Conceptually, mainframes and all other computers have two types of physical storage: Internal and external.

• Physical storage located on the mainframe processor itself. This is called processor storage, real storage or central storage; think of it as memory for the mainframe.
• Physical storage external to the mainframe, including storage on direct access devices, such as disk drives and tape drives. This storage is called paging storage or auxiliary storage.

The primary difference between the two kinds of storage relates to the way in which it is accessed, as follows:

• Central storage is accessed synchronously with the processor. That is, the processor must wait while data is retrieved from central storage.
• Auxiliary storage is accessed asynchronously. The processor accesses auxiliary storage through an input/output (I/O) request, which is scheduled to run amid other work requests in the system. During an I/O request, the processor is free to execute other, unrelated work.

As with memory for a personal computer, mainframe central storage is tightly coupled with the processor itself, whereas mainframe auxiliary storage is located on (comparatively) slower, external disk and tape drives. Because central storage is more closely integrated with the processor, it takes the processor much less time to access data from central storage than from auxiliary storage. Auxiliary storage, however, is less expensive than central storage. Most z/OS® installations use large amounts of both.