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(nasm.info)Section B.4.19

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B.4.19. `CBW', `CWD', `CDQ', `CWDE': Sign Extensions
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     CBW                           ; o16 98               [8086]
     CWDE                          ; o32 98               [386]

     CWD                           ; o16 99               [8086]
     CDQ                           ; o32 99               [386]

   All these instructions sign-extend a short value into a longer one,
by replicating the top bit of the original value to fill the extended
one.

   `CBW' extends `AL' into `AX' by repeating the top bit of `AL' in
every bit of `AH'. `CWDE' extends `AX' into `EAX'. `CWD' extends `AX'
into `DX:AX' by repeating the top bit of `AX' throughout `DX', and
`CDQ' extends `EAX' into `EDX:EAX'.