/* Test to see controller with at last one attached drive is there. * Returns a bit for each possible drive found (0x01 for drive 0, * 0x02 for drive 1). * Logic: * - If a status register is at 0xff, assume there is no drive here * (ISA has pull-up resistors). Similarly if the status register has * the value we last wrote to the bus (for IDE interfaces without pullups). * If no drive at all -> return. * - reset the controller, wait for it to complete (may take up to 31s !). * If timeout -> return. * - test ATA/ATAPI signatures. If at last one drive found -> return. * - try an ATA command on the master. */