Unit Control Block

In IBM mainframe operating systems from the OS/360 and successors line, a Unit Control Block (UCB) is a memory structure, or a control block, that describes any single input/output peripheral device (unit), or an exposure (alias), to the operating system. Certain data within the UCB also instructs the Input/Output Supervisor (IOS) to use certain closed subroutines in addition to normal IOS processing for additional physical device control.

Some other operating systems have similar structures.


During initial program load (IPL) of current[lower-alpha 1] MVS systems, the Nucleus Initialization Program (NIP) reads necessary information from the I/O Definition File (IODF) and uses it to build the UCBs. The UCBs are stored in system-owned memory, in the Extended System Queue Area (ESQA). After IPL completes, UCBs are owned by Input/Output Support. Some of the information stored in the UCB are: device type (disk, tape, printer, terminal, etc.), address of the device (such as 1002), subchannel identifier and device number, channel path ID (CHPID) which defines the path to the device, for some devices the volume serial number (VOLSER), and a large amount of other information, including OS Job Management data.

While the contents of the UCB has changed as MVS evolved, the concept has not. It is a representation to the channel command processor of an external device. Inside every UCB is a representation of a subchannel information block, that is used in the SSCH assembler instruction (put in the IRB, for input, or put in the ORB, for output),[1] to start a chain of channel commands, known as CCWs. CCWs are queued onto the UCB with the STARTIO macro interface,[2] although that reference does NOT discuss the STARTIO macro as that macro instruction is NOT an IBM-supported interface, not withstanding the fact that that interface has remained the same for at least the past three decades. The STARTIO interface will either start the operation immediately, should the Channel Queue be empty, or it will queue the request on the Channel Queue for deferred execution. Such deferred execution will be initiated immediately when the request is at the head of the queue and the device becomes available, even if another program is in control at that instant. Such is the basic design of Input/Output Supervisor (IOS).

The UCB evolved to be an anchor to hold information and states about the device. The UCB currently has five areas used for an external interface: Device Class Extension, UCB Common Extension, UCB Prefix Stub, UCB Common Segment and the UCB Device Dependent Segment.[3] Other areas are internal use only. This information can be read and used to determine information about the device.

In the earliest implementations of this OS, the UCBs (foundations and extensions) were assembled during SYSGEN, and were located within the first 64 KB of the system area, as the I/O device lookup table consisted of 16-bit Q-type (i.e. relocatable) addresses. Subsequent enhancements allowed the extensions to be above the 64-kilobyte (65,536 bytes) line, thereby saving space for additional UCB foundations below the 64-kilobyte line and also thereby preserving the architecture of the UCB lookup table (converting a CUu to a UCB foundation address).

Handling parallel I/O operations

UCBs were introduced in the 1960s with OS/360. Then a device addressed by UCB was typically a moving head hard disk drive or a tape drive, with no internal cache. Without it, the device was usually grossly outperformed by the mainframe's channel processor. Hence, there was no reason to execute multiple input/output operations at the same time, as these would be impossible for a device to physically handle. In 1968 IBM introduced the 2305-1 and 2305-2 fixed-head disks, which had 8 exposures (alias addresses) per disk, and the OS/360 support provided a UCB per exposure in order to permit multiple concurrent channel programs. Similarly, later systems derived from OS/360 required an additional UCB for each allocated virtual volume in a 3850 Mass Storage System (MSS) and for each exposure on a 3880-11, 3880-13 and their successors.

Workload Manager and UCBs

When originally implemented, operating system had no real way to determine if a waiting I/O was more or less important than any other waiting I/Os. I/Os to a device were handled first in, first out. Workload Manager (WLM) was introduced in MVS/ESA 5.1. OS/390 added "smart" I/O queuing. It allowed the operating system, using information provided to WLM by the systems programmer, to determine which waiting I/Os were more, or less, important than other waiting I/Os. WLM would then, in a sense, move a waiting I/O further up, or down, in the queue so when the device was no longer busy, the most important waiting I/O would get the device next. WLM improved the I/O response to a device for the more important work being processed. However, there was still the limit of a single I/O to a single UCB/device at any one time.

Parallel Access Volumes (PAVs)

Since only one set of channel commands or I/O could be run at one time. This was fine in the 1960s when CPUs were slow and I/O could only be processed as fast as CPUs could process it. As systems matured and CPU speed greatly surpassed I/O input capacity, access to the device that was serialized at the UCB level became a serious bottleneck.

