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CDQ

Extends a dword register into a qword value across two registers. It copies the most significant bit (sign bit) of the source register into the destination register to perform sign-extension.

The table after the description covers what the source and destinations can be.

DO NOT support LOCK

This instruction is available in 64-bit mode and compatibility mode. It specifically targets the eax register as the source and utilizes edx as the implicit destination for the upper 32 bits of the resulting sign-extended value.

The instruction is essential for preparing the dividend for the IDIV instruction. Failure to use CDQ (or CQO for 64-bit registers) before IDIV will result in an incorrect quotient or a #Z / #O exception because IDIV expects the dividend to be spread across edx:eax (for 32-bit) or rdx:rax (for 64-bit).