In computer science, a data structure is a data organization, management and storage format that enables efficient access and modification. More precisely, a data structure is a collection of data values, the relationships among them, and the functions or operations that can be applied to the data.
IBM mainframes are large computer systems produced by IBM since 1952. During the 1960s and 1970s, IBM dominated the large computer market. Current mainframe computer in IBM's line of business computers are developments of the basic design of the IBM System/360.
An operating system (OS) is system software that manages computer hardware and software resources and provides common services for computer programs.
An SRB may be considered, in the abstract, to be a highly optimized Task Control Block (TCB), one which has few, if any, associated resources other than access to the processor itself. All system resources which are utilized under an SRB must be accessed through the use of "branch entries", some of which are new entries to traditional system services which were formerly accessed exclusively using SVC instructions (which an SRB may not employ for any purpose other than abnormally terminating itself in which case SVC 13, ABEND, may be used, however the "branch entry" to ABTERM is really more appropriate).
The Task Control Block (TCB) is an instance of a Process control block in an IBM System/360 architecture and successors operating system environment.
A Supervisor Call instruction (SVC) is a hardware instruction in the System/360 family of IBM mainframe computers up to contemporary zSeries used to cause an interrupt to request a service from the operating system. The system routine providing the service is called an SVC routine. SVC is a specific implementation of a system call.
When employed by the Start Input/Output interface, an SRB is always paired with an Input/Output Supervisor Block (IOSB).
An Input/Output Supervisor Block (IOSB) is a data structure employed exclusively by the Start Input/Output interface of MVS/370 and successor IBM mainframe operating systems. The IOSB describes the I/O request and passes parameters to and receives responses from the Input/Output Supervisor (IOS).
When otherwise employed, an SRB facilitates inter-address-space communication in general, and inter-application communication in particular.
In computing, an address space defines a range of discrete addresses, each of which may correspond to a network host, peripheral device, disk sector, a memory cell or other logical or physical entity.
In computer science, inter-process communication or interprocess communication (IPC) refers specifically to the mechanisms an operating system provides to allow the processes to manage shared data. Typically, applications can use IPC, categorized as clients and servers, where the client requests data and the server responds to client requests. Many applications are both clients and servers, as commonly seen in distributed computing. Methods for doing IPC are divided into categories which vary based on software requirements, such as performance and modularity requirements, and system circumstances, such as network bandwidth and latency.
SRBs may also be employed for intra-address-space processes, where the highest possible performance is required, and in this case the necessary resources are first acquired under a TCB (usually the "job step" TCB), before the SRBs are SCHEDULEd (i.e., are presented to the system dispatcher to compete for processor resources).
It is conceivable that an address space may have but one TCB (again, the "job step" TCB) but tens or hundreds or even thousands of SRBs, with the SRBs performing almost all of the work in the address space, and the TCB merely synchronizing the SRBs and responding to communications from the system operator.
For purposes of such synchronization, the TCB will usually issue a WAITR, SVC 1, specifying a list of Event Control Blocks (ECBs; one ECB per SRB, plus one for the system operator), and each SRB will indicate its completion to the TCB by using a "branch entry" to the POST system service (which is normally SVC 2, but in this special case would be a call to the address contained in CVT0PT01), and specifying the ECB which is associated with its SRB, and possibly a "message" to the TCB. The "message", should it be present, is often placed in the lowest 24 bits of the ECB, and which is otherwise unused. The highest eight bits are used by the system.
Disk device access and network device access is available to SRBs using the "improved control interval processing" feature of VSAM and the "fast path" feature of VTAM, respectively.
History
With the introduction of MVS/370 and successor systems, a whole new environment was introduced: the Service Request Block (SRB), which generally has a higher priority than any TCB, and, indeed, which itself has two distinct priorities: a Global SRB (priority over all local address space SRBs and TCBs) and a Local SRB (priority over only the local address space TCBs); and MVS's dispatcher must manage all of these with absolute consistency across as many as two processors (MVS/370) and as many as sixteen processors (successor systems). More than sixteen processors are available on some z/System models. z/System support for zIIP has dramatically increased the use of SRBs (over and above their traditional Start Input/Output usage).
In IBM System z9 mainframes, the System z Integrated Information Processor (zIIP) is a special purpose processor. It was initially introduced to relieve the general mainframe central processors (CPs) of specific DB2 processing loads, but currently is used to offload other z/OS workloads as described below. The idea originated with previous special purpose processors, the zAAP, which offloads Java processing, and the IFL, which runs Linux and z/VM but not other IBM operating systems such as z/OS, DOS/VSE and TPF. A System z PU is "characterized" as one of these processor types, or as a CP, or SAP. These processors do not contain microcode or hardware features that accelerate their designated workloads. Instead, by relieving the general CP of particular workloads, they often lead to a higher workload throughput at reduced license fees.
