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I'm trying to round an input double using a specified rounding mode in in-line assembly in C. To do so, I need to grab the FPU control word using fstcw and then change the bits in the word. Unfortunately I'm encountering an error on the very first line:

double roundD(double n, RoundingMode roundingMode) {
    asm("fstcw      %%ax       \n"

        ::: "ax");      //clobbers

    return n;
}

The assembler error I receive is:

Error: operand type mismatch for 'fstcw'.

I'm under the impression this code snippet should store the FPU control word, which is 16 bits in length, in the AX register, which is also 16 bits in length. Just to be sure, I also tested the above code with the EAX register instead of AX, and received the same error.

What might I be missing here? Please let me know if any further information is needed.

Sep Roland
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tschuy
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    So I just googled it, and apparently `fstcw` wants to store things in memory, not a register. – melpomene Feb 26 '18 at 03:58
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    According to [this](http://www.website.masmforum.com/tutorials/fptute/fpuchap3.htm) the destination can be either a location in memory or the AX register. – tschuy Feb 26 '18 at 04:01
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    No, your link says exactly the opposite: "**fstcw *Dest*** [...] *This instruction stores the content of the Control Word register at a 16-bit WORD memory address (Dest).*" and "*[`fstsw`] is the only FPU instruction which can transfer information directly from one of the FPU's registers to one of the CPU's general purpose registers.*". – melpomene Feb 26 '18 at 04:05
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    If this happens to be for the hullspeed assignment then you might want to look at this: https://stackoverflow.com/questions/35515378/assembly-code-fsqrt-and-fmul-instructions . if the hullspeed assignment means nothing to you, that answer may give you some information. if the hullspeed assignment is what this is for (we had another hullspeed question recently) then that answer may be of more interest to you. – Michael Petch Feb 26 '18 at 06:22

1 Answers1

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fstcw (control word) only works with a memory destination operand, not register.

Perhaps you're getting mixed up with fstsw (status word) which has a separate form (separate opcode) where the destination is AX instead of specified by an addressing mode.

That was helpful to efficiently branch based on an FP compare result (before fcomi to compare into EFLAGS existed), which happens more often than anything with the control word. That's why there's an AX-destination version of fnstsw but not fnstcw.

And BTW, you can set the rounding mode using C. #include <fenv.h>

Or far better, if SSE4.1 is available, use roundsd (or the intrinsic) to do one rounding with a custom rounding mode, without setting / restoring the SSE rounding mode (in MXCSR, totally separate from the x87 rounding mode).

Peter Cordes
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