Mainframe Architecture

A mainframe is a high-performance computer made for high-volume, processor-intensive computing. Mainframes were the first commercially available computers. They were produced by vendors like IBM, Unisys, Hitachi, Bull, Fujitsu, and NEC. But IBM always was the largest vendor – it still has 90% market share in the mainframe market.

Mainframes used to have no interactive user interface. Instead, they ran batch processes, using punched cards, paper tape, and magnetic tape as input, and produced printed paper as output. In the early 1970s, most mainframes got interactive user interfaces, based on terminals, simultaneously serving hundreds of users.

While the end of the mainframe is predicted for decades now, mainframes are still widely used. Today’s mainframes are still relatively large (the size of a few 19" racks), but they don’t fill-up a room anymore. They are expensive computers, mostly used for administrative processes, optimized for handling high volumes of data.

The latest IBM z13 mainframe, introduced in 2015, can host up to 10TB of memory and 141 processors, running at a 5GHz clock speed. It has enough resources to run up to 8000 virtual servers simultaneously.


Mainframes are highly reliable, typically running for years without downtime. Much redundancy is built in, enabling hardware upgrades and repairs while the mainframe is operating without downtime. Sometimes a separate system is added to the mainframe which primary job it is to continuously check the mainframe’s health. When a hardware failure is detected, automatically an IBM engineer is called, sometimes even without the systems managers knowing it!

All IBM mainframes are backwards compatible with older mainframes. For instance, the 64 bits mainframes of today can still run the 24-bit System/360 code from the early days of mainframe computing. Much effort is spent in ensuring all software continues to work without modification.

Mainframe architecture
A mainframe consists of processing units (PUs), memory, I/O channels, control units, and devices, all placed in racks (frames). The architecture of a mainframe is shown below.


The various parts of the architecture are described below.

Processing Units
In the mainframe world the term PU (Processing Unit) is used instead of the more ambiguous term CPU. A mainframe has multiple PUs, so there is no central processing unit. The total of all PUs in a mainframe is called a Central Processor Complex (CPC).

The CPC resides in its own cage inside the mainframe, and consists of one to four so-called book packages. Each book package consists of processors, memory, and I/O connections, much like x86 system boards.

Mainframes use specialized PUs (like the quad core z10 mainframe processor) instead of off-the-shelf Intel or AMD supplied CPUs.

All processors in the CPC start as equivalent processor units (PUs). Each processor is characterized during installation or at a later time, sometimes because of a specific task the processor is configured to do. Some examples of characterizations are:

Processor unit (PU) Task
Central processors (CP) Central processors are the main processors of the system that can be used to run applications running on VM, z/OS, and ESA/390 operating systems.
CP Assist for Cryptographic Function (CPACF) CPACF assists the CPs by handling workload associated with encryption/decryption.
Integrated Facility for Linux (IFL) IFL assists with Linux workloads: they are regular PUs with a few specific instructions that are needed by Linux.
Integrated Coupling Facility (ICF) This facility executes licensed internal code to coordinate system tasks.
System Assisted Processor (SAP) A SAP assists the CP with workload for the I/O subsystem, for instance by translating logical channel paths to physical paths.
IBM System z Application Assist Processors (zAAP) Used for Java code execution
zIIP Processing certain database workloads
Spares Used to replace any CP or SAP failure

Main Storage
Each book package in the CPC cage contains from four to eight memory cards. For example, a fully loaded z9 mainframe has four book packages that can provide up to 512 GB of memory.

The memory cards are hot swappable, which means that you can add or remove a memory card without powering down the mainframe.

Channels, ESCON and FICON
A channel provides a data and control path between I/O devices and memory.

Today’s largest mainframes have 1024 channels. Channels connect to control units, either directly or via switches. Specific slots in the I/O cages are reserved for specific types of channels, which include the following:

  • Open Systems Adapter (OSA) – this adapter provides connectivity to various industry standard networking technologies, including Ethernet
  • Fiber Connection (FICON) - this is the most flexible channel technology. With FICON, input/output devices can be located many kilometers from the mainframe to which they are attached.
  • Enterprise Systems Connection (ESCON) - this is an earlier type of fiber-optic technology. ESCON channels can provide performance almost as fast as FICON channels, but at a shorter distance.

The FICON or ESCON switches may be connected to several mainframes, sharing the control units and I/O devices.

The channels are high speed – today’s FICON Express16S channels provide up to 320 links of 16 Gbit/s each.

