11. CBW, CWD, CDQ Instructions

• The CBW (convert byte to word) instruction extend s the sign bit of AL into the AH register. This preserves the number 's sign:

      .DATA
      byte_val SBYTE -101
      .CODE
      mov al, byte_val ; AL = 9Bh
      cbw              ; AX = FF9Bh
  

  Note that both 9Bh and FF9Bh both equal decimal -101, the only difference is the storage size.

• The CWD (convert word to doubleword) instruction extends the sign bit of AX into the DX register:

      .DATA
      word_val SWORD -101 ; FF9Bh
      .CODE
      mov ax, word_val    ; AX = FF9Bh
      cwd                 ; DX:AX = FFFFh:FF9Bh
  

• The CDQ (convert doubleword to quadword) instruction extends the sign bit of EAX into the EDX register:

      .data
      dword_val SDWORD -101
      .code
      mov eax, dword_val  ; EAX = FFFFFF9Bh
      cdq                 ; EDX:EAX = FFFFFFFh:FFFFFFF9Bh