26. Sign Extending Unsigned Value

• If sign extension was not what you had in mind, that is, if you need to extend the unsigned value, explicitly set the higher register to zero:

          .DATA
          mem8    BYTE    251
          mem16   WORD    251
          .CODE
          .
          .
          .
          mov     al, mem8  ; Load 251 (FBh) from 8-bit memory
          sub     ah, ah    ; Zero upper half (AH)
  
          mov     ax, mem16 ; Load 251 (FBh) from 16-bit memory
          sub     dx, dx    ; Zero upper half (DX)
  
          sub     eax, eax  ; Zero entire extended register (EAX)
          mov     ax, mem16 ; Load 251 (FBh) from 16-bit memory