Операционные системы IBM System/360: от DOS/360 до VSE/SP

IBM mainframe operating system designed for use with smaller machines
the family of operating systems for IBM System/360 computers

Disk Operating System/360, also DOS/360, or simply DOS, is the discontinued first member of a sequence of operating systems for IBM System/360, System/370 and later mainframes. It was announced by IBM on the last day of 1964, and it was first delivered in June 1966. In its time, DOS/360 was the most widely used operating system in the world.

The Basic Operating System (BOS) was an early version of DOS and TOS which could provide usable functionality on a system with as little as 8 KB of main storage and one 2311 disk drive.

TOS/360 (Tape Operating System/360, not a DOS as such and not so called) was an IBM operating system for the System/360, used in the early days around 1965 to support the System/360 Model 30 and similar platforms. TOS, as per the "Tape" in the name, required a tape drive. It shared most of the code base and some manuals with IBM's DOS/360. TOS went through 14 releases, and was discontinued when disks such as the IBM 2311 and IBM 2314 became more affordable at the time of System/360, whereas they had been an expensive luxury on the IBM 7090.

DOS/360 was the primary operating system for most small to midsize S/360 installations.

DOS/VS was released in 1972. The first DOS/VS release was numbered "Release 28" to signify an incremental upgrade from DOS/360. It added virtual memory in support of the new System/370 series hardware. It used a fixed page table which mapped a single address space of up to 16 megabytes for all partitions combined. DOS/VS increased the number of partitions (separate simultaneous programs) from three (named Background, Foreground 1 and Foreground 2) to five (BG and F1 through F4) and allowed a system wide total of fifteen subtasks. DOS/VS was succeeded by DOS/VSE through z/VSE.

DOS/VSE was introduced in 1979 as an "extended" version of DOS/VS to support the new 4300 processors. The 4300 systems included a feature called ECPS:VSE that provided a single level storage for both the processor and the I/O channels. DOS/VSE provided support for ECPS:VSE, but could also run on a System/370 without that feature. VSE was the last free version of DOS.

VSE/Advanced Functions (VSE/AF) is a product that adds new device support and functionality to DOS/VSE. Many installations installed VSE/AF using products such as VSE System Installation Productivity Option/Extended (VSE System IPO/E), which combines DOS/VSE, VSE/AF and various other products.

SSX/VSE ("Small System Executive") was an attempt by IBM to simplify purchase and installation of VSE by providing a pre generated system containing the OS and the most popular products. SSX was released in 1982, and later replaced by VSE/SP. SSX was sold by IBM as a bundle of 14 component products (Advanced Functions/VSE, VSE/POWER, ACF/VTAME, VSE/VSAM, CICS/DOS/VS, DOS/VS, Sort/Merge, VSE/ICCF, VSE/OCCF, VSE/IPCS, DOS/COBOL, Back Up/Restore, Space Management, VSE/DITTO), and originally would only agree to offer the individual products separately via RPQ, although IBM later agreed to add those products individually to its price list under pressure from ISVs who claimed that the bundling violated antitrust laws.

In 1986 IBM released VSE/SP ("System Product") in conjunction with the announcement of the 9370 processors. VSE/SP replaced SSX/VSE and bundled VSE with the most popular VSE program products such as VSE/AF, ACF/VTAM, CICS, and POWER/VS. VSE/SP supported only 24 bit addresses, despite customer requests to provide an XA (31 bit) version.

VSE/ESA was a 31 bit DOS/VSE version, which was released in 1990 with support for up to 384 MB of real storage. It provided up to twelve static partitions and allowed VSE/POWER and ACF/VTAM to be run in private address spaces. It introduced a new feature called dynamic partitions which could allow up to 150 concurrent jobs, each in its own address space. Version 1 could run in either ESA or 370 mode, with the ESA mode also supporting XA hardware with limitations. Version 2 (1995) only supported ESA mode with ESA hardware. Version 2 added support for multiprocessing, through the new Turbo Dispatcher, which permits different partitions to execute simultaneously on different processors. A partition can only run on one processor at a time, which mostly limits the multiprocessing to multitasking. Up to ten processors are theoretically supported ("tolerated"), but up to four are effectively utilized. Those limits remain in the last z/VSE.