An SRB is a highly-optimized instance of a Process control block in these MVS/370 and successor systems. A TCB is a general-purpose instance of a Process control block in these OS/360 and successor systems.
Process Control Block is a data structure in the operating system kernel containing the information needed to manage the scheduling of a particular process. The PCB is "the manifestation of a process in an operating system."
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Related Research Articles
Multiple Virtual Storage, more commonly called MVS, was the most commonly used operating system on the System/370 and System/390 IBM mainframe computers. It was developed by IBM, but is unrelated to IBM's other mainframe operating systems, e.g., VSE, VM, TPF.
Virtual Storage Access Method (VSAM) is an IBM DASD file storage access method, first used in the OS/VS1, OS/VS2 Release 1 (SVS) and Release 2 (MVS) operating systems, later used throughout the Multiple Virtual Storage (MVS) architecture and now in z/OS. Originally a record-oriented filesystem, VSAM comprises four data set organizations: Key Sequenced Data Set (KSDS), Relative Record Data Set (RRDS), Entry Sequenced Data Set (ESDS) and Linear Data Set (LDS). The KSDS, RRDS and ESDS organizations contain records, while the LDS organization simply contains a sequence of pages with no intrinsic record structure, for use as a memory-mapped file.
Job Control Language (JCL) is a name for scripting languages used on IBM mainframe operating systems to instruct the system on how to run a batch job or start a subsystem.
This article discusses support programs included in or available for OS/360 and successors. IBM categorizes some of these programs as utilities and others as service aids; the boundaries are not always consistent or obvious. Many, but not all, of these programs match the types in utility software.
The Job Entry Subsystem (JES) is a component of IBM's mainframe operating systems that is responsible for managing batch workloads. In modern times, there are two distinct implementations of the Job Entry System called JES2 and JES3. They are designed to provide efficient execution of batch jobs.
The Houston Automatic Spooling Priority Program, commonly known as HASP, is an extension of the IBM OS/360 operating system and its successors providing extended support for "job management, data management, task management, and remote job entry."
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.
An access method is a function of a mainframe operating system that enables access to data on disk, tape or other external devices. They were introduced in 1963 in IBM OS/360 operating system. Access methods provide an application programming interface (API) for programmers to transfer data to or from device, and could be compared to device drivers in non-mainframe operating systems, but typically provide a greater level of functionality.
In IBM mainframe operating systems, basic partitioned access method (BPAM) is an access method for libraries, called partitioned datasets (PDSes) in IBM terminology. BPAM is used in OS/360, OS/VS2, MVS, z/OS, and others.
In IBM mainframe operating systems, Execute Channel Program (EXCP) is a supervisor call for low-level device access, where the programmer is responsible for providing a channel program—a list of device-specific commands (CCWs)—to be executed by I/O channels, control units and devices. EXCP is more specifically described in the OS System Programmer's Guide.
OS/360, officially known as IBM System/360 Operating System, is a discontinued batch processing operating system developed by IBM for their then-new System/360 mainframe computer, announced in 1964; it was heavily influenced by the earlier IBSYS/IBJOB and Input/Output Control System (IOCS) packages. It was one of the earliest operating systems to require the computer hardware to include at least one direct access storage device.
Basic Direct Access Method, or BDAM is an access method for IBM's OS/360 and successors computer operating systems on System/360 and later mainframes. BDAM "consists of routines used in retrieving data from, and storing data onto, direct access devices." BDAM is available on OS/360, OS/VS2, MVS, z/OS, and related high-end operating systems.
Single Virtual Storage (SVS) refers to Release 1 of Operating System/Virtual Storage 2 (OS/VS2); it is the successor system to the MVT option of Operating System/360. OS/VS2 (SVS) was a stopgap measure pending the availability of MVS, although IBM provided support and enhancements to SVS long after shipping MVS.
The IBM Administrative Terminal System, also known as ATS/360, provided text- and data-management tools for working with documents to users of IBM System/360 systems.
In MVS/370 and successor versions of IBM mainframe operating systems, Start Input/Output (STARTIO) is a macro instruction and a "branch entry" for low-level device access, where the programmer is responsible for providing a list of device-specific CCWs, that is, a channel program, to be executed by I/O channels, control units and devices and a number of "exits", several of which may be immediate returns to the Input/Output Supervisor (IOS). Invokers of STARTIO must be in supervisor mode and key 0. STARTIO interfaces directly with the IOS component of MVS.
In IBM mainframe operating systems, Execute Channel Program in Real Storage (EXCPVR) is a supervisor call for low-level device access, where the programmer is responsible for providing a list of device-specific CCWs, that is, a channel program, to be executed by I/O channels, control units and devices. Additionally, the programmer is expected and required to ensure that the input/output data areas are in real storage and are "fixed", either prior to invoking EXCPVR or via the page fix appendage..
