On Tuesday 10 September 2002 02:47 am, Lukas Ruf wrote:
> > > > __asm rdtsc
> > > > __asm sub eax, time_low // Find the difference
> > > > __asm sub edx, time_high
> > > >
> > > > __asm div mhz // Unsigned divide EDX:EAX by mhz
> > > >
> > > > __asm mov time, eax
> > > > printf("Seconds: %u\n", time);
> > > >
> > > > What exactly does the div line do?
> > >
> > > divides the number of cycles measured by the number of hertz which
> > > leads to the seconds consumed ,-)
> >
> > I know that's what it does,
> > but what is the dividend? Is it the 64-bit number created by combining
> > eax and edx? I don't understand where the dividend on that line is coming
> > from. And if it is a 64-bit number, how do I change the mull assembly in
> > do_fast_getttimeoffset to do the same thing?
>
> From the Intel Instruction Set Reference Manual, page 219:
> (http://developer.intel.com/design/pentium4/manuals/245471.htm)
>
> Operand Size Dividend Divisor Quotient Remainder Maximum Quotient
> Word/byte AX r/m8 AL AH 255
> Doubleword/word DX:AX r/m16 AX DX 65,535
> Quadword/doubleword EDX:EAX r/m32 EAX EDX 232 1
>
> - If you read the rdtsc, the counters are stored in EDX:EAX.
> - If you read the lines I posted, the dividend is stored in EDX:EAX
> - If you read further these lines, the divisor may be a register or a
> memory location with size of 32bit.
> - If you have any further questions, please ask.
>
> --lpr
So in this case:
__asm__("mull %2"
:"=a" (eax), "=d" (edx)
:"rm" (fast_gettimeoffset_quotient),
"0" (eax));
So this inline assembly does
Move the memory location at eax into %eax
%edx:%eax = %eax * fast_gettimeoffset_quotient
It then moves %edx to the memory location edx
and %eax to the memory location eax.
The real result should be held at this point in edx unless the result
was too large to hold in %edx alone. Am I right?
Anton
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