Introduction to the New Mainframe: Large-Scale Commercial Computing

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.