COA课件06.ppt

William Stallings Computer Organization and Architecture6th Edition Chapter 6 External Memory 外部存储器 Types of External Memory Magnetic Disk 磁盘 RAID Removable Optical 光盘 CD-ROM CD-Recordable (CD-R) CD-R/W DVD Magnetic Tape 磁带 §6.1 Magnetic Disk Disk substrate(底层) coated with magnetizable material (iron oxide…rust) Substrate used to be aluminium(铝) Now glass Improved surface uniformity(一致,均匀) Increases reliability(可靠性) Reduction in surface defects Reduced read/write errors Lower flight heights (See later)(较低的飞行高度) Better stiffness(硬度好) Better shock/damage resistance (震动/损坏) Read and Write Mechanisms：电磁互变 Recording and retrieval via conductive coil(感应线圈) called a head 磁头 May be single read/write head or separate ones During read/write, head is stationary(固定的), platter rotates Write 电变磁 Current through coil produces magnetic field Pulses(脉冲) sent to head Magnetic pattern recorded on surface below Read (traditional) 磁变电 Magnetic field moving relative to coil produces current Coil is the same for read and write Read (contemporary) Separate read head, close to write head Partially shielded magneto resistive (MR) sensor Electrical resistance depends on direction of magnetic field High frequency operation Higher storage density and speed Inductive Write MR Read Data Organization and Formatting Concentric rings or tracks 同心圆/磁道 Gaps between tracks Reduce gap to increase capacity Same number of bits per track (variable packing density) Constant angular velocity 恒角速度 Tracks divided into sectors 扇区 Minimum block size is one sector May have more than one sector per block Disk Data Layout Disk Velocity 磁盘速度 Bit near centre of rotating disk passes fi