History of IBM CKD Controllers
| Industry | Computer hardware |
|---|---|
| Founded | April 7, 1964 |
| Products | Storage Control Units (SCUs), Directors, Integrated File Adapters |
| Website | www |
Starting with the 1964 introduction of the System/360, IBM's mainframe computers accessed count key data (CKD) subsystems through channels linked to distinct Storage Control Units (SCUs), each connected to Direct Access Storage Devices (DASD), commonly hard disk drives. This architecture persisted in IBM's high-end mainframes up to the IBM Z series. Conversely, entry-level models employed integrated attachments that combined SCU and channel functionalities, referred to as Integrated File Adapters.
The System/360 selector channel was later replaced by the System/370 block multiplexor channel, which retained compatibility by functioning as a selector channel for older subsystems.
Over time, the SCU transformed into Directors and Controllers, often known as "A-units" or "A-Boxes," each containing a controller alongside at least one DASD. Within the IBM System cabinet, an Integrated Storage Control (ISC) serves as a Director. A single Director could support from one up to four A-units, while multiple standard DASD units, designated "B-units," could be connected to an A-unit.
The sections below provide a chronological listing of IBM mainframe CKD storage controls organized by announcement date. These are grouped into conventional storage controls, director-type controls, and integrated controls that connect an A-unit.
Chronological summary
| Controller Manual |
Announcement Date |
Notes | |
|---|---|---|---|
| 2820 SCU | A22-6895 | 4/7/1964 | exclusively used with the IBM 2301 drum |
| 2841 SCU | A26-5988-0 | 4/7/1964 | |
| 2314 DASF | A26-3599 | 4/23/1965 | SCU bundled with nine drives, similar design to 2841 |
| System/360 Model 25 DAC | A24-3510 | 1/4/1968 | |
| 2314-A1 SCU | A26-3599 | circa 9/1969 | Unbundled version of 2314 DASF; SCU resembles 2841 |
| 2844 Aux SCU with 2314 | A26-3599 | circa 1968 | offers dual path functionality, comparable to 2841 |
| 2835-1,2 SCUs | GA26-1589 | 1/28/1970 | resembles 3830-1 with parallel read channels |
| 3830-1 SCU | GA26-1592 | 6/30/1970 | |
| System/370 Model 145 IFA | A26-3599 | 9/23/1970 | |
| 2314-B1 SCU | A26-3599 | 12/14/1970 | Designed for 2319B DASD, similar to 2841 |
| 3830-2 SCU | GA26-1617 | Aug 1972 | single director configuration |
| Integrated Storage Control Unit | GA26-1620 | 8/2/1972 | a repackaged version of the 3830-2 |
| S/370 125-0, -2 3330/3333 Direct Disk Attachment | 10/4/1972 | ||
| S/370 115-0, -2, 125-0, -2 3340/3344 Direct Disk Attachment | GA33-1506 | 3/17/1973 | |
| 3830-3 SCU and Integrated Storage Controller with Staging Adapter |
GA32-0036 | 10/9/1974 | single director; utilized with the 3850 MSS |
| S/370 138 Integrated File Adapter | GA24-3632 | 6/30/1976 | |
| 4321/4331 DASD Adapter for 3340/3344 | GA33-1526 | 1/30/1979 | |
| 3880-1, 2, 3 SCU | GA26-1661 | 6/1/1980 | features dual directors |
| 3880-11, 13 SCU | GA32-0061 | 10/21/1981 | equipped with dual directors and large cache memory |
| 3880-4 SCU | GA26-1661 | 1983 | included for reference only; not a CKD director |
| 4361 DASD Adapter for 3340/3344 | GA33-1566 | 9/10/1984 | |
| 3880-21, 23 SCU | GA32-0081 | 10/18/1984 | dual directors with extensive cache |
| 3380-CJ2 Direct Channel Attached DASD | GC26-4497 | 9/1/1987 | combines an SCU and two DASDs in one unit; additional DASDs can be connected |
| 3990-1,2,3 SCUs | GA32-0099 | 11/14/1989 | dual directors; model 3 includes cache memory |
| 3990-6 SCU | GA32-0274 | 5/19/1993 | features dual caching directors |
Conventional storage controls
A Storage Control Unit (SCU), often referred to simply as a storage control, interfaces with a System/360 or System/370 channel and administers one or more logically distinct devices. Both the control unit and the attached devices adhere to the System/360/370 Input/Output architecture, which permits either component to be busy or report status independently during I/O processes.
Traditional DASD storage controls feature proprietary interfaces tailored to specific DASD models or series. In 1972, IBM introduced a standardized interface between DASD and control units, utilized by Director type storage controls.
2820
The IBM 2820 Storage Control Unit connects to one or two selector channels and supports from one up to four 2301 Drum Storage devices. While conceptually resembling the IBM 2841, it differs by employing parallel recording channels and does not support the same device types as the 2841.
