Architecture
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. Unlike other operating systems, 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.
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.
Read more about this topic: ICL VME
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