Struggling with conversion to AMD64/Linux syscalls in inline asm

[Jie Zhang]

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