Specifying the size of the data space

When you create a data space, you tell the system on the BLOCKS parameter how large to make that space, the largest size being 524,288 blocks. (The product of 524,288 times 4K bytes is 2 gigabytes.) The addressing range for the data space depends on the processor. If your processor does not support an origin of zero, the limit is actually 4096 bytes less than 2 gigabytes. Before you code BLOCKS, you should know two facts about how an installation controls the use of virtual storage for data spaces and hiperspaces.

An installation can set limits on the amount of storage available for each address space for all data spaces and hiperspaces that have a storage key of 8 through F. If your request for a data space would cause the installation limit to be exceeded, the system rejects the request with a nonzero return code and a reason code.
An installation sets a default size for data spaces and hiperspaces; you should know this size. If you do not use the BLOCKS parameter, the system creates a data space with the default size.

If you create the data space with a storage key of 0 through 7, the system does not check the size against the total storage already used for data spaces and hiperspaces. If you create the data space with a storage key of 8 through F, the system adds the initial size of the space to the cumulative total of all data spaces and hiperspaces for the address space and checks this total against the installation limit for an address space.

For information on the IBM® defaults and how to change them, see Limiting data space use.

The BLOCKS parameter allows you to specify a maximum size and initial size value.

The maximum size identifies the largest amount of storage you will need in the data space.
An initial size identifies the amount of the storage you will immediately use.

As you need more space in the data space, you can use the DSPSERV EXTEND macro to increase the size of the available storage, thus increasing the storage in the data space that is available for the program. The amount of available storage is called the current size. (At the creation of a data space, the initial size is the same as the current size.) When it calculates the cumulative total of data space and hiperspace storage, the system uses the current size of the data space.

If you know the default size and want a data space smaller than or equal to that size, use the BLOCKS=maximum size or omit the BLOCKS parameter.

If you know what size data space you need and are not concerned about exceeding the installation limit, set the maximum size and the initial size the same. BLOCKS=0, the default, establishes a data space with the maximum size and the initial size both set to the default size.

If you do not know how large a data space (with storage key 8 - F) you will eventually need or you are concerned with exceeding the installation limit, set the maximum size to the largest size you might possibly use and the initial size to a smaller amount, the amount you currently need.

Use the NUMBLKS parameter to request that the system return the size of the data space it creates for you. You would use NUMBLKS, for example, if you did not specify BLOCKS and do not know the default size.

Figure 1 shows an example of using the BLOCKS parameter to request a data space with a maximum size of 100,000 bytes of space and a current size of 20,000 bytes.

Figure 1. Example of Specifying the Size of a Data Space

As your program uses more of the data space storage, it can use DSPSERV EXTEND to extend the current size. Extending the current size of a data space describes extending the current size and includes an example of how to extend the current size of the data space in Figure 1.