Hello,

I have some trouble understanding exactly how the FPU works.

I have the following code and what would help me is to have it explained:


Could you tell me:
1. what are the mathematical operations that are being performed, i.e. on the instruction "n", the value "x" is multiplied by the the value "y", and that sort of things.
2. what FPU registers are being used on each line since none is explicitly used other than on the fifth line.

Thank you in advance.

fildpi ; loads Pi onto the top of the stack (st0)
fild dword [audio_frequency] ; loads the audio frequency onto the top of the stack (st0), Pi is moved down to st1
fidiv word [channel_phaze] ; converts channel_phaze to a double precision FP value; divide st0 (audio_frequency) by channel_phaze and store results on top of stack (st0)
fimul word [frequency] ; converts frequency to a double precision FP value and multiplies st0 by freqency and stores the result in st0
fmul st0,st1 ; multiplies st0 by st1 (pi) and stores the result in st0
fsin ; calculates the sine of st0 in radians and stores the result in st0
fimul word [amplitude] ; converts to double precision and multiplies by st0 storing the result in st0
fistp word [di] ; store the contents of st0 in the memory location [di]; remove st0 from the stack

mathematically this performs the following operation

di = Amplitude * sin((π * frequency)/channel_phaze)

It's exactly what I wanted. Thanks very much. Just one more thing, why has pi been moved down to st1? Is it because the "fild" instruction decrements the TOP register pointer?

Instead of "fmul st0, st1" you could use the fmulp (with no arguments) this muliplies st0 by st1 and pops the stack then stores the result in st0. Doing this would leave your FPU stack clean for the next operation.

thanks madmatt for the optimization

mattst88, shouldn't the operation be rather:
amplitude * sin (pi * ((audio_frequency / channel_phaze) * frequency))
or am I wrong?

Yes, it should. My mistake.



Yes, this is exactly what is happening. Since Pi is stored in st0 (the top of the stack) and something else is being pushed onto the stack, Pi is no longer the top. It is now referenced as st1 and the new item is now the top, st0.