Re: Struggling with conversion to AMD64/Linux syscalls in inline asm

[Jie Zhang <jie dot zhang at analog dot com>]

On 01/25/2010 04:47 PM, Jie Zhang wrote:
> On 01/25/2010 04:39 PM, me wrote:
> > On Mon, 25 Jan 2010 12:18:51 +0800
> > Jie Zhang<jie.zhang@analog.com> wrote:
> >
> > > On 01/24/2010 10:51 PM, me wrote:
> > > > 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)
> > >
> > > This statement does not tell gcc the connections between registers
> > > and C variables. So there is no guarantee that those variables will
> > > be put into the registers you expect by the above statement.
> > I'm grateful for your comment but don't really understand it.
> > i.e. If...
> > > :"=b"(test_str), "=a"(test_int)
> > > :"b"(fl)
> > didn't connect the c variable "fl" to the register "rbx" then I'm not
> > sure how
Oops. I overlooked there is only on register in class 'a' for AMD64. So 
it definitely connects the register with the variable. Sorry!


Jie