Putting a 64 bit word into two 32 bit registers and vice versa

bobcory · #1 20-Jul-2009, 15:34

This is one of those simple questions that is going to embarrass me once I know the answer but here goes

I want to use 64 bit words in VC++ as follows eg

__int64 iTest=0xAAAABBBBCCCCDDDD;

I then want to very simply put iTest into a couple of 32 bit registers using inline asm so that, for example,

eax == AAAABBBB

and

edx = CCCCDDDD

I then want to mess with eax and edx (eg doing shifts) and then put them back into iTest

Speed is absolutely crucial as this is part of a program that will run for days if i am not careful

Many thanks
Bob

Edit PS
I note that the disassembler does the following

323: __int64 i64Test=0xaaaabbbbccccdddd;
10003FE8 mov dword ptr [ebp-18h],0CCCCDDDDh
10003FEF mov dword ptr [ebp-14h],0AAAABBBBh

so I guess that is the answer (ie copy the memory locations to the registers)

Edit PPS
This does it:

__int64 iTest=0xaaaabbbbccccdddd;
__int64 *iptTest=&iTest;
_asm{
mov ecx, iptTest
mov edx, [ecx]
mov eax, [ecx+4]
}

Anybody got anything quicker or more elegant?

bobcory · #2 21-Jul-2009, 10:20

I think I have got this sussed now. There is very little stuff on the web re __int64 and inline asm so this may be helpful to somebody one day

Code:

int Long64BitIntTo32BitIntsLocal(__int64 lngWord, unsigned int *intFront, unsigned int *intEnd){
  //Extract all the bits from a 64 bit word and put them into two 32 bit words
  //Written 21st July 2009

  //Store the address of the 64 bit word
  __int64 *iptPointer=&lngWord;

  //Move the two sections to the two 32 bit ints
  _asm{
    mov esi, iptPointer          ;Put the Address of the 64 bit word into esi
    
    mov eax, dword ptr[esi]      ;Put the VALUE of the BACK 32 bits into eax ...
    mov edi, intEnd              ;Put the ADDRESS of BACK 32 bits into edi
    mov dword ptr[edi], eax      ;Put the VALUE into *intEnd to

    mov eax, dword ptr[esi+4]    ;Put the VALUE of the FRONT 32 bits into eax ...
    mov edi, intFront            ;Put the ADDRESS of FRONT 32 bits into edi
    mov dword ptr[edi], eax      ;Put the VALUE into *intFront to
  }

  return 0;
}

Call it like this

Code:

  unsigned int intFront;
  unsigned int intEnd;
 
  __int64 iTest=0xaaaabbbbccccdddd;
  
  Long64BitIntTo32BitIntsLocal(iTest, &intFront, &intEnd);

  //Note that intFront and intEnd are OUTPUT ie this is the result we wanted!

I find passing pointers to function calls a bit tricky as it seems to me that if you declare a pointer "eg int * ptrPointer" it doesn't exist until it does something so function calls don't seem to work if you do the following:

Code:

  //THIS WILL NOT WORK
  unsigned int *intFront;
  unsigned int *intEnd;
 
  __int64 iTest=0xaaaabbbbccccdddd;
  
  Long64BitIntTo32BitIntsLocal(iTest, intFront, intEnd);

The above version of Long64BitIntTo32BitIntsLocal is deliberately complicated to show the use of pointers in asm!

This is an easy version of Long64BitIntTo32BitIntsLocal

Code:

int Long64BitIntTo32BitIntsLocalX(__int64 lngWord, unsigned int *intFront, unsigned int *intEnd){
  //Extract all the bits from a 64 bit word and put them into two 32 bit words
  //Written 21st July 2009

  //Store the address of the 64 bit word
  __int64 *iptPointer=&lngWord;

  //Set up temporary stores
  unsigned int int1;
  unsigned int int2;

  //Move the two sections to the temporary stores
  _asm{
    mov ecx, iptPointer       ;Put the Address of the 64 bit word into ecx
    mov eax, [ecx]            ;Put the contents into eax ...
    mov int2, eax             ;... and then into store
    mov eax, [ecx+4]          ;Put the contents into eax ...
    mov int1, eax             ;... and then into store
  }

  //Put the contents in the right place
  *intFront=int1;
  *intEnd  =int2;

  return 0;
}

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Putting a 64 bit word into two 32 bit registers and vice versa

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