IDIV operation in assembly (understanding)

when I have the the operation

IDIV ecx

in assembly, then i have read that the that the value in edx:eax is divided by the operand ecx. I also know that the quotient is stored in eax and the remainder in edx.

so but what exactly is the value in edx:eax ??

Can someone explain it to me?

edit: i also did not understand the reason of “cdq” operation which come always before the IDIV operation although I have read some pages about that.

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