VME: История и Эволюция Мейнфрейм Операционной Системы ICL

Mainframe operating system by ICL

VME (Virtual Machine Environment) is a mainframe operating system developed by the UK company International Computers Limited (ICL, now part of the Fujitsu group). Originally developed in the 1970s (as VME/B, later VME 2900) to drive ICL's then new 2900 Series mainframes, the operating system is now known as OpenVME incorporating a Unix subsystem, and runs on ICL Series 39 and Trimetra mainframe computers, as well as industry standard x64 servers.

When New Range was first launched in October 1974, its operating system was referred to as "System B". By the time it was first delivered it had become "VME/B". VME/K (originally known internally as "System T" for "Tiny") was developed independently (according to Campbell Kelly, "on a whim of Ed Mack"), and was delivered later with the smaller mainframes such as the 2960. At the time VME/B was still plagued with performance and reliability problems, and the mainly American management team had misgivings about it. There was also a project known as System D, which was an advanced, highly modular operating system designed for configuring and building customized near real time applications. Although it was used to bid on a system for British Rail it was subsequently cancelled because of engineering resource constraints. ICL had sold a large system to the European Space Agency to process data from Meteosat at its operation centre in Darmstadt. A bespoke variant of VME/K, known as VME/ESA was developed on site to meet the customer's requirements. Following a financial crisis in 1980, new management was brought into ICL (Christophor Laidlaw as chairman, and Robb Wilmot as managing director). An early decision of the new management was to drop VME/K. Thus in July 1981 "VME2900" was launched: although presented to the customer base as a merger of VME/B and VME/K, it was in reality the VME/B base with a few selected features from VME/K grafted on. This provided the opportunity to drop some obsolescent features, which remained available to customers who needed them in the form of the "BONVME" option. The "2900" suffix was dropped at System Version 213 (SV213) when ICL launched Series 39 in 1985 as the successor to the original 2900 series; and the "Open" prefix was added after SV294. VME became capable of hosting applications written originally for Unix through a UNIX System V Release 3 based subsystem, called VME/X, adapted to run under VME and using the ASCII character encoding. In 2007 Fujitsu announced a VME version run as a hosted subsystem, called superNova, within Microsoft Windows, or SUSE or Red Hat Enterprise Linux on x86 64 hardware. Fujitsu intended to support VME on customer computers until 2020. In 2020 Fujitsu transferred 13 HM Revenue and Customs applications from their computers onto Fujitsu's virtual managed VME hosting platform. As of 2021, the Department for Work and Pensions completely replaced its VME based systems, following the completion of its award winning VME R replacement programme. https://www. computerweekly. com/news/252498414/DWP completes in house VME replacement project

VME is structured as a set of layers, each layer having access to resources at different levels of abstraction. Virtual resources provided by one layer are constructed from the virtual resources offered by the layer below. Access to the resources of each layer is controlled through a set of Access Levels: in order for a process to use a resource at a particular access level, it must have an access key offering access to that level. The concept is similar to the "rings of protection" in Multics. The architecture allows 16 access levels, of which the outer 6 are reserved for user level code. Orthogonally to the access levels, the operating system makes resources available to applications in the form of a Virtual Machine. A Virtual Machine can run multiple processes. In practice, a VME Virtual Machine is closer to the concept of a process on other operating systems, while a VME process is more like a thread. The allocation of resources to a virtual machine uses a stack model:
when the stack is popped, all resources allocated at that stack level are released. Calls from an application to the operating system are therefore made by a call that retains the same process stack, but with a change in protection level; the resulting efficiency of system calls is one of the features that makes the architecture competitive. Communication between Virtual Machines is achieved by means of Events (named communication channels) and shared memory areas. The hardware architecture also provides semaphore instructions INCT (increment and test) and TDEC (test and decrement). Files and other persistent objects are recorded in a repository called the Catalogue. The file naming hierarchy is independent of the location of a file on a particular tape or disk volume. In days where there was more need for offline storage, this made it easy to keep track of files regardless of their location, and to move files between locations without renaming them. As well as files, the Catalogue keeps track of users and user groups, volumes, devices, network connections, and many other resources. Metadata for files can be held in an object called a File Description. The Catalogue was probably the first example of what would later be called an entity relationship database. Interrupts are handled by creating a new stack frame on the stack for the relevant process, handling the interrupt using this new environment, and then popping the stack to return to the interrupted process. Run time exceptions, referred to as contingencies, are captured by the Object Program Error Handler (OPEH), which can produce a report (equivalent to a stack trace), either interactively or written to a journal.

