Quick, beginner MASM register question - DX:AX

Question

Hello, I am currently studying for an exam I'll have on x86 assembly.

I didn't have much luck googling for ":", too common of a punctuation mark :/

IDIV - Signed Integer Division

Usage: IDIV src

Modifies flags: (AF,CF,OF,PF,SF,ZF undefined)

Signed binary division of accumulator by source. If source is a byte value, AX is divided by "src" and the quotient is stored in AL and the remainder in AH. If source is a word value, DX:AX is divided by "src", and the quotient is stored in AL and the remainder in DX.

Taken from "Intel Opcodes and Mnemonics"

What does DX:AX mean?

Thanks a lot for your time :)

Answer 1

It's a pair of registers: DX and AX.

The numerator itself is a double word. The upper word of the numerator should be stored at DX, the lower one in AX.

Answer 2

DX:AX is the 32-bit value to use as the numerator of your division. The most significant 16 bits are held in DX, the least significant in AX. It's a way of specifying a 32-bit value in an otherwise 16-bit environment.