History of IBM CKD Controllers

Beginning with its 1964 System/360 announcement, IBM's mainframes initially accessed CKD (Count key data) subsystems via a channel connected to separate Storage Control Units (SCUs) with attached Direct Access Storage Devices (DASD), typically a hard disk drive. This practice continued in IBM's larger mainframes thru System/370; however low end systems generally used lower cost integrated attachments where the function of the SCU was combined with that of the channel, typically called an Integrated File Adapter.

The System/360 selector channel was followed by the System/370 block multiplexor channel which could operate as a selector channel to allow attachment of legacy subsystems.

The SCU evolved into a Director and Controller, the latter typically labelled an "A-unit" (or A-Box") with the controller and at least one DASD physically in an A-unit. An Integrated Storage Control (ISC) is a Director within the cabinet of an IBM System. A Director could attach from one to four A-units. One or more conventional DASD, now labeled a "B-unit" could attach to an A-unit

The following sections list in order of announcement IBM mainframe CKD storage controls, categorized as conventional storage controls, director type storage controls and integrated controls attaching an A-unit.

Chronological summaryEdit

CKD storage controls in chronological order[1]
2820 SCU A22-6895[2] 4/7/1964 similar to 2841 with parallel read channels
2841 SCU A26-5988-0[3] 4/7/1964
2314 DASF A26-3599[4] 4/23/1965 Bundled SCU with 9 drives, similar to 2841
System 360 Model 25 DAC A24-3510[5] 1/4/1968
2314-A1 SCU A26-3599[4] 9/1969 est.[6] Unbundled 2314 DASF, SCU similar to 2841
2844 Aux SCU with 2314 A26-3599[4] 1968 est.[c] provides dual path, similar to 2841
2835-1,2 SCUs GA26-1589[7] 1/28/1970 similar to 3830-1 with parallel read channels
3830-1 SCU GA26-1592[8] 6/30/1970
System 370 Model 145 IFA A26-3599[4] 9/23/1970
2314-B1 SCU A26-3599[4] 12/14/1970 For 2319B DASD, similar to 2841
3830-2 SCU GA26-1617[9] Aug 1972 single director
Integrated Storage Control Unit GA26-1620 8/2/1972 A repackaged 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, used with 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[10] 6/1/1980 dual director
3880-11, 13 SCU GA32-0061[11] 10/21/1981 dual directors, large cache
3880-4 SCU GA26-1661[10] 1983 For reference only, NOT a CKD director
4361 DASD Adapter for 3340/3344 GA33-1566 9/10/1984
3880-21, 23 SCU GA32-0081[12] 10/18/1984 dual directors, large cache
3380-CJ2 Direct Chanel Attached DASD GC26-4497 9/1/1987 An SCU and two DASD in one unit; additional DASD can be attached
3990-1,2,3 SCUs GA32-0099 11/14/1989 dual directors, model 3 has cache
3990-6 SCU GA32-0274 5/19/1993 dual caching directors

Conventional storage controlsEdit

A storage control, frequently called a Storage Control Unit, attaches to a System/360 (or System/370) channel one or more logically separate devices. The control unit and the device each comporting to System/360 and/or System/370 Input/Output architecture. Since the control unit and attached devices are logically distinct either or both can be busy when an I/O is initiated and either can present ending status when it becomes free.[13]

Conventional DASD storage controls listed in this section have different interfaces to the attached DASD, unique to a DASD or DASD family. In 1972, IBM introduced a standard DASD to control interface used with Director type storage controls.[14]


The IBM 2820 Storage Control Unit (SCU) connects to one or two selector channels and can attach from one to four 2301 Drum Storage units.[2]


The IBM 2841 Storage Control Unit[3][15] was a Direct Access Storage Device (DASD) control unit introduced with System/360, first shipped in 1965 and offered until at least 1975.

The 2841 was a microprogrammed[16] control unit "intended for use in controlling access to a disk or strip file or a slow-speed drum storage unit." It connected to one or two standard System/360 channels,[17] or could also be attached to an IBM 1130 or IBM 1800 Data Acquisition and Control System to add support for 2311 disks.

