27. Sign Extending with MOVSX and MOVZX

• The 80386/486/Pentium processors provide instructions that move and extend a value to a larger data size in a single step.

• MOVSX moves a signed value into a register and sign-extends it with 1.

• MOVZX moves an unsigned value into a register and zero-extends it with zero.

          mov     bx, 0C3EEh  ; Sign bit of bl is now 1: BH == 1100 0011, BL == 1110 1110
          movsx   ebx, bx     ; Load signed 16-bit value into 32-bit register and sign-extend
                              ; EBX is now equal FFFFC3EEh
          movzx   dx, bl      ; Load unsigned 8-bit value into 16-bit register and zero-extend
                              ; DX is now equal 00EEh
  

• MOVSX and MOVZX instructions usually execute much faster than the equivalent CBW, CWD, CWDE, and CDQ.