Chapter 1. PHYSICAL FILES

CHAPTER OBJECTIVES

Upon completion of this chapter, you should be able to

  • Explain the hierarchy of data: file, records, and fields.

  • Describe the differences between an alphanumeric (character) field, zoned decimal field, and packed decimal field.

  • Explain the EBCDIC (Extended Binary Coded Decimal Interchange Code) representation for storing data.

  • Explain and demonstrate how Data Description Specifications (DDS) are used to describe files.

  • Describe the differences between arrival sequence and keyed sequence access paths.

HIERARCHY OF DATA

Programming languages such as COBOL/400 provide the computer with the instructions to perform specific tasks. Normally, these instructions include accessing data. Data are unorganized raw facts.

To process data in an organized way, data areas are set aside in the computer's memory for fields, records, and files. A field is a group of storage positions reserved for a specific data item. For example, if a retail organization wanted to store data about their employees, it would probably want to include the following data items: employee number, store number, employee name, department number, hourly rate, hours worked, sales, and so on. Each data item describes one specific element of the employee and is stored as a field. Suppose an employee's number is 864955834. Enough storage positions would be set aside to store this value in the employee number field.

Examine the employee number field in Figure 1.1 and observe that the value 864955834 ...

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