Operating_systems-page64 - Jelena Mamenko Operating Systems...

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Jelena Mamčenko Operating Systems Lecture Notes on Operating Systems 64 DOS simply can’t do without I/O buffering, which represents the simplified variant of cashing. That’s why it is represented with the compact code: anticipatory reading and intermediate writing aren’t executed. However the purpose of I/O buffering is not just to minimize the access time to the same data, but also to extract the logical records from the physical records and vice- versa – to form physical records from the logical records. Physical record is a portion of data, which is transferred between RAM and external memory (for disks – contents of a sector). Logical record is a portion of data, inquired by a program or outputted by it. The tools of the I/O buffering allow reading of the physical record for only one time, even if there are several logical records in it needed for a certain program’s performance. Analogically, physical record is written to disk only after it is formed from several logical records. Without the I/O buffering tools the reading of each
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