Problem while compiling 64bit instruction

[mitulshr]

Hi All,

I have 2 doubts/problem

1) I am writing assembly code to integrate with Visual Studio 13.
    Snippet :
   mov      rax, 0
   db      0x49,0x0F,0xC7,0xF8    ;will transfer data to r8 register
   jnc      .1
   mov      [rcx], r8
   inc      rax
   ret
.1
   ret
     
 I am getting following error:
       
error: impossible combination of address sizes

 If I comment out the statement I am getting following error:


 error: instruction not supported in 32-bit mode


 I have tried compiling using Command Line line as well as in Visual Studio.


Doubt:2)


 
 I have 2 identical .asm file(copy pasted). One of them is compiling successfully but other one giving error.
 What could be the possible reason for this?? Again I have tried both - Visual Studio and Command Line.


Please let me know if you any further information or clarification.

Thanks... 

[Frank Kotler]

Hi mitulshr,

I am not familiar with Visual Studio. Nasm does not seem to have a problem with your code, either encoded as bytes or as:

Code: [Select]

rdseed r8
Is this error supposed to be coming from Nasm?

Your "Doubt:2" does not seem possible. Either the two .asm files are  not identical or the compilation processes are not identical. (the randomness isn't supposed to occur until runtime )

I'm inclined to think that this is "not a Nasm problem". Sorry I can't help you.

Best,
Frank

[mitulshr]

Hi Frank Kotler,

Thanks for your response.

Doubt: 1)
             rdseed is not being recognized by nasm
             and I am getting these errors even if try to compile it command line.
             I am using "NASM version 2.09.10 compiled on Jul 15 2011"
           
            Edit:
            Just found the documentation about rdseed [C.1.17 Version 2.10.02]
           But, there was nothing about rdrand!!!

Doubt: 2)
             This is the most surprising  thing I have seen in my coding history. I do believe there must be some thing I am missing
             But, I literally copy pasted the code!!!

[Frank Kotler]

Well "rdseed" is pretty new, I guess. I would expect a string of "db"s to work anyway. As for "doubt 2"... different line endings, maybe? I'm surprised that would cause a problem. What errors are you seeing there?

Best,
Frank

[mitulshr]

Hi Frank,

Doubt 1)
               Now, I have downloaded the latest version and rdrand and rdseed instruction is working
               But, I am still getting the same errors. But, I tried to compile it with -fwin64 flag, and now its giving me same error for 32 bit instruction
               Is it problem that you cannot compile 32bit and 64bit instructions in same .asm file??

Doubt 2)
             A restart in Windows did the trick!!!

[Frank Kotler]

Well you can't use 64-bit instructions in 32-bit code, of course. You can't push/pop 32-bit registers in 64-bit code. There may be other 32-bit instructions that are not allowed... I can't think of any. I'm not very experienced with 64-bit code. What error are you getting?

Best,
Frank