How to move ST(0) to EAX?

Question

Hullo, I am learning x86 FPU assembly, and I have got a simple question I cannot find answer for:

How to move value from ST(0) ( top of the FPU stack ) to EAX ?

also:
is this code correct:

; multiply (dot) two vectors of 3 floats passed by pointers as arg 1 arg 2
; passings are ok I think, but not sure if multiplies-adds are ok

    push    ebp                                     
    mov     ebp, esp                                
    mov     eax, dword [ebp+8H]                     
    mov     edx, dword [ebp+0CH]                    

    fld     qword [eax]                             
    fmul    qword [edx]                             
    fld     qword [eax+4H]                          
    fmul    qword [edx+4H]                          
    fld     qword [eax+8H]                          
    fmul    qword [edx+8H]                          
    faddp   st1, st(0)                              
    faddp   st1, st(0)                            
    fstp    qword [ebp+10H]     ; here I vould prefer 'mov eax, st0'

    pop     ebp                                   
    ret                                           

Answer 1

There is no real reason why you should. Remember that EAX is only a 32-bit register, while all the FPU registers are 80 bits in width, because the FPU does calculations on 80-bit floats by default. Therefore, moving data from the FPU register to a general purpose register will cause data loss. If you really want to do something like that, try this (assuming that you've got some stack space available) :

sub esp, 4           ; or use space you already reserved
fstp dword [esp]
mov eax, [esp]       ; or better,  pop eax
add esp, 4

This instruction sequence will round the float currently on the top of the FPU stack to a 32-bit one, then write it to a temporary stack location, load the float (binary32) bit pattern into EAX, and clean up the used stack space.

---

This is almost never what you want. The standard calling conventions return float / double / long double values in st(0), so that's where a C compiler will expect a double foo() function to leave the value. (Or xmm0 with SSE/SSE2).

You only need this if you want to do integer manipulation / tests on FP bit-patterns. (i.e. to implement type-punning in C like memcpy from a float to uint32_t). e.g. for the famous but now mostly obsolete fast approximate inverse-sqrtf magic-number hack used in Quake source code.

Answer 2

There are no x87 instructions to move floating point values between the FPU registers and the CPU registers.

You have to use memory as the intermediate.