Inline Assembly - Display a register in decimal using printf?

Question

I just had a really quick question that I saw someone mention something about in another question, but I didn't want to necro-post on it.

I'm coding in inline assembly with c++, and need to display a register value in decimal. I was searching ways to do this, and saw someone mention "If you're using inline c, just call printf." But they didn't go much further into explanation on it than that.

Is it possible the call printf can be used to get a register value in decimal format without needing to write a conversion section of the code? And if so, how would that work? Say after some computations to a user entered integer, the value now lies in the AX register. Would I simply put call printf in the code after it? Or does it print values from the stack? Or is it maybe even possible to do something like:

AX printf

I apologize for my ignorance on this, our book does not cover inline assembly, and I'd like to avoid having to write a massive segment of code to convert if I can. Plus I can't really seem to find answers on how exactly printf works. Thank you for any help, I really appreciate it!

Answer 1

The easiest way to accomplish this is to use inline assembler to copy your register to some variable, and then print that variable.

short registerValue;
__asm mov registerValue, ax;
printf("ax: %hd", registerValue);

The exact assembler invocation will depend on your compiler and syntax; the above likely won't work with a compiler other than cl.

If you want to actually call printf from assembler, you'll need to figure out it's calling convention and how that calling convention passes variadic function arguments.

Answer 2

Depending on the compiler, there may be predefined pseudo-symbols which directly access the registers. This was especially convenient with Turbo C and its descendants:

 _some_magic_function ();
 printf ("es:bx = %0x:%0x\n", _ES, _BX);