A.14 CBW, CWD, CDQ, CWDE: Sign Extensions

CBW                           ; o16 98               [8086] 
CWD                           ; o16 99               [8086] 

CDQ                           ; o32 99               [386] 

CWDE                          ; o32 98               [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.

CWD extends AX into DX:AX by repeating the top bit of AX throughout DX.
CWDE extends AX into EAX, and 

CDQ extends EAX into EDX:EAX.