Compiled object code is maintained in a format called OMF (Object Module Format), which is both the compiler output and the format used by the loader. Various compilers are available, as well as utilities, notably the Collector, which links the code in several OMF modules into a single module, for more efficient loading at run time, and the Module Amender, which allows patching of the instructions in an OMF module to fix bugs, using assembly language syntax.

As a result of ICL's heavy involvement with delivery of computer services to the UK Public Sector, in particular those with special security requirements such as OPCON CCIS, it was an early entrant into the market for Secure Systems. VME formed a core of ICL's activities in the Secure Systems arena. It had the advantage that as the last large scale operating system ever designed, and one built from scratch, its underlying architecture encompassed many of the primitives needed to develop a Secure System, in particular the hardware assisted Access Control Registers (ACR) to limit to privileges that could be taken by any process (including Users). This led to the UK Government's Central Computing and Telecommunications Agency (CCTA) funding Project Spaceman in the mid 1980s for ICL Defence Technology Centre (DTC) to develop an enhanced security variant of VME. ICL launched this as a pair of complementary products, with the commercial release being called High Security Option (HSO), and the public sector release, including Government Furnished Encryption (GFE) technologies, being called Government Security Option (GSO). HSO and GSO were formally tested under the CESG UK (Security) Evaluation Scheme, one of the predecessors to ITSEC and Common Criteria, and in doing so became the first mainstream operating system to be formally Certified.

The Series 39 range introduced Nodal Architecture, a novel implementation of distributed shared memory that can be seen as a hybrid of a multiprocessor system and a cluster design. Each machine consists of a number of nodes, and each node contains its own order code processor (CPU) and main memory. Virtual machines are typically located (at any one time) on one node, but have the capability to run on any node and to be relocated from one node to another. Discs and other peripherals are shared between nodes. Nodes are connected using a high speed optical bus, which is used to provide applications with a virtual shared memory. Memory segments that are marked as shared (public or global segments) are replicated to each node, with updates being broadcast over the inter node network. Processes which use unshared memory segments (nodal or local) run in complete isolation from other nodes and processes.

VME was originally written almost entirely in S3, a specially designed system programming language based on Algol 68R (however, VME/K was written primarily in the SFL assembly language). Although a high level language is used, the operating system is not designed to be independent of the underlying hardware architecture: on the contrary, the software and hardware architecture are closely integrated. From the early 1990s onwards, some entirely new VME subsystems were written partly or wholly in the C programming language. From its earliest days, VME was developed with the aid of a software engineering repository system known as CADES, originally designed and managed by David Pearson (computer scientist) and built for the purpose using an underlying IDMS database. CADES is not merely a version control system for code modules: it manages all aspects of the software lifecycle from requirements capture, design methodology and specification through to field maintenance. CADES was used in VME module development to hold separate definitions of data structures (Modes), constants (Literals), procedural interfaces and the core algorithms. Multiple versions ('Lives') of each of these components could exist. The algorithms were written in System Development Language (SDL), which was then converted to S3 source by a pre processor. Multiple versions of the same modules could be generated.

The primary language used for developing both the VME operating system itself and other system software such as compilers and transaction processing monitors is S3. This is a high level language based in many ways on Algol 68, but with data types and low level functions and operators aligned closely with the architecture of the 2900 series. An assembly language SFL (System Function Language) is also available. This was used for the development of VME/K, whose designers were not confident that a high level language could give adequate performance, and also for the IDMS database system on account of its origins as a third party product. SFL was originally called Macro Assembler Programming LanguagE (MAPLE), but as the 2900 architecture was being positioned as consisting of high level language machines the name was changed at the request of ICL Marketing. It had been developed as a part of the toolkit for System D, which was subsequently cancelled. Related families of assemblers for other architectures (CALM xx running under VME, PALM xx developed in Pascal and running on various hosts) were developed for internal use. Neither S3 nor SFL was ever promoted as a commercial development tool for end user applications, as neither were normally delivered as a standard part of the operating system, nor were they explicitly marketed as products in their own right. Both SFL and S3 were however available as options to user organisations and third parties who had a specific need for them.