Structure of the format-1 global areas

Format-1 global areas 1, 2, and 3 are called GL1, GL2, and GL3 in the following text.

Figure 1 represents the layout of the format-1 global area for a base-only system with a single I-stream; that is, a system with no other subsystems and residing in a uniprocessor.

Storage protection limits access to one or more storage locations by preventing writing or reading, or both. A storage protection key is an indicator associated with one or more areas of storage. An entry must have a matching key to use a particular area of storage; therefore, an area of storage with a key that is different from an active entry is protected.

The smallest segment of storage affected by a protection key is 4096 bytes (4 KB). Each format-1 global area, therefore, starts on a 4-KB boundary and is a multiple of 4 KB.

GL2 is assigned the same storage protection key as the active entry control block (ECB). GL1 and GL3 are assigned a different key so that the ECB must issue a macro before any updates can be performed. After the ECB has completed an update, a second macro must be issued to return the key to its original setting. These two macros, GLMOD and FILKW, are discussed in Using the format-1 global area by applications.