_DEV
CSR_WRITE_1(chan->cmd + _DEV, ATA_DEV_OBS); /* ??? */
CSR_WRITE_1(chan->cmd + _DEV, ATA_DEV_LBA);
CSR_WRITE_1(chan->cmd + _DEV, ((bno >> 24) & 0xf) | ATA_DEV_LBA);