Control units
A control unit is similar to an expansion card in an x86 or midrange system. It contains logic to work with a particular type of I/O device, like a printer or a tape drive.

Some control units can have multiple channel connections providing multiple paths to the control unit and its devices, increasing performance and availability.

Control units can be connected to multiple mainframes, creating shared I/O systems. Sharing devices, especially disk drives, is complicated and there are hardware and software techniques used by the operating system to control updating the same disk data at the same time from two independent systems.

Control units connect to devices, like disk drives, tape drives, and communication interfaces. Disks in mainframes are called DASD (Direct Attached Storage Device), which is comparable to a SAN (Storage Area Network) in a midrange or x86 environment.

This entry was posted on Vrijdag 04 September 2015

Earlier articles

Quantum computing

My Book

Security bij cloudproviders wordt niet beter door overheidsregulering

Passend Europees cloudinitiatief nog ver weg

Data Nederlandse studenten in cloud niet grootschalig toegankelijk voor bedrijven VS

VS kan nog steeds Europese data Microsoft opeisen ondanks nieuwe regels

The cloud is as insecure as its configuration

Infrastructure as code

DevOps for infrastructure

Infrastructure as a Service (IaaS)

(Hyper) Converged Infrastructure

Object storage

Software Defined Networking (SDN) and Network Function Virtualization (NFV)

Software Defined Storage (SDS)

What's the point of using Docker containers?

Identity and Access Management

Using user profiles to determine infrastructure load

Public wireless networks

Supercomputer architecture

Desktop virtualization

Stakeholder management

x86 platform architecture

Midrange systems architecture

Mainframe Architecture

Software Defined Data Center - SDDC

The Virtualization Model

What are concurrent users?

Performance and availability monitoring in levels

UX/UI has no business rules

Technical debt: a time related issue

Solution shaping workshops

Architecture life cycle

Project managers and architects

Using ArchiMate for describing infrastructures

Kruchten’s 4+1 views for solution architecture

The SEI stack of solution architecture frameworks

TOGAF and infrastructure architecture

The Zachman framework

An introduction to architecture frameworks

How to handle a Distributed Denial of Service (DDoS) attack

Architecture Principles

Views and viewpoints explained

Stakeholders and their concerns

Skills of a solution architect architect

Solution architects versus enterprise architects

Definition of IT Architecture

What is Big Data?

How to make your IT "Greener"

What is Cloud computing and IaaS?

Purchasing of IT infrastructure technologies and services

IDS/IPS systems

IP Protocol (IPv4) classes and subnets

Introduction to Bring Your Own Device (BYOD)

IT Infrastructure Architecture model

Fire prevention in the datacenter

Where to build your datacenter

Availability - Fall-back, hot site, warm site

Reliabilty of infrastructure components

Human factors in availability of systems

Business Continuity Management (BCM) and Disaster Recovery Plan (DRP)

Performance - Design for use

Performance concepts - Load balancing

Performance concepts - Scaling

Performance concept - Caching

Perceived performance

Ethical hacking

Computer crime

Introduction to Cryptography

Introduction to Risk management

The history of UNIX and Linux

The history of Microsoft Windows

Engelse woorden in het Nederlands

Infosecurity beurs 2010

The history of Storage

The history of Networking

The first computers

Cloud: waar staat mijn data?

Tips voor het behalen van uw ITAC / Open CA certificaat

Ervaringen met het bestuderen van TOGAF

De beveiliging van uw data in de cloud

Proof of concept

Een consistente back-up? Nergens voor nodig.

Measuring Enterprise Architecture Maturity

The Long Tail

Open group ITAC /Open CA Certification

Human factors in security

Google outage

SAS 70

De Mythe van de Man-Maand

TOGAF 9 - wat is veranderd?

Landelijk Architectuur Congres LAC 2008

InfoSecurity beurs 2008

Spam is big business

De zeven eigenschappen van effectief leiderschap

Een ontmoeting met John Zachman

Persoonlijk Informatie Eigendom

Archivering data - more than backup

Sjaak Laan

Recommended links

Genootschap voor Informatie Architecten
Ruth Malan
Gaudi site
XR Magazine
Esther Barthel's site on virtualization
Eltjo Poort's site on architecture


XML: RSS Feed 
XML: Atom Feed 


The postings on this site are my opinions and do not necessarily represent CGI’s strategies, views or opinions.


Copyright Sjaak Laan