Allocating Data Sets; Specifying A Multivolume Indicator; Determining Exchange Type - IBM 5280 Programmer's Manual

Allocating Data Sets
To allocate the data set(s), use the diskette label maintenance utility
(SYSLABEL), which prompts you for the information needed for allocation.
Your response to the exchange type, the record size, the number o/records,
and the delete character prompts will depend on how you code your program
and other considerations. This is discussed in the sections that follow.
Specifying a Multivolume Indicator
If your data set is located on more than one diskette volume, you must specify
a multivolume indicator during allocation. See "Multivolume Record
Processing" in Chapter 5 for the conventions to follow in specifying
multivolumes.
Determining Exchange Type
You should be aware of the data set exchange types you can allocate for the
most efficient operation of your program.
SYSLABEL will prompt you to enter one of the following exchange types:
basic, H, or I. The data exchange you select determines the structure of your
data set as discussed below.
Unblocked and unspanned (basic and H) means that records are not blocked
together and that the records must each start on sector boundaries. Each
record is one block of data. On the 5280, unblocked and unspanned records
cannot be longer than sectors.
l28-Byte Sectors
A
B
c
o
E
2
3 4
5
100-Byte Records
In the preceding example, each 100-byte record starts at the beginning of a
128-byte sector boundary. The remaining 28 bytes in each sector are not
available for data storage. Some of the potential storage space is thus wasted.
The basic and H exchange types use the unblocked and unspanned data
structure.
Blocked and spanned (I) means that records are blocked together, and that
sector boundaries are not necessarily related to record positions.
Chapter 8. Guide for COBOL Program Execution 8- 5