How to issue system calls without a C library

X86:
int sys_write(int fd, void *buf, int count)
{
    int ret;

    asm("int $0x80" : "=a" (ret) :
        "0" (__NR_write), "b" (fd), "c" (buf), "d" (count));
    return ret;
}

00000000 <sys_write>:
   0:   53                      push   %ebx
   1:   b8 04 00 00 00          mov    $0x4,%eax
   6:   8b 54 24 10             mov    0x10(%esp),%edx
   a:   8b 4c 24 0c             mov    0xc(%esp),%ecx
   e:   8b 5c 24 08             mov    0x8(%esp),%ebx
  12:   cd 80                   int    $0x80
  14:   5b                      pop    %ebx
  15:   c3                      ret
  16:   8d 76 00                lea    0x0(%esi),%esi
  19:   8d bc 27 00 00 00 00    lea    0x0(%edi),%edi

ARM (OABI):
#define __sys2(x) #x
#define __sys1(x) __sys2(x)
#define __syscall(name) __sys1(__NR_##name) ""

int sys_write(int fd, void *buf, int count)
{
    register long ret __asm__("r0");
    register long __r0 __asm__("r0") = (long)fd;
    register long __r1 __asm__("r1") = (long)buf;
    register long __r2 __asm__("r2") = (long)count;

    asm("swi " __syscall(write) : "=r" (ret):
        "0" (__r0), "r" (__r1), "r" (__r2));
    return ret;
}

00000000 <sys_write>:
   0:   ef900004        svc     0x00900004
   4:   e12fff1e        bx      lr

With EABI parameter passing is simplified
Syscall number is passed in r7
Arguments are r0 onward (unchanged)