Conceptually, mainframes and all other computers have two types of physical storage:
– The physical storage located on the mainframe processor itself. This is called processor storage or real storage; think of it as memory for the mainframe.
– The physical storage external to the mainframe, including storage on direct access devices, such as disk drives and tape drives. This storage is called auxiliary storage.
The primary difference between the two kinds of storage relates to the way in which it is accessed, as follows:
– Real storage is accessed synchronously with the processor. That is, the processor must wait while data is retrieved from real 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 real storage is tightly coupled with the processor itself, whereas mainframe auxiliary storage is located on (comparatively) slower, external disk and tape drives.
Because real storage is more closely integrated with the processor, it takes the processor much less time to access data from real storage than from auxiliary storage. However, the processor is free to do other work while waiting for an I/O request to be satisfied.
Auxiliary storage is less expensive than real storage, so it provides the capability for many jobs to be running while keeping real storage costs down.