#define LINUX   1
#define OPENBSD 16
#define FREEBSD 32
#define NETBSD  64

#ifndef SUPPORT_VECTOR
#define SUPPORT_VECTOR (LINUX | FREEBSD | NETBSD | OPENBSD)
#endif

#define SupportsLinux()   (SUPPORT_VECTOR & LINUX)
#define SupportsFreebsd() (SUPPORT_VECTOR & FREEBSD)
#define SupportsOpenbsd() (SUPPORT_VECTOR & OPENBSD)
#define SupportsNetbsd()  (SUPPORT_VECTOR & NETBSD)

#define IsLinux()   (SupportsLinux() && os == LINUX)
#define IsFreebsd() (SupportsFreebsd() && os == FREEBSD)
#define IsOpenbsd() (SupportsOpenbsd() && os == OPENBSD)
#define IsNetbsd()  (SupportsNetbsd() && os == NETBSD)

__attribute__((__noreturn__)) static void Exit(int rc, int os) {
  asm volatile("syscall"
               : /* no outputs */
               : "a"(IsLinux() ? 60 : 1), "D"(rc)
               : "memory");
  __builtin_unreachable();
}

static int Write(int fd, const void *data, int size, int os) {
  char cf;
  int ax, dx;
  asm volatile("clc\n\t"
               "syscall"
               : "=a"(ax), "=d"(dx), "=@ccc"(cf)
               : "0"(IsLinux() ? 1 : 4), "D"(fd), "S"(data), "1"(size)
               : "rcx", "r11", "memory", "cc");
  if (cf) ax = -ax;
  return ax;
}

static int Main(int argc, char **argv, char **envp, long *auxv, int os) {
  Write(1, "hello world\n", 12, os);
  return 0;
}

__attribute__((__noreturn__)) void Start(long di, long *sp) {
  long *auxv;
  int i, os, argc;
  char **argv, **envp, *page;

  // detect freebsd
  if (SupportsFreebsd() && di) {
    os = FREEBSD;
    sp = (long *)di;
  } else {
    os = 0;
  }

  // extract arguments
  argc = *sp;
  argv = (char **)(sp + 1);
  envp = (char **)(sp + 1 + argc + 1);
  auxv = (long *)(sp + 1 + argc + 1);
  for (;;) {
    if (!*auxv++) {
      break;
    }
  }

  // detect openbsd
  if (SupportsOpenbsd() && !os && !auxv[0]) {
    os = OPENBSD;
  }

  // detect netbsd
  if (SupportsNetbsd() && !os) {
    for (; auxv[0]; auxv += 2) {
      if (auxv[0] == 2014 /* AT_EXECFN */) {
        os = NETBSD;
        break;
      }
    }
  }

  // default operating system
  if (!os) {
    os = LINUX;
  }

  Exit(Main(argc, argv, envp, auxv, os), os);
}