The IBM 2841 Storage Control Unit was initially capable of attaching combinations of up to eight of following DASD:

The standard offering was a 2841 SCU with a number of 2311 DASD up to eight.[15] With less than eight units of a single type other units can be intermixed, for example a 2841 with two 2303 Drum Storages supported up to six additional 2321 Data Cells.

Features of the 2841 included two-channel switch, file scan, and record overflow special features.[18]

DASD devices introduced after the 2841 used different storage controls as discussed in the following sections.

2314 familyEdit

In IBM 2314 family of SCUs and DASDs, the SCUs connect to one or two selector channels and can attach from one to nine drives, at most eight online plus one optional service drive which is accessible from the SCU for maintenance purposes. The model numbers reflect packaging differences but otherwise all versions of the 2314 family are functionally the same. The models are:

  • 2314 Direct Access Storage Facility - Model l: A bundle of an SCU and nine drives, two four drive modules and one one drive module.
  • 2314 Storage Control Unit Model A1 - This SCU was announced initially as a part of an A Series DASF and shortly thereafter unbundled. The unbundled DASD models were the one drive 2312 Disk Storage, the four drive 2313 Disk Storage and the two drive 2318 Disk Storage. One to nine drives could be attached as in the Model A1 DASF.
  • 2314 Storage Control Unit Model B1 - Part of the B series, this SCU attaches a three drive 2319-B1 Disk Storage and optionally one or two additional, three drive 2319-B2 Disk Storages.
  • 2844 Auxiliary Storage Control - This SCU is a second 2314 SCU attached to one set of 2314 DASD allowing simultaneous access to any two of the attached DASD.[4]


The 2835 storage Control connects to one or two block multiplexer channels and attaches one or two 2305 Fixed Head storage Modules.[7]

3830 Model 1Edit

The 3830 Model 1 Storage Control Unit connects to one, two or four channels and can attach one to eight 3330 DASD spindles.[8]

3380 Model CJ2Edit

The 3380 Model CJ2 provides both the storage functions of one 3380 DASD (two devices) and the storage control functions of a 3990 model I (two paths to A-units) in a single unit. It connects to two, four or eight block multiplexor channels. Up to three 3380 'B' units (models BJ4 or BK4 only) can be attached to the 3380 Model CJ2; providing the head-of-storage-facility function for up to 14 devices.[19]

Director type storage controlsEdit

The IBM Storage Control Unit evolved into a Director and Controller, the latter typically packaged and labelled as an "A-unit" (or "A-Box") with the controller and at least one DASD physically in an A-unit. A DASD unit without controller then became known as a "B-unit".

The terms "A-unit" (or "A-Box") and "B-unit" were not initially used, instead units containing the controller and DASDs were given product type numbers, specifically 3333 for the A-unit containing 3330 DASD and 3333-11 for the A-unit containing two double capacity 3330-11 DASDs. The use of the suffixes "A" and "B" in the model number to designate an A-unit and B-units began with the 3340 DASD with the 3340 Model A2 and 3340 B2, respectively. This practice of using the digits of a suffix to a DASD Model number to represent the type of unit and the number of DASD in the unit continued for all subsequent CKD DASD, for example, the 3380 Model A4 has one controller and four access mechanisms in one unit, the 3380 Model AA4 has two controllers and four access mechanisms in one unit while the 3380 Model B4 has four access mechanisms.

The combination of an A-unit with one or more optional B-units is a "string." "String switch" is an optional feature on most A-units that allows the controller in the A-unit to be switched between two directors, thereby providing additional paths into a string of DASD.

The 3830 Model 2 storage control and associated Integrated Storage Controls announced in August 1972[20] were the first instance of a storage control as a director for A-Units.[14]

3830 Model 2 and 3Edit

The 3830-2 connects to one, two or four block multiplexer channels and can attach any combination of up to four A-units, i.e., any combination of 3333s, 3340-A2s, or 3350-A2/A2Fs up to a maximum of 32 physical drives (up to 6 additional drives attached to each A-unit).[9]

The Integrated Storage Control for the System/370 Models 158 and 168 functions identically to the 3830-2.

