Think of a data object as a contiguous string of bytes organized into blocks, each 4096 bytes long. The first block contains bytes 0 to 4095 of the object, the second block contains bytes 4096 to 8191, and so forth.

Your program references data in the object by identifying the block or blocks that contain the desired data. Window services makes the blocks available to your program by mapping a window in your program storage to the blocks. A window is a storage area that your program provides and makes known to window services. Mapping the window to the blocks means that window services makes the data from those blocks available in the window when you reference the data. You can map a window to all or part of a data object depending on the size of the object and the size of the window. You can examine or change data that is in the window by using the same instructions that you use to examine or change any other data in your program storage.

The following figure shows the structure of a data object and shows a window mapped to two of the object’s blocks.

Figure 1. Structure of a Data Object