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| The doubling of transistor capacity every 18-24 months. |
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| A rule stating that the performance enhancement possible with a given improvement is limited by the amount that the improved feature is used. It is a quantitative version of the law of diminishing returns. |
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| Performance = 1/Execution time. |
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| Execution time for a program = CPU clock cycles for a program*Clock cycle time. |
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| Cycles per instruction. (CPU clock cycles/Instruction count). |
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| The process of positioning a read/write head over the proper track on a disk. |
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| Thousands of concentric circles that make up the surface of a magnetic disk. |
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| Segments that make up a track on a magnetic disk; a sector is the smallest amount of information that is read or written on a disk. |
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| A rigid rotating disk on which data is stored in a disk drive. |
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| The time required for the desired sector of a disk to rotate under the read/write head; usually assumed to be half the rotation time. |
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| The process of periodically checking the status of an I/O device to determine the need to service the device. |
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Definition
| To indicate to the processor that an I/O device needs attention. |
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Definition
| A mechanism that provides a device controller with the ability to transfer data directly to or from the memory without involving the processor. |
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Definition
| No redundancy, data spread over multiple disks (striping). Improves performance for large accesses (video-editors). |
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Definition
| Mirroring, uses twice as many disks as RAID0. When data is written to one disk, that data is also written to a redundant disk – thus there are always two copies of the data. (Most expensive of the RAIDs). |
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Definition
| Error detecting and correcting code, not used anymore. |
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Definition
| Bit-interleaved parity. Writes/reads go to all disks in the group plus one extra – the parity disk. |
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Term
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Definition
| Block-interleaved parity. Similar to RAID3 except it stripes at the block level instead of the byte level. Parity disk must be updated with each write. |
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| Distributed Block-interleaved Parity. Spreads parity information throughout all disks so there is no bottleneck for writes as with RAID4. |
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Definition
| Extends RAID5 by adding additional parity block, thus it uses block-level striping with two parity blocks distributed across all member disks. Super good data protection. |
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| The minimum unit of information that can be either present or not present in a cache. |
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| A field in a table used for a memory hierarchy that contains the address information required to identify whether the associated block in the hierarchy corresponds to a requested word. |
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| Block found at current level in memory. |
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| A scheme in which writes always update both the cache and the next lower level of the memory hierarchy, ensuring that data is always consistent between the two. |
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Definition
| A scheme that handles writes by updating values only to the block in the cache, then writing the modified block to the lower level of the hierarchy when the block is replaced. |
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| The table containing the virtual to physical address translations in a virtual memory system. The table, which is stored in memory, is typically indexed by the virtual page number; each entry in the table contains the physical page number for that virtual page if the page is currently in memory. |
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Term
| Translation Look-aside Buffer |
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Definition
| A cache that keeps track of recently used address mappings to try to avoid an access to the page table. |
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Definition
| An event that occurs when an accessed page is not present in main memory. |
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| The virtual address is an address that corresponds to a location in virtual space and is translated by address mapping to a physical address when memory is accessed. |
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Definition
| The physical address is it’s actual address in memory. |
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