The 3830-3 only attaches up to four 3333s and 3350-A2/A2F/C2/C2Fs but has additional functions to support the 3850 Mass Storage System.[21] The 3350 drives cannot be used as staging drives when in native mode.

The Integrated Storage Controller with Staging Adapter functions identically to the 3830-3.

3880 familyEdit

Conventional directorsEdit

The IBM 3880 Storage Control Models 1, 2 and 3 have two directors per cabinet. Each director can attach to one, two, four or eight block multiplexor channels. The Model 4 has one director and only attaches 3370 (FBA) and 3375 (CKD) strings.[10]

Each storage director is limited to specific devices.[10]: 1-2–1-3  Through use of diskettes, each storage director can be initialized to attach exclusively one of the supported types of strings. The first box on a string must be an A-unit, and the remaining boxes must be compatible B-units, or for the last 3350 in a 3350 string, a 3350-C2. IBM allowed field upgrades among Models 1. 2 and 3. Directors can attach up to four intermixed 3330 or 3350 A-units or four 3340 A-units or two 3380 A-units [10]

Caching directorsEdit

Each 3380 Storage Control, Model 11, Model 13, Model 21 and Model 23 has two directors; each director can attach to one, two or four block multiplexor channels.

Large semiconductor buffers, called subsystem storage were added to the 3380 Storage Controls in the Models 13[12] and 23 for caching and the Models 11[11] and 21 for paging.

Caching in semiconductor buffers was first introduced in DASD CKD subsystems by Memorex[22] (1978) and StorageTek[d] (1981).

The 3880 Model 13 has two caching storage directors that access subsystem storage; a larger portion of subsystem storage is the cache which is used to store active data for quick access; a smaller portion of the storage is the directory which is used to locate the data stored in the cache. The cache storage director attaches only one or two 3380 A-units each of which can in turn attach up to three 3380 B-units for a total of 16 devices. Because each 3380 DASD has two actuators the 3880 Model 13 can have up to 32 device addresses.[12] The Model 23 increased the cache size and somewhat improved performance but otherwise performed the same functions.[23]

Paging is well established in computer with dedicated CKD paging devices going back to the drums included in the S/360 announcement. The 1978 StorageTek 4305[24] was the first CKD device using semiconductor memory for paging.

The 3880 Model 11 has two storage directors, director 1 can access subsystem storage for paging and director 2 is for nonpaging. Director 1 attaches one string of up to eight 3350 DASDs. Director 2 attaches any combination of from one to four 3330 A-units or 3350 A-units, each A-unit attaching up to three additional B-units for up to 32 DASDs.[11]

3990 familyEdit

All 3990's are dual directors and each director can attach to four or eight block multiplexor channels. Other characteristics of specific models include:

  • Multiple A-unit paths: Prior to the 3390 all directors had a single path to associated A-units; each director in the 3390 Models 2, 3 and 6 has dual paths to A-units.
  • Attached DASD A-units: The 3990 Model 1 can attach up to four 3380 A-units. The 3390 Models 2, 3 and 6 can intermix up to four 3380 and 3390 A-units
  • Cache: In the Models 3 and 6 one director has a cache and one director has nonvolatile storage

The Model 1 can be field upgraded to a Model 2 or 3. The Model 6 has improved reliability, availability and serviceability but is otherwise functionally the same as the Model 3.[25]

System 360 Model 25 integrated attachmentEdit

On January 4, 1968, IBM announced the System 360 Model 25 which included the "Disk Attachment Control" (DAC) attaching up to four 2311 DASD. This integrated attachment simulates both a channel and a storage control unit between the CPU and attached 2311 units. By merging the functions the DAC eliminates the need for a channel and a separate storage control. There are no differences between a 2311 connected to a 2841 SCU and one connected to the DAC.[5]

2319A integrated attachmentEdit

On September 23, 1970 IBM announced as part of its System/370 Model 145[26] a new "Integrated File Adapter" along with a new 2319-A1 DASD. On March 8, 1981 support of the 2319-A1 was announced with an Integrated File Adapter as part of the announcement of the System/370 Model 135.[27]

The 2319-A1 contained three 2314 class disk drives plus a piece of the storage control from the system unit. Conventional 2314 type DASD, e.g., 2312, 2313 and/or 2318, could be attached to the 2319-A1.

