A method of allocating a basic input/output system to a shadow memory applied to a computer including a plurality of equipment having an operation read only memory and a shadow memory. The method includes executing the preliminary initialization of the equipment to acquire the capacity of the operation read only memory of each equipment after being initialized. Then the equipment is re-initialized according to the order of the capacity of each operation read only memory after being initialized from smallest capacity to largest capacity to allocate the shadow memory more effectively and increase the number of equipment to be enabled.