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modern operating systems 2nd edition phần 4

Các hoạt động khôi phục sẽ ghi đè lên cơ sở dữ liệu hiện có trên Exchange Server MS. Một bản sao lưu ngoại tuyến của cơ sở dữ liệu MS Exchange Server cần được phục hồi Lưu ý: Không để gián đoạn các hoạt động khôi phục lại. | 272 INPUT OUTPUT CHAP. 5 that van be inserted into an expansion slot. The mechanical component is the device itself This arrangement is shown in Fig. 1-5. The controller card usually has a connector on it. into which a cable leading to the device itself can be plugged. Many controllers can handle two four or even eight identical devices. If the interface between the controller and device is a standard interface either an official ANSI IEEE or ISO standard or a de facto one then companies can make controllers or devices that fit that interface. Many companies for example make disk drives that match the IDE or SCSI interface. The interface between the controller and the device is often a very low-level interface. A disk for example might be formatted with 256 sectors of 512 bytes per track. What actually comes off the drive however is a serial bit stream stalling with a preamble then the 4096 bits in a sector and finally a checksum also called an Error-Correcting Code ECC . The preamble is written when the disk is formatted and contains the cylinder and sector number the sector size and similar data as well as synchronization information. The controller s job is to convert the serial bit stream into a block of bytes and perform any error correction necessary. The block of bytes is typically first assembled bit by bit in a buffer inside the Gonưolĩer. After its checksum has been verified and the block declared to be eiTor free it can then be copied to main memory. The controller for a monitor also works as a bit serial device at an equally low level. It reads bytes containing the characters to be displayed from memory and generates the signals used to modulate the CRT beam to cause it to write on the screen. The controller also generates the signals for making the CRT beam do a horizontal retrace after it has finished a scan line as W ell as the signals for making it do a vertical retrace after the entire screen has been scanned. If it were not for the CRT controller