Since it could not connect to director type storage control the 2319-A1 was not an "A-unit" as that term came to be used by IBM.

Integrated controls attaching A-unitsEdit

Direct disk attachmentsEdit

S/370 125-0 and -2Edit

Direct attachment of the 3333/3330 DASD Subsystem or the 3340 DASF Subsystem or the 3340/3344 DAS Subsystem (3125-2 only) is provided. Depending on model and features up to 16 spindles can be attached. In a 3340 Subsystem the 3340 mdl A2 and its attached 3340 drives can be shared with another S/370, except 3115-0 or 3125-0, via the String Switch capability (#9315).[28]

S/370 115-0Edit

Direct attachment of the 3340 DASF Subsystem or the 3340/3344 DAS Subsystem (3115-2 only) is provided. Depending on the Model and feature up to eight drives can be attached. In a 3340 DASF Subsystem, via the string switch feature (#9315), the 3340 Model A2 can be shared with another S/370, except 3115-0 and 3125-0.[28]

S/370 138 Integrated File AdapterEdit

One or two 3330 or 3340 A-units can be attached to the Integrated File Adapter of the S/370 Model 138. Up to three appropriate B-units may be attached to each A-unit. If 3344 B-units are installed, they may only installed in the first of the two possible strings with up to three 3340 and/or 3344 B-units in any combination.[29]

4321/4331 DASD Adapter for 3340/3344Edit

One or two DASD adapters are available (depending upon model) for attachment of one 3340 A-unit and with up to three additional 3340 or 3344 B-units providing up to eight devices. String Switch feature in the 3340 A-unit allows sharing of 3340/3344 DASD with another IBM processor or director that supports the A-units and string switching.[28]

4361 DASD Adapter for 3340/3344Edit

Up to four 3340 A-units can be attached to an optional DASD Adapter; to each 3340 A-unit can be attached from one to three 3340 B-units or 3344 B-units for a total of 32 DASD in four strings of eight. String Switch feature in the 3340 A-unit allows sharing of 3340/3344 DASD with another IBM processor or director that supports the A-units and string switching.[28]


  1. ^ Manual numbers are generic; links are to the latest version as indicated by the highest dash number found on line.
  2. ^ Unless otherwise noted dates are from IBM's Storage product profiles or Mainframes product profiles
  3. ^ This model number appears in Datamation’s December 1968 edition
  4. ^ STK 8890 CyberCache for STK 3350 compatibles