2841
The IBM 2841 Storage Control Unit was introduced in 1965 as a microprogrammed DASD controller for the IBM System/360 and remained available through at least 1975.
Its primary function was to manage disk, strip file, or slower drum storage access. The 2841 connected to one or two standard System/360 channels and could also interface with an IBM 1130 or an IBM 1800 Data Acquisition and Control System to support 2311 disks.
Originally, the 2841 accommodated up to eight DASD units from these types:
- IBM 2302 Disk Storage, Models 3 and 4 (maximum of four Model 3 or two Model 4)
- IBM 2311 Disk Storage, Model 1 (up to eight)
- IBM 2321 Data Cell (up to eight)
- IBM 7320 Drum Storage (up to eight; discontinued in 1965)
- IBM 2303 Drum Storage (up to two)
A typical setup included a single 2841 SCU controlling up to eight 2311 DASD devices. Mixed device configurations were possible—for instance, combining two 2303 Drum Storages with as many as six additional 2321 Data Cells.
Notable features encompassed a two-channel switch, file scanning capabilities, and record overflow handling.
Subsequent DASD models employed different storage controls, which are discussed in later sections.
2314 family
The IBM 2314 series of SCUs and DASDs connect via one or two selector channels and support between one and nine drives—commonly eight operational drives plus one reserved for service and maintenance. Although model numbers reflect packaging variations, their operational functions are largely consistent. Prominent models include:
- 2314 Direct Access Storage Facility – Model l: Integrates an SCU with nine drives configured as two modules of four drives each plus one single-drive module.
- 2314 Storage Control Unit Model A1 – Initially part of an A Series DASF but later offered separately; supports from one up to nine drives including the 2312, 2313, and 2318 Disk Storage units.
- 2314 Storage Control Unit Model B1 – Designed for the B series; connects a three-drive 2319-B1 Disk Storage along with an option for one or two additional three-drive 2319-B2 Disk Storages.
- 2844 Auxiliary Storage Control – Functions as a second 2314 SCU attached to a group of 2314 DASD units, allowing concurrent access to any two connected DASDs.
2835
The 2835 Storage Control interfaces with one or two block multiplexer channels and connects to one or two 2305 Fixed Head storage modules.
3830 Model 1
The 3830 Model 1 Storage Control Unit supports connection to as many as four channels and accommodates between one and eight 3330 DASD spindles.
3380 Model CJ2
The 3380 Model CJ2 combines the storage capabilities of a single 3380 DASD (comprising two devices) with the control features of a 3990 Model I into a unified unit. It links to two, four, or eight block multiplexer channels. Additionally, it can manage up to three 3380 'B' units (models BJ4 or BK4), enabling control over as many as 14 devices as the primary controller for the storage facility.
Director type storage controls
IBM’s Storage Control Units evolved into Directors and Controllers, where controllers were commonly packaged as "A-units," integrating both the controller and DASD. Storage devices lacking controller functionality were designated as "B-units."
Originally, these terms corresponded to product type numbers—for example, 3333 denoted an A-unit paired with 3330 DASD. The "A" and "B" suffixes were introduced with the 3340 DASD series (3340 Model A2 and B2). This naming convention was maintained for CKD DASD models such as the 3380 Model A4 (featuring one controller and four access mechanisms), AA4 (two controllers), and B4 (four access mechanisms).
Combining an A-unit with optional B-units forms what is known as a "string." Most A-units offer an optional "string switch," which allows the controller to alternate between two directors, providing additional pathways into the DASD string.
The 3830 Model 2 storage control and Integrated Storage Controls, introduced in August 1972, were pioneers in functioning as directors for A-units, allowing a single disk control unit to efficiently manage multiple drives.
3830 Model 2 and 3
The 3830-2 connects with one, two, or four block multiplexer channels and supports up to four A-units—any mix of 3333s, 3340-A2s, or 3350-A2/A2Fs—with a maximum of 32 physical drives (plus up to six additional drives per A-unit).
The Integrated Storage Control designed for System/370 Models 158 and 168 operates identically to the 3830-2.
The 3830-3 supports up to four 3333s and 3350-A2/A2F/C2/C2Fs but also incorporates features for the 3850 Mass Storage System. However, the 3350 drives are not capable of serving as staging drives in native mode.
The Integrated Storage Controller equipped with a Staging Adapter functions similarly to the 3830-3.
Conventional directors
The IBM 3880 Storage Control Models 1, 2, and 3 each include two directors per cabinet, with each director connecting to one, two, four, or eight block multiplexer channels. In contrast, Model 4 contains only one director and supports attachment exclusively to strings composed of 3370 (FBA) and 3375 (CKD) devices.
