Messages in this thread | | | Date | Wed, 12 Dec 2007 18:41:15 -0800 | From | "H. Peter Anvin" <> | Subject | Re: [RFT] Port 0x80 I/O speed |
| |
Rene Herman wrote: > On 12-12-07 06:23, Kyle McMartin wrote: > >> On Wed, Dec 12, 2007 at 12:31:18AM +0100, Rene Herman wrote: >>> asm volatile ("rdtsc": "=A" (tsc)); >> >> rdtsc returns a 64-bit value in two 32-bit regs, you need to do >> >> inline unsigned long long rdtsc(void) >> { >> unsigned int lo, hi; >> asm volatile ("rdtsc": "=a" (lo), "=d" (hi)); >> return (unsigned long long)hi << 32 | lo; >> } >> >> as in msr.h, otherwise you'll only be looking at the value in %rax. > > On 32-bit, "=A" is edx:eax. Not sure what the point is in not letting it > be that on 64-bit in fact, but yes, the thing should be compiled as 32-bit.
On 64-bit, "=A" is rdx:rax, which means that a 64-bit value ends up in rax only (a 128-bit value, which gcc does support on 128-bit platforms, would be in rdx:rax.)
gcc can't deal internally with values that span partial registers.
-hpa
| |