About register constraint in gcc inline assembly

["Roach" <lcz90@xxxxxxxxxxxxx>]

Hi there,

I got a question about register constraint in kernel source.
Here is the cut of bios32_service()

597         __asm__("lcall (%%edi); cld"
598                 : "=a" (return_code),
599                   "=b" (address),
600                   "=c" (length),
601                   "=d" (entry)
602                 : "" (service),
603                   "1" (0),
604                   "D" (&bios32_indirect));

In line 602, its register constraint is a NULL string. I can't find how
compiler arranges the registers here in gcc.info.

Does anyone know the further information?

Kindly regards,
Roach