // Buffer length
     mov RSI, hip;              // Message buffer
     mov EDI, 1;                // Stdout file descriptor (0x01)
     mov RAX, 0x2000004;        // write syscall number (0x01 on 
Linux)
     syscall;                   // Make the syscall
   }

   return 0;
}
```

Compiling it with:

```
dmd -betterC -ofhello hello.d
```

Results in this on MacOS (Intel):

```
▶ objdump -d -M intel ./main

./main:	file format mach-o 64-bit x86-64

Disassembly of section __TEXT,__text:

0000000100000fcc <_main>:
100000fcc: 55                          	push	rbp
100000fcd: 48 8b ec                    	mov	rbp, rsp
100000fd0: 48 83 ec 10                 	sub	rsp, 0x10
100000fd4: 48 8d 05 25 00 00 00        	lea	rax, [rip + 0x25]     
   ## 0x100001000
100000fdb: 48 89 45 f0                 	mov	qword ptr [rbp - 
0x10], rax
100000fdf: 48 c7 45 f8 08 00 00 00     	mov	qword ptr [rbp - 
0x8], 0x8
100000fe7: 48 8b 55 f8                 	mov	rdx, qword ptr [rbp - 
0x8]
100000feb: 48 8b 75 f0                 	mov	rsi, qword ptr [rbp - 
0x10]
100000fef: bf 01 00 00 00              	mov	edi, 0x1
100000ff4: b8 04 00 00 02              	mov	eax, 0x2000004
100000ff9: 0f 05                       	syscall
100000ffb: 31 c0                       	xor	eax, eax
100000ffd: c9                          	leave
100000ffe: c3                          	ret
```

While I wrote the syscall number to RAX, `objdump` shows it as 
`eax`, not sure why?!

> By the way: syscall numbers on MacOS are apparently unstable 
> and the "correct" way to call the kernel is via C's stdlib.

Anyway, I thought this was cool and as I couldn't find anything 
much about ASM in D, I thought this may be useful to others.