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1. a) • A block is a group of records. A block is referred to as the UNIT of TRANSFER

• In computer files as when a record is searched / updated the whole block containing the record is transferred to main store (not just the record)

b) Blocks are used to save time. They save time in 2 ways

• It is quicker to search in main store, therefore the more data that can be placed in main store (rather than disk) the quicker it is to search

• The Indexes can be smaller thus making them quicker to search

Block size is limited by the amount of main store available

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2.a) The payroll file is an Indexed sequential file, this means it can be accessed in 2 ways

i) sequentially – when all records need updating - In the monthly pay run the transactions (time Information) would be merged with the main file sequentially to create a new (updated) main file and printouts of payslips

ii) Using the index – when few records need updating – If something someone's record changes e.g. address

For a monthly payroll run option 1 would be best because all records are processed

Page 132.b) • The record needs to be inserted into the correct place because the file is organised

sequentially

• The key of the record is compared to the primary index . This locates a secondary index

• The key of the record is compared to the secondary index . This locates a tertiary index

• The tertiary index locates the block where the record needs to be added

• This block is loaded into main memory.

• The record is inserted and the indexes are updated to show that the record is now there.

• (if there is not sufficient room for all of the record in this block then some of the data is placed in an overflow area on disk and a pointer is inserted to show this)

• The block is written back to the appropriate location on disk

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2.c)• The key of the record is compared to the primary index . This locates a secondary

index

• The key of the record is compared to the secondary index . This locates a tertiary index

• The tertiary index locates the block where the record needs to be added is found

• The block containing the record is read to main store.

• The block is quickly searched in main store and the record found

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3.a)• A hierarchy of indexes is when a very large index is split into different levels

• A Primary index is used to load a secondary index which locates and loads a third index

• . • The third index points to the address in the file of the block containing the record

Example of a Hierarchy Index Structure

Index High Key

1 1000

2 2000

3 3000

4 4000

5 5000

Address Key

5212 3101

5212 3102

5212 3103

5212 3104

5213 3105

5213 3106

Index High Key

72 3100

73 3200

74 3300

75 3400

76 3500

77 3600

4

Searching for 3105

73

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4. • Over time a large file will become ‘untidy’ and inefficient. This can be due to gaps left

by deleted records or overflow from inserted records

• A Housekeeping routine is used to rewrite the file and reorganise the indexes , getting rid of gaps left by deleted records and removing overflow by reorganising the blocks.