The Silicon Graphics hardware implementation of the EISA bus is bridged onto the GIO bus, which does not support a locked cycle. The general form of locked bus cycles is not supported in the Silicon Graphics implementation of EISA. An EISA card cannot lock the bus nor can software in the IRIX kernel lock the EISA bus.
A device driver in the IRIX kernel can perform a software-controlled read-modify-write cycle, as on a VME bus, using the pio_*_rmw() kernel functions. See ("Using the PIO Map in Functions"). This function ensures that no other software accesses the EISA bus during the read-modify-write operation.