Early mainframe designs help explain the terminology that still permeates mainframe discussions.

Figure 1 presents a conceptual diagram of an early mainframe design: an S/360™ system.

Figure 1. Conceptual S/360

The central processor contains the processors, memory, control circuits, and interfaces for channels. A channel provides an independent data and control path between I/O devices and memory. Early systems had up to 16 channels; the today's largest mainframe machines can have over 1000 channels.

Channels connect to control units. A control unit contains logic to work with a particular type of I/O device. For example, a control unit for a printer would have much different internal circuitry and logic than a control unit for a tape drive. Some control units can have multiple channel connections providing multiple paths to the control unit and its devices.

Control units connect to devices, such as disk drives, tape drives, communication interfaces, and so forth. The division of circuitry and logic between a control unit and its devices is not defined, but it is usually more economical to place most of the circuitry in the control unit.

The channels in Figure 1 are parallel channels (also known as bus and tag channels, named for the two heavy copper cables they use). A parallel channel can be connected to a maximum of eight control units. Most control units can be connected to multiple devices; the maximum depends on the particular control unit, but 16 is a typical number.

The disk drive marked with a Y in the figure can be addressed as 171, 571, or 671 because it is connected through three channels. By convention the device is known by its lowest address (171), but all three addresses could be used by the operating system to access the disk drive. Multiple paths to a device are useful for performance and for availability. When an application wants to access disk 171, the operating system will first try channel 1. If it is busy (or not available), it will try channel 5, and so forth.

Figure 1 contains another S/360 system with two channels connected to control units used by the first system. This sharing of I/O devices is common in all mainframe installations. Tape drive Z is address A31 for the first system, but is address 331 for the second system. Sharing devices, especially disk drives, is not a simple topic and there are hardware and software techniques used by the operating system to control exposures such as updating the same disk data at the same time from two independent systems.

Today's mainframe designs are more complex than shown in Figure 1; differences include:

• Parallel channels are not available on the newest mainframes and are slowly being displaced on older systems.
• Parallel channels have been replaced with ESCON® (Enterprise Systems CONnection) and FICON® (FIber CONnection) channels. These channels connect to only one control unit or, more likely, are connected to a director (switch) and are optical fibers.
• Current mainframes have more than 16 channels and use two hexadecimal digits as the channel portion of an address.
• Channels are generally known as CHPIDs (channel path identifiers) or PCHIDs (physical channel identifiers) on later systems, although the term channel is also correct. The channels are all integrated in the main processor box.

The device address seen by software is more correctly known as a device number (although the term address is still widely used) and is indirectly related to the control unit and device addresses.