By now, you should know whether your program needs the kind of virtual storage that a data space or hiperspace offers. Only by understanding the differences between the two types of spaces, can you decide which one most appropriately meets your program's needs, or whether the program can use them both.

The main difference between data spaces and hiperspaces is the way a program references data. A program references data in a data space directly, in much the same way it references data in an address space. It addresses the data by the byte, manipulating, comparing, and performing arithmetic operations. The program uses the same instructions (such as load, compare, add, and move character) that it would use to access data in its own address space. To reference the data in a data space, the program must be in the ASC mode called access register (AR) mode. Pointers that associate the data space with the program must be in place and the contents of ARs that the instructions use must identify the specific data space.

Figure 1 shows a program in AR mode using a data space. The CLC instruction compares data at two locations in the data space; the MVC instruction moves the data at location D in the data space to location C in the address space.

Figure 1. Accessing data in a data space

In contrast, a program does not directly access data in a hiperspace. MVS™ provides a system service, the HSPSERV macro, to transfer the data between an address space and a hiperspace in 4K byte blocks. The HSPSERV macro read operation transfers the blocks of data from a hiperspace into an address space buffer where the program can manipulate the data. The HSPSERV write operation transfers the data from the address space buffer area to a hiperspace for storage. You can think of hiperspace storage as a high-speed buffer area where your program can store 4K byte blocks of data.

Figure 2 shows a program in an address space using the data in a hiperspace. The program uses the HSPSERV macro to transfer an area in the hiperspace to the address space. While the data is in the address space, the program compares the values at locations A and B, and uses the MVC instruction to move data at location D to location C. After it finishes using the data in those blocks, the program transfers the area back to the hiperspace. The program could be in either primary or AR ASC mode.

Figure 2. Accessing data in a hiperspace

With one HSPSERV invocation, the program can transfer data in more than one area between the hiperspace and the address space.