  1. ^ The tables within each section are extracted from IBM System/370 Reference Summary GX20-1850-7, IBM Corp., February 1989, with System/360 SCUs information added from reference manuals.
  2. ^ a b IBM System/360 Component Descriptions - 2820 Storage Control and 2301 Drum Storage (PDF). Third Edition. IBM. September 1968. A22-6895-2.
  3. ^ a b IBM System/360 Component Descriptions - 2841 Storage Control Unit, 2302 Disk Storage, Models 3 and 4, 2311 Disk Storage Drive, 2321 Data Cell Drive, Model 1, 7320 Drum Storage (PDF). First Edition. IBM. A26-5988-0.
  4. ^ a b c d e f IBM System/360 Component Descriptions 2314 Direct Access Storage Facility and 2844 Auxiliary Storage Control (PDF). Seventh Edition. IBM. November 1971. GA26-3599-6.
  5. ^ a b IBM System/360 Model 25 Functional Characteristics (PDF) (First ed.). IBM. January 1968. A24-3S10-0.
  6. ^ "multi-disc drive". Datamation: 206. September 1969. IBM is now offering from one to eight active disc drives for its 2314 direct access storage facility
  7. ^ a b Reference Manual for IBM 2835 Storage Control and IBM 2305 Fixed Head Storage Module (PDF). Third Edition. IBM. August 1971. GA26-1589-2.
  8. ^ a b Reference Manual for IBM 3830 Storage Control Model 1 and IBM 3330 Disk Storage (PDF). Sixth Edition. IBM. November 1976. GA26-1592-5.
  9. ^ a b Reference Manual for IBM 3830 Storage Control Model 2 (PDF). Sixth Edition. IBM. April 1977. p. 11. GA26-1592-5.
  10. ^ a b c d e IBM 3880 Storage Control Models 1, 2, 3 and 4 Description Manual (PDF) (Tenth ed.). IBM. September 1987. GA26-1661-9.
  11. ^ a b c IBM (March 1982). IBM 3880 Storage Control Model 11 Description (PDF) (First ed.). GA32-0061-0.
  12. ^ a b c IBM (June 1982). IBM 3880 Storage Control Model 13 Description (PDF) (First ed.). GA32-0067-0.
  13. ^ see IBM System/360 architecture, Unit Status
  14. ^ a b Mancke, Richard B.; Fisher, Franklin M.; McKie, James W. (July 1980). "Historical Narrative, US vs IBM, Exhibit 14791". US Government. pp. 334, 1051–2. Retrieved April 10, 2016. The new attachment strategy created a single interface that permitted the use of a single disk control unit, the 3830 Model II, for the 370 disk drives.
  15. ^ a b IBM System/360 Component Descriptions - 2841 Storage Control Unit and Associated DASD (PDF). Eighth Edition. IBM. December 1969. A26-5988-7.
  16. ^ IBM Field Engineering Theory of Operation 2841 Storage Control (Stage 2) (PDF). IBM. April 1988. SY26-4000-3.
  17. ^ Thoburn, F. J. (November 1970). "A Transmission Control Unit for High-speed Computer-to-computer Communication". IBM Journal of Research and Development: 614–619. CiteSeerX doi:10.1147/rd.146.0614.
  18. ^ IBM Corporation (1964). IBM System/360 System Summary (PDF). p. 27. A22-6810-0.
  19. ^ ...IBM 3380 DASD Direct Channel Attach Model CJ2, VM, Between-Release Support Information
  20. ^ Brock, Gerald (July 1974). "US Senate Committee on the Industrial reorganization act". US Govt. Printing Office. p. 97 (5667). Retrieved April 11, 2016. In August, 1972, the 3830 controller was withdrawn from the market and replaced by the 3830 II. The 3830 II was similar to the 3830 except that it could control up to sixteen drives instead of eight, and part of the control electronics was missing. The missing part was put into a modified 3330 box, called the 3333.
  21. ^ IBM (March 1982). IBM 3880 Storage Control Model 11 Description (PDF) (First ed.). GA32-0061-0.
  22. ^ "Now Memorex fills the gap in your system's performance." Datamation Advertisement, August 1978, p. 85-6
  23. ^ "It's how IBM's newest cache makes DASD faster". Datamation: 119. November 1984.
  24. ^ Cornwell, Michael (September 2009). "Innovative Solid State Storage Architecture" (PDF). www.snia.org. SNIA. Retrieved December 31, 2015. StorageTek 4305 1978 First Solid State Disk
  25. ^ IBM 3990/9390 Storage Control Introduction. Storage Subsystem Library (Ninth ed.). IBM. October 1996. GA32-0098-08.
  26. ^ System/370 Model 145 Announcement Letter
  27. ^ System/370 Model 135 Announcement Letter
  28. ^ a b c d Universal Sales Manual Hardware Products. IBM. February 15, 1994.
  29. ^ IBM System/370 Input/Output Systems Configurator (PDF). IBM. April 1988. GA22-7002-27. Retrieved January 1, 2016.

External linksEdit