IBM released z/VSE 3.1 in 2005. This change in naming reflected the new "System z" branding for IBM's mainframe product line, but did not represent a fundamental change in architecture from VSE/ESA 2.7 which preceded it. In particular, it did not support the new 64 bit z/Architecture, running only in 31 bit mode even on 64 bit capable machines. z/VSE 4.1 released in 2007 introduced support for 64 bit real addressing, with up to 8 GB of memory. However, while parts of the supervisor run in 64 bit mode, it only provides 31 bit virtual address spaces to problem state applications. As of 2011 one estimate placed the number of sites using z/VSE at around 4,000.

DOS/360 required a System/360 CPU (model 25 and above) with the standard instruction set (decimal and floating point instruction sets optional). The minimum memory requirement was 16 KB; storage protection was required only if multiprogramming was used. A 1052 Model 7 printer keyboard, either a selector or multiplexor channel, and at least one disk drive was required — initially a 2311 holding 7.25 MB. A card reader, card punch and line printer were usually included, but magnetic tape drives could be substituted. To further reduce memory usage, the supervisor employed overlays called transients that were read into one of two reserved transient areas as required. Physical transients were loaded into the 556 byte A Transient area to handle hardware errors (ERPs), record error specific data (OBR/MDR) on IJSYSRC, and issue error messages. All A Transient module names began with Logical transients were loaded into the 1200 byte B Transient area to provide common program services like OPEN and CLOSE for LIOCS. All B Transient module names began with
The use of and prefixes ensured rapid loading of transients because their names were stored first in the directory. DOS/VS added Machine Check and Channel Check Handlers, which were another set of transients all starting with and executing in the Recovery Transient area. This was done as part of the reliability, availability, and serviceability (RAS) enhancements for the System/370. Before this addition, machine checks caused termination of the program running and channel checks caused termination of the program accessing the device, at the time of the error.

Like OS/360, initial releases of DOS could run only one program at a time. Later versions of "real" DOS were able to run up to three programs concurrently, in separate memory partitions, supported by the same hardware memory protection features of the more scalable OS/360 operating system. These were identified as BG (background), F1 (foreground 1) and F2 (foreground 2). Multiprogramming was an optional feature of DOS/360, selectable at system generation. A later SYSGEN option allowed batch operation run in either FG partition. Otherwise foreground programs had to be manually started by the computer operator. DOS/VS allowed up to seven concurrent programs, although five or six was a more common number due to the smaller scale of the hardware usually hosting DOS systems. Both DOS and DOS/VS allow the number of partitions to be set at IPL (Initial Program Load), the IBM term for Boot load.

Executable programs were stored in a Core Image Library. While running, DOS could not reclaim space as programs were deleted or replaced with newer versions. When the Core Image Library became full, it had to be compressed by a utility program, and this could halt development work until it was complete. Many shops simply froze changes for a day, compressed the CIL "off line", and IPLed with the new Core Image Library at the beginning of a business day. A relocatable library for linkable object programs and a source statement library for assembler macros and include text were also supported. Installations could define additional private relocatable and source statement libraries on other disk volumes.

DOS/360 offered Basic Telecommunications Access Method (BTAM) and Queued Telecommunications Access Method (QTAM). BTAM was primitive and hard to use by later standards, but it allowed communication with almost any type of terminal, which was a big advantage at a time when there was little standardization of communications protocols. The simplicity of its API also allowed the relatively easy interface of external communications processors, which facilitated DOS/360 machines becoming nodes in the multi tier networks of large organizations. Conversely, QTAM users did not need as much knowledge about individual devices because QTAM operated at the logical level using the OPEN/CLOSE/GET/PUT macros.

