Beginning with its 1964 System/360 announcement, IBM's mainframes initially accessed count key data (CKD) 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 IBM Z; 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.
An Integrated Storage Control (ISC) is a Director within the cabinet of an IBM System.
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.
[18] DASD devices introduced after the 2841 used different storage controls as discussed in the following sections.
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.
[7] The 3830 Model 1 Storage Control Unit connects to one, two or four channels and can attach one to eight 3330 DASD spindles.
The Integrated Storage Controller with Staging Adapter functions identically to the 3830-3.
[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.
Caching in semiconductor buffers was first introduced in DASD CKD subsystems by Memorex[22] (1978) and StorageTek[d] (1981).
By merging the functions the DAC eliminates the need for a channel and a separate storage control.
[5] 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.
[27] The 2319-A1 contained three 2314 class disk drives plus a piece of the storage control from the system unit.
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.
In a 3340 subsystem the 3340 model 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).
[29] 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.