How much slower is the FPU compared with the CPU?

[Astro]

Hi,

As the subject. Does it run at clock speed?

Best regards,
Robin.

[clive]

As the subject. Does it run at clock speed?

These days it is certainly synchronous and tightly coupled to the CPU. How exactly the design is pipelined will depend on the implementation, but the speed/latency is measured in CPU cycles.

The demarcation between FPU and SSE units may also have changed over time.

-Clive

[clive]

As I recall the were sections of the P4 floating point unit that ran at half speed. I'll see if I can find a cite.

EDIT : I might be thinking that the Integer units were twice as fast, so relatively the FPU was slower.

The MindShare series was quite useful, Intel tends to keep internal details secret
http://www.amazon.com/exec/obidos/ASIN/032124656X/hsecrets-20

-Clive

[Astro]

Hi Clive,

I seem to remember something about integer vs. floating point performance, but can't remember enough about it. :(

Thanks for the book reference!

Best regards,
Robin.

[dedndave]

the FPU is pretty fast
especially if you compare doing the equiv floating point math
a while back, i tried my best to beat Ray - lol
i came close   :P

http://www.masm32.com/board/index.php?topic=11999.msg91486#msg91486

that was just a simple integer conversion problem

[FORTRANS]

Hi,

   This had some code for comparing FDIV and DIV.  You
could expand it to test the operations you are interested
in.

Steve

http://www.masm32.com/board/index.php?topic=12992.msg100662#msg100662