Parallel Access Volume (PAV) allow UCBs to clone themselves to allow multiple I/O to run simultaneously. With appropriate support by the DASD hardware, PAV provides support for more than one I/O to a single device at a time. To maintain backward compatibility, operations are still serialized below the UCB level. But PAV allows the definition of additional UCBs to the same logical device, each using an additional alias address. For example, a DASD device at base address 1000, could have alias addresses of 1001, 1002 and 1003. Each of these alias addresses would have their own UCB. Since there are now four UCBs to a single device, four concurrent I/Os are possible. Writes to the same extent, an area of the disk assigned to one contiguous area of a file, are still serialized, but other reads and writes occur simultaneously. The first version of PAV the disk controller assigns a PAV to a UCB. In the second version of PAV processing, Work Load Manager (WLM) reassigns a PAV to new UCBs from time to time. In the third version of PAV processing, with the DS8000 controller series, each I/O uses any available PAV with the UCB it needs.

The net effect of PAVs is to decrease the IOSQ time component of disk response time, often to zero. As of 2007, the only restrictions to PAV are the number of alias addresses, 255 per base address, and overall number of devices per logical control unit, 256 counting base plus aliases.

Static versus dynamic PAVs

There are two types of PAV alias addresses, static and dynamic. A static alias address is defined, in both DASD hardware and z/OS, to refer to a specific single base address. Dynamic means that the number of alias addresses assigned to a specific base address fluctuates based on need. The management of these dynamic aliases is left to WLM, running in goal mode (which is always the case with supported levels of z/OS). On most systems that implement PAV, there is usually a mixture of both PAV types. One, perhaps two, static aliases are defined for each base UCB and a bunch of dynamic aliases are defined for WLM to manage as it sees fit.

As WLM watches over the I/O activity in the system, WLM determines if there a high-importance workload is delayed due to high contention for a specific PAV-enabled device. Specifically, for a disk device, base and alias UCBs must be insufficient to eliminate IOS Queue time. If there is high contention, and WLM estimates doing so would help the workload achieve its goals more readily, it will try to move aliases from another base address to this device.

Another problem may be certain performance goals are not being met, as specified by WLM service classes. WLM will then look for alias UCBs that are processing work for less important tasks (service class), and if appropriate, WLM will re-associate aliases to the base addresses associated with the more important work.


WLM's actions in moving aliases from one disk device to another take a few seconds for the effects to be seen. For many situations this is not fast enough. HyperPAVs are much more responsive because they acquire a UCB from a pool for the duration of a single I/O operation, before returning it to the pool. There is no delay waiting for WLM to react.

Further, because with HyperPAV the UCB is acquired for only the duration of a single I/O, a smaller number of UCBs are required to service the same workload, compared to Dynamic PAVs. For large z/OS images UCBs can be a scarce resource. So HyperPAVs can provide some relief in this regard.

Other operating systems

A similar concept in Unix-like systems is a kernel's devinfo structure, addressed by a combination of major and minor number through a device node.

Digital's VMS operating system uses an identically named structure, the UCB, for similar purposes. A UCB is created for each I/O device. The data in the UCB includes the device's unit number (a part of the device name) and a listhead to which pending I/O requests may be queued. The UCB may have a device-driver defined extension in which the driver can keep driver-defined data that is instantiated for each device.[4]

The device object in the I/O subsystem of Windows NT-family operating systems is another very similar structure.


  1. In older systems, the UCBs were part of the Nucleus and were assembled during the SYSGEN process.


  1. "z/Architecture Principles of Operation". PubLibZ.Boulder.IBM.com. IBM. 2004-05-04. p. 14.3.9. Retrieved 2017-01-03.
  2. "MVS Programming: Authorized Assembler Services Reference" (PDF). PubLibZ.Boulder.IBM.com (12th ed.). IBM. September 2009. Retrieved 2017-01-03.
  3. "z/OS Release 11 MVS Data Areas" (PDF). PubLibZ.Boulder.IBM.com. IBM. 2009. Retrieved 2017-01-04.
  4. Goldenberg, Ruth; Saravanan, Sara (1994). OpenVMS AXP Internals and Data Structures. Digital Press. p. 753. ISBN 978-1555581206. The executive creates a unit control block (UCB) for each I/O device attached to the system.
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