CBW: Convert Signed Byte to Word: This instruction converts a signed byte to a signed word. In other terms, it copies the sign bit of a byte to be converted to all of the bits in the higher byte of the result word. The byte to be converted might be in the AL. The result will be stored in the AX. It does not affect any flag.

CWD: Convert Signed Word to Double Word: This instruction copies the sign bit of the AX register to all the bits of the DX register. This operation is to be done before signed division. It does not  make affect  on any flag.