28 Introduction to the New Mainframe: Large-Scale Commercial Computing
compatibility with the previous architecture, for example z/VSE™ runs in 2
31
addressability); the common areas extended above the line.
The latest advance has been the introduction of addressability to 2
64
and the
topmost 31-bit address became referred to as the
bar. The 24-bit and 31-bit
architectures allow both program instructions and data to be in any part of the
storage.
In 64-bit addressing, however, only data can reside above the bar. The rationale
for this is to allow more data to be held in main storage, which will decrease the
time to access it, and therefore increase the capacity of the system.
This may appear to restrict the combined size of programs in the address space
to somewhat less than 2 gigabytes. However, the architecture also allows for
programs to be executed, subject to security rules, in other address spaces,
which lifts this restriction.
2.7.4 Real storage management
main storage, in this context referred to as real storage, is managed by a Real
Storage Manager (RSM™) in elements of 4 kilobytes, known as
frames. Each
frame of real storage holds a page of virtual storage.
Real storage is not directly addressed by application programs. RSM uses tables
to establish a link between a virtual address and a real address. A hardware
mechanism known as Dynamic Address Translation (DAT) converts the virtual
addresses seen by a program into real addresses that the CPs can access.
The majority of programs have sections that are frequently executed and have
data objects that they refer to on a regular basis. The RSM, in common with other
systems, tries to keep as many of the pages of programs and data that have
been used in storage, as a performance aid. Eventually, all of real storage will be
full of virtual pages. To accommodate new ones, the pages that have not been
used recently are copied to a disk file. This is referred to as a
page-out, and if the
virtual page is referred to at a later time, it will be paged in. Avoiding page-in and
page-out activity aids the performance of a program and it is therefore a capacity
requirement to have sufficient real storage to achieve this.
2.8 Architecture, running work, and capacity
A z/OS system is capable of driving the servers’ processors at 100% for
sustained periods of time. This is not difficult on any system if there is sufficient
resources for workload. The problem is to make sure that the high-importance
units of work achieve their objectives.

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