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Every character occupies storage of one byte space in computer memory. Financial institutions like banks are computerized. A bank may store data of its customers on a central computer. This would include details such as the full-name of the customer, his residential address, his social security number, his contact details and his account number. Data such as the customer name consist of a sequence or a group of characters. Associated fields such as the customer name, address, the account number etc.
A record represents data for a single instance. A customer record tells you everything about one customer. A record may be divided into several fields. Assume that the customer name field can be 10 characters long, the customer address can have upto 30 characters, the contact details field can span 10 characters, the social security number 09 digits and the account number upto 21 digits.
Recollect that each character on the mainframe computer occupies one byte of memory space. If a collection of records are stored together, say 1000 customer records of the bank, it is called a file. A file is then, just a sequence of records. IBM mainframes use the term dataset instead of file. Mass storage devices are used to permanently store data. It does not lose its contents even when the electrical power is cut-off. Mass storage devices can store humungous volumes of data.
Sequential access means that the mainframe system must search the storage device from the beginning, till the desired data is found. This is like playing a Bollywood music cassette tape on your Sony Walkman. Direct access implies that the mainframe system can directly locate the desired data on the storage device. This is like reading a topic in a reference book. If I would like to read about optical isomers in Organic Chemistry’s book written by Morrison Boyd, I’d check the entry optical isomers in the index. In the index, the entry optical isomer had the page number 257 against it. Faraday’s laws of electro-magnetic theory in Physics say that when a magnet is moved past a magnetic field, it can be magnetised, usually created by an electro-magnet.