All DOS job control statements began with "" in card columns one and two except end of job which was "", end of data, "", and comments , "". (In the description that follows the character "" represents a single blank.) The statement indicates "the beginning of control information for a job." The format is // JOB <jobname> <comments>. must be one to eight alphanumeric characters to identify the job. are ignored. The statement identifies a program to be executed as a job step. "All control statements necessary for execution must be processed" before the statement is read. The format is // EXEC <program>
The statement "can be used to allow for operator action between job steps." The format is // PAUSE <comment>. The comment is used to provide a message to the operator. The statement may be used to display a message to the operator. The format is The end of data statement marks the end of data in the input stream. The format is /*. Any data on the statement following the blank is ignored. The end of job statement marks the end of a job, and may indicate the end of data to be flushed if the job terminates abnormally. The format is /&. Any data on the statement following the blank is ignored. The statement specifies values of system options that apply to this job. The format is // OPTION <option1>[,<option2> ]. The statement "is used to assign a logical I/O unit to a physical device." The format is // ASSGN SYSxxx,<device>[,<tape option>]. SYSxxx indicates a logical unit such as SYS001 or SYSIPT. is either "X'cuu'" to indicate a physical device (channel and unit), "IGN" for ignore, or "UA" for unassigned. specifies either tape mode settings such as density, parity, etc., or "ALT" to indicate an alternate device. The statement resets specified I/O unit assignments to their permanent values. The format is // RESET <option>. may be "SYS" to reset all system logical unit assignments, "PROG" to reset all programmer assignments, "ALL" to reset all assignments, or "SYSxxx" to reset the assignment for the logical unit "SYSxxx", for example SYS002. The statement instructs the system to print a listing of all specified I/O assignments currently in effect. The format is // LISTIO <option>. is "SYS" to list all system assignments, "PROG", "F1", or "F2" to list all assignments for the background or specified foreground partition, "ALL", "SYSxxx", "X'cuu'", "UNITS" to list all assigned units, 'UA" to list all unassigned units, or "DOWN" to list all units marked as inoperative. The statement issues command to a magnetic tape unit. The format is // MTC <opcode>,SYSxxx[,<nn>]. is a function such as "FSF" to forward space one file or "REW" to rewind the tape. is a number that can specify the number of times the operation is to be performed, such as forward space two files. The statement provides disk or tape volume label information for standard label checking. The format is // VOL SYSxxx,<volume>. DOS originally provided the statement for tape label information and the and statements for disk label and extent information. At least as early as 1968 the statement had been replaced by and the statement by These statements used numerous positional parameters and had fairly high information densities.

DOS JCL was designed for parsing speed and simplicity; the resulting positional syntax was significantly more cryptic than OS/360 keyword driven job control.

Early DOS included no spooling sub system to improve the efficiency of punched card and line printer I/O. By the late 1960s both IBM and aftermarket vendors began filling this void. IBM's spooler was an option called POWER, and Software Design, Inc., an independent software company, sold a spooler called GRASP.

DOS/360 had no relocating loader, so programmers had to link edit a separate executable version of each program for each partition, or address space, in which the program was likely to be run. Alternatively assembler language programs could be written as self relocating, but that imposed additional complexity and a size penalty, albeit a small one. Large DOS shops with multiple machines and multiple partition layouts often wrote their own relocating loader to circumvent this issue.

The DOS/360 application programming interface was incompatible with OS/360. High level language programs written for DOS needed to be compiled and linked before they could be used with OS/360. Minor differences between compilers of DOS as opposed to OS sometimes required modifications to programs. The port in the other direction however was more challenging. Since OS/360 had significantly more features supported in its API, any use of those features would have to be removed from programs being ported to DOS. This was less of a problem for programmers working in high level languages such as COBOL. Assembler programs, on the other hand, tended to utilize those very features more often and usually needed greater modification to run on DOS.