Struggling with conversion to AMD64/Linux syscalls in inline asm

[me <mgbg25171 at blueyonder dot co dot uk>]

I'm trying to convert some 32 bit linked asm (which runs fine) 
into 64 bit inline asm (g++) but am stumbling at the first hurdle
ie I'm getting -14 returned as the file descriptor for /dev/vcsa1.

Here's my attempt.

[code]
//My amd64 inline asm version 
#include <iostream>
using namespace std;

void my_fn(){
   const char * fl =       "/dev/vcsa1";
   const char * test_str = "          "; //make same size as fl
   int test_int = -1; //ie initialised to fail unless replaced by value 

   //1 open /dev/vcsa1 & return file descriptor in rax  
   __asm__ __volatile__(
   ".equ sys_open, 2\n"
   ".equ O_RDWR, 02\n" 
   "mov $sys_open, %%rax\n" 
   "mov $O_RDWR, %%rcx\n" 
   "mov $0600, %%rdx\n" //read/write for user in x86. Not sure for AMD64?
   "syscall\n" 
   :"=b"(test_str), "=a"(test_int)
   :"b"(fl)
   ); 
  
   //test file name in rbx and returned file descriptor
   cout << test_str << endl; // /dev/vcsa1                     pass
   cout << test_int << endl; // 2 before syscall -14 after ie  fail  
}

int main(){
   my_fn();
   return 0;
}
[/code]

I'd be grateful if someone could tell me where I'm going wrong.