chap file processing and external sorting reading blocks stored on slowersecondary memory (such as on the disk drivethe disk stores the complete contents of the virtual memory blocks are read into main memory as demanded by memory accesses naturallyprograms using virtual memory techniques are slower than programs whose data are stored completely in main memory the advantage is reduced programmer effort because good virtual memory system provides the appearance of larger main memory without modifying the program example consider virtual memory whose size is ten sectorsand which has buffer pool of five buffers associated with it we will use lru replacement scheme the following series of memory requests occurs after the first five requeststhe buffer pool will store the sectors in the order because sector is already at the frontthe next request can be answered without reading new data from disk or reordering the buffers the request to sector requires emptying the contents of the least recently used bufferwhich contains sector the request to sector brings the buffer holding sector ' contents back to the front processing the remaining requests results in the buffer pool as shown in figure example figure illustrates buffer pool of five blocks mediating virtual memory of ten blocks at any given momentup to five sectors of information can be in main memory assume that sectors and are currently in the buffer poolstored in this orderand that we use the lru buffer replacement strategy if request for sector is then receivedthen one sector currently in the buffer pool must be replaced because the buffer containing sector is the least recently used bufferits contents will be copied back to disk at sector the contents of sector are then copied into this bufferand it is moved to the front of the buffer pool (leaving the buffer containing sector as the new least-recently used bufferif the next memory request were to sector no data would need to be read from disk insteadthe buffer containing sector would be moved to the front of the buffer pool when implementing buffer poolsthere are two basic approaches that can be taken regarding the transfer of information between the user of the buffer pool and