In IBM mainframe operating systems , Execute Channel Program ( EXCP ) is a macro generating a system call , implemented as a Supervisor Call instruction , 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 for OS/360 and successors is more specifically described in the OS System Programmer's Guide.; EXCP for DOS/360 and successors is more specifically described in DOS Supervisor and I/O Macros. This article mostly reflects OS/360 through z/OS ; some details are different for TOS/360 and DOS/360 through z/VSE .
17-533: Using EXCP, legacy devices and legacy datasets may be operated on with relatively high performance. EXCP devices are OPENed (that is, are made available to the application) by specifying the Data Control Block (DCB) for OS and the DTFPH for DOS. For OS/360 through z/OS, the program provides an Input/Output Block (IOB) to EXCP; if the program executes an EXCP to multiple IOBs, the system processes them in
34-508: A Data Control Block ( DCB ) is a description of a dataset in a program. A DCB is coded in Assembler programs using the DCB macro instruction (which expands into a large number of "define constant" instructions). High level language programmers use library routines containing DCBs. A DCB is one of the many control blocks used in these operating systems. A control block is a data area with
51-554: A merge of the constants and variables specified in the DD JCL statement, and the dataset label for existing magnetic tape and direct-access datasets, into the DCB, replacing the zero values with actual, non-zero values. A control block called the JFCB (Job File Control Block) initially holds the information extracted from the DD statement for the dataset. The results of the merge are stored in
68-472: A predefined structure, very similar to a C struct , but typically only related to system's functions. A DCB may be compared to a FILE structure in C, but it is much more complex, offering many more options for various access methods . The control block acted as the Application programming interface between Logical IOCS and the application program and usually was defined within (and resided within)
85-453: Is a sequence of Channel Command Words (CCWs), which may be executed sequentially unless a branch, called Transfer In Channel (TIC), is executed, or the channel returns a status modifier . Each CCW is a doubleword (eight bytes) as follows: The data address (8-31) is the 24-bit address of a main storage buffer area to or from which the data is to be transferred. The flag bits (32-36) are defined as follows: The count field (48-63) indicate
102-674: Is always in Task Control Block (TCB) mode, The normal mode for applications, as EXCP is a Type 1 SVC . In MVS/370 and subsequent instances of the OS, the EXCP processor invokes STARTIO to schedule execution of the channel program. Although the EXCP processor's back-end is always in Service Request Block (SRB) mode, the back-end contains emulation code which allows the appendages developed for earlier, pre-MVS instances of
119-587: Is forward- and backward-chained to the DCB, and EXCP checks the chaining as a system security measure, as the DCB resides in unprotected user storage whereas the DEB resides in protected system storage, Subpool 253; in OS/VS1 and OS/VS2 EXCP uses DEBCHK to ensure that the DEB was created by OPEN. For DASD, EXCP checks that the seek address in the IOB is within one of the extents, and uses a Set File Mask to, e.g. indicate whether
136-497: The EXCP context) allows authorized programs to override or augment many of the system security and data integrity checks. Most of these appendages are supported for compatibility with earlier instances of the OS, but the functions of several have been modified or extended for MVS. The appendages are specified in the DCB as the last two characters of the module name IGG019xx, where xx = WA to Z9, inclusive. These module names are reserved for installation-written appendages. Any other name of
153-435: The IOB and request either that EXCP reissue the request or that EXCP terminate it. EXCP is sometimes confused with a direct access storage device access method, but it is not for direct access storage devices exclusively; rather, it is a general purpose low-level device access interface which supports any legacy device type and any legacy dataset organization. For accessing all device types and all dataset organizations,
170-634: The JFCB which may also be written into the DSCB during the CLOSE process, thereby making the dataset definition permanent. An example is the BLKSIZE= variable, which may be (and usually is) specified in the DCB as zero. In the DD statement, the BLKSIZE is specified as a non-zero value and this, then, results in a program-specified LRECL (logical record length) and a JCL-specified BLKSIZE (physical block size), with
187-549: The OS, specifically for EXCP, to function largely as before and thereby to appear to be in TCB mode, for which these appendages were originally designed. This distinction can complicate conversion of certain roll-your-own access methods and applications to MVS. System/360 channel programs are a sequence of commands which are executed by the channel and the I/O device. Channel programs can be located anywhere in main storage. The channel program
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#1732794089404204-428: The application program itself. The addresses of I/O subroutines would be resolved during a linkedit phase after compilation or else dynamically inserted at OPEN time. The equivalent control block for IBM DOS/360 , DOS/VSE and z/VSE operating systems is a "DTF" (Define the file) Many of the constants and variables contained within a DCB may be left blank (i.e., these default to zero). The OPEN process results in
221-434: The extent is cylinder or track oriented, whether write is permitted; this prevents a seek out of the specified track (cylinder). If the programmer tries to read past the end of the track (cylinder), a unit check occurs and error recovery restarts the channel program with the next track (cylinder) of the current extent. If the operation goes beyond the end of the extent, EXCP calls the end of extent appendage, which may update
238-560: The form IGG019xx is reserved for use by IBM access methods. Appendages must reside in SYS1.SVCLIB (SYS1.LPALIB in SVS or later instances of the OS). Normally, when a program OPENs a DCB for EXCP, OPEN creates a Data Extent Block (DEB) containing each extent for the first volume of the associated dataset; however, for parallel mount OPEN creates a DEB containing all extents for all volumes. Each DEB
255-1014: The more general Start Input/Output interface (STARTIO) is available in MVS/370 and subsequent instances of the OS although it is not an officially supported interface. A variation of EXCP is Execute Direct Access Program (XDAP), which is used for accessing direct access storage only, and is provided for reading and updating existing direct access storage records. XDAP cannot be used to add records, but XDAP can be utilized along with BSAM or BPAM to effect record additions to such datasets, and several OS components utilize XDAP in this manner (the Linkage Editor, IEWL, in particular). EXCP may also be used to access communications devices attached to IBM 2701 , 2702 and 2703 communications controllers and IBM 370x or Amdahl 470x front-end processors (and their respective follow-ons) operating in emulator mode (EP) or partitioned emulator mode (PEP). EXCP's front-end
272-470: The number of bytes to be transferred by this command. All eight bits of commands that initiate I/O operations are passed to the device. "The modifier bits specify to the device how the command is to be executed." For example, for the IBM 2305 DASD the write command ('MMMMMM01'BX) may have the following values: Data Control Block In IBM mainframe operating systems , such as OS/360 , MVS , z/OS ,
289-410: The order that they were requested. For DASD the IOB includes a seek address, IOBSEEK , in the format MBBCCHHR, where M is the extent, BB is the bin for a data cell, CCHH is the cylinder and head, and R is the record number. "An appendage is a programmer-written routine that provides additional control over I/O operations during channel program execution." A comprehensive list of appendages ( exits in
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