Each director is restricted to particular device types and can be configured via diskettes to attach solely one supported string type. Strings start with an A-unit followed by compatible B-units; notably, the final device in a string may be a 3350-C2. Field upgrades are available between Models 1, 2, and 3. Directors can connect up to four mixed A-units comprising either 3330 or 3350 units, four 3340 A-units, or two 3380 A-units.
Directors with caching capabilities
The 3380 Storage Control Models 11, 13, 21, and 23 each incorporate two directors, which connect to one, two, or four block multiplexer channels.
Models 13 and 23 introduced large semiconductor buffers known as "subsystem storage" for caching purposes, whereas Models 11 and 21 utilized these buffers primarily for paging functions.
The concept of caching within semiconductor buffers was initially implemented in DASD CKD subsystems by Memorex in 1978 and later by StorageTek in 1981.
In Model 13, two caching directors access the subsystem storage—a primary cache for frequently used data and a smaller directory cache to track cached information. Each caching director connects to one or two 3380 A-units, with each A-unit supporting up to three B-units, allowing a maximum of 16 devices. Considering dual actuators per DASD device, Model 13 can address up to 32 devices. Model 23 expanded the cache capacity and enhanced performance while preserving similar operational characteristics.
Paging has been a well-established feature in CKD systems, with dedicated paging devices tracing back to S/360 drum units. StorageTek’s 4305 (1978) was the first CKD device utilizing semiconductor memory specifically for paging.
Model 11 consists of two directors: director 1 manages subsystem storage dedicated to paging and connects one string with up to eight 3350 DASDs; director 2 controls nonpaging devices, attaching between one and four mixed A-units (3330 or 3350), each supporting up to three B-units—totaling up to 32 DASDs.
IBM 3990 series
Every model in the 3990 family features dual directors, each interfacing with four or eight block multiplexer channels. Notable attributes include:
- Multiple paths to A-units: Unlike earlier designs with a single path, Models 2, 3, and 6 of the 3390 provide dual access paths to A-units.
- Supported DASD A-units: The Model 1 supports up to four 3380 A-units; Models 2, 3, and 6 accommodate a combination of up to four A-units from the 3380 and 3390 series.
- Caching capabilities: Models 3 and 6 incorporate one director equipped with cache memory while the other utilizes nonvolatile storage.
The Model 1 can be upgraded in the field to either Model 2 or Model 3. Model 6 enhances reliability, availability, and serviceability but offers functionality equivalent to Model 3.
System/360 Model 25 integrated disk attachment
Introduced on January 4, 1968, the System/360 Model 25 featured the "Disk Attachment Control" (DAC), which supports up to four attached 2311 DASD units. This DAC combines channel and storage control functions into a single unit between the CPU and disk drives, removing the need for separate components. Performance and compatibility with the 2311 remain consistent whether connected through the DAC or via a conventional storage control unit.
2319A integrated attachment
On September 23, 1970, IBM released the System/370 Model 145 equipped with an "Integrated File Adapter" paired with the newly introduced 2319-A1 DASD. Support for this adapter was later extended to the System/370 Model 135 in March of 1981.
The 2319-A1 integrated three disk drives similar to those in the 2314 series along with built-in storage control functionality. It also supported traditional DASD models like the 2312, 2313, and 2318. However, since it lacked compatibility with director-based storage controls, it was not designated as an "A-unit" within IBM's classification system.
S/370 125-0 and 125-2
These units enable direct linkage to the DASD subsystem, including the (3125-2 exclusively), with support for as many as 16 spindles depending on the setup. The 3340 model A2 and its drives may be shared among S/370 systems (excluding models 3115-0 or ) through the String Switch (#9315) functionality.
S/370 115-0
This model allows direct connection to either the 3340 DASF Subsystem or the DAS Subsystem (available only on 3115-2). Depending on the specific model and installed options, it supports up to eight drives. The string switch feature (#9315) of the 3340 DASF Subsystem permits sharing of the 3340 Model A2 with another S/370 system, except for and .
S/370 138 Integrated File Adapter
The Integrated File Adapter of the S/370 Model 138 supports connection of one or two 3330 or 3340 A-units. Each A-unit can accommodate up to three B-units; when using 3344 B-units, these must be placed exclusively in the first string, permitting a maximum combination of three 3340 and/or 3344 B-units.
4321/4331 DASD Adapter for 3340/3344
Depending on the version, one or two DASD adapters connect one 3340 A-unit along with up to three additional 3340 or 3344 B-units, allowing for a total of eight devices. The string switch feature on the 3340 A-unit facilitates sharing of DASD with other IBM processors or directors that support this capability.
4361 DASD Adapter for 3340/3344
An optional DASD Adapter supports up to four 3340 A-units, each capable of attaching one to three B-units (either 3340 or 3344), resulting in a total capacity of 32 DASD units across four strings of eight devices each. The string switch function enables sharing with other IBM processors or directors compatible with this technology.