Question

I have been looking around on NASM tutorials and I have noticed that in all the references to the DIV instruction, when discussing 32-bit division, say something along the lines of:

DIV ECX    ; EDX:EAX / ECX

What does the EDX:EAX mean? Why are two registers being divided by one register?

Thanks in advance

Answer 1

This is a spanned register, or register pair, its used for 64-bit math in this case (so you can use a 64-bit quotient, IIRC this was added to allow arbitrary point arithmatic).
EDX contains the high DWORD and sign, EAX the low DWORD.

The same logic is used for returning 64-bit results. Also, it should be noted, this has nothing to do with NASM, its part of the x86 architecture (which also defines 32-bit pairs, like DX:AX when using 16-bit instructions).