A single System z9® mainframe can have up to 1024 individual channels for input and output (I/O) connectivity. This capacity is one factor that contributes to the mainframe's legendary scalability.

Although less complex than a real system with more channels and I/O devices, Figure 1 illustrates key concepts related to I/O configurations and capacity.

Figure 1. Recent system configuration

Briefly, partitions create separate logical machines in the central processor complex (CPC). ESCON® and FICON® channels are logically similar to parallel channels but they use fiber connections and operate much faster. A modern system might have 100-200 channels or channel path identifiers (CHPIDs). Key concepts partly illustrated here include the following:

• ESCON and FICON channels connect to only one device or one port on a switch.
• Most modern mainframes use switches between the channels and the control units. The switches may be connected to several systems, sharing the control units and some or all of its I/O devices across all the systems.
• CHPID addresses are two hexadecimal digits.
• Multiple partitions can sometimes share CHPIDs. Whether this is possible depends on the nature of the control units used through the CHPIDs. In general, CHPIDs used for disks can be shared.
• An I/O subsystem layer exists between the operating systems in partitions (or in the basic machine if partitions are not used) and the CHPIDs.

An ESCON director or FICON switch is a sophisticated device that can sustain high data rates through many connections. (A large director might have 200 connections, for example, and all of these can be passing data at the same time.) The director or switch must keep track of which CHPID (and partition) initiated which I/O operation so that data and status information is returned to the right place. Multiple I/O requests, from multiple CHPIDs attached to multiple partitions on multiple systems, can be in progress through a single control unit.

The I/O control layer uses a control file known as an IOCDS (I/O Control Data Set) that translates physical I/O addresses (composed of CHPID numbers, switch port numbers, control unit addresses, and unit addresses) into device numbers that are used by the operating system software to access devices. This is loaded into the Hardware Save Area (HSA) at power-on and can be modified dynamically. A device number looks like the addresses for early S/360™ machines except that it can contain three or four hexadecimal digits.

Many users still refer to these as "addresses" although the device numbers are arbitrary numbers between x'0000' and x'FFFF'. Today's mainframes have two layers of I/O address translations between the real I/O elements and the operating system software. The second layer was added to make migration to newer systems easier.

Modern control units, especially for disks, often have multiple channel (or switch) connections and multiple connections to their devices, as shown in Figure 2. They can handle multiple data transfers at the same time on the multiple channels. Each device will have a unit control block (UCB) in each z/OS® image.

Figure 2. Device addressing