10. Jumps Based on a Value of Zero

• A program can test register for zero value by OR instruction:

          or      bx, bx          ; Is BX = 0?
          jz      is_zero         ; Jump if so
  

  This code is functionally equivalent to:

          cmp     bx, 0           ; Is BX = 0?
          je      is_zero         ; Jump if so
  

  but produces smaller and faster code, since it does not use an immediate number as an operand. The same technique also lets you test a register’s sign bit:

          or      dx, dx          ; Is DX sign bit set?
          js      sign_set        ; Jump if so
  

• Note that none of the above OR instructions change original value in the register.