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CWD

Extends a signed word (16-bit) value in the AX register to a signed double word (32-bit) value across the EAX register. It copies the sign bit of AX into the upper 16 bits of EAX.

The following table covers what the source and destinations can be.

DO NOT support LOCK

This instruction is available in 32-bit protected mode and 64-bit mode. In 64-bit mode, it operates specifically on the AX and EAX registers to maintain compatibility with 32-bit code. It does not affect the upper 32 bits of the RAX register.

The user SHALL ensure that CWD is used specifically for sign-extending a 16-bit integer to 32-bit before performing operations like IDIV EAX to avoid incorrect results due to improper sign extension. Failure to sign-extend will result in the upper 16 bits of EAX containing stale data, leading to an incorrect quotient or a #Z / #O exception.