General Points

The CP component of VM/370 is a program that runs on a real 370 to provide the illusion of several slower 370's. While most computers can simulate other computers, CP only simulates privileged instructions. Due to the architecture of the 370 it is possible to let the 370 hardware execute the non-privileged instructions directly but yet give control to CP when a privileged instruction is encountered in such a way that CP can provide the illusion that the instruction was executed.

{spec}Two possible projects for Gnosis would be to provide virtual 370's or to provide virtual Gnosis's.

See (p1,vm) about providing a virtual machine of Gnosis.

Constructing another Gnosis is of less interest because Gnosis, unlike the 370, already multiplexes its facilities among clients.

The architecture of Gnosis attempts to carry this concept one step in a new direction. Given the 370, CP must simulate the entire privileged machine in order to provide any virtual function. Given Gnosis, selected facilities that would normally be provided by Gnosis directly, can be replaced by virtual or (_synthetic) facilities. The advantages of this are two-fold: it is less effort to construct the smaller virtual facility and that the overhead is associated only with facilities so replaced. See (p3,sub-regime) on more about how to do this.

There are several reasons why one substitutes synthetic things for the "real" ones. Virtual 370's are less expensive than real ones. Virtual 370's may have more core than the real 370. {MVS will run on a virtual 370 under CP on a 135, but MVS will not run on a 135 without CP.} A virtual 370 may have logical features unavailable on the real machine such as channel to channel adapters, printers and such. Virtual 370's may have features that are designed and added by the installation such as DICOM. VIO {virtual I/O} in MVS is usually faster and less expensive than the real I/O that it replaces.

Another aspect of this question is "why should the synthetic things mimic the 'real' ones?". There are two answers here: there are programs available that may have been designed with the real facility in mind; it is convenient to have the same program run in those cases where the real facility is available and has appropriate attributes and also run, perhaps less efficiently, when a real facility must be replaced by a synthetic one.

This section lists some "synthesizable" facilities of Gnosis.

If a segment key designates a node with no segment keeper and no other program is attending to the segment with a node key to the segment node, then there are a fixed set of valid addresses the segment supports. A segment keeper is in a position to create the illusion of a static segment when in truth the pages are materialized only when the segment key holder comes to the point of accessing those pages. This materialization might consist of: Fetching the page from a 3850, Fetching the page from another city via Tymnet, De-compressing the data from some compressed format, Computing the contents of the page.

The ability of the segment keeper to monitor stores into the segment allows the segment keeper to signal someone else of this fact. Given two 370's in different cities each running Gnosis and connected by a network, it is possible to build one logical segment that appears to simultaneously exist in the two cities. This is accomplished as follows: A segment node is created in each system with segment keepers that communicate with each other. A given page of the segment is either: {1} in just one city with read-write access there or {2} in both cities with read only access there. The segment keepers can adjust the state of each page in accordance with the page faults registered for the real segments. For example, if a program in one city tries to store into a page that is read only there, that program is stopped until the segment keeper in the remote city has removed the page from the remote segment. The segment keeper will have to send portions of pages over the communications lines to make the illusion whole. This technique is similar to what Tymshare now uses to make copies of the master user directory available to the network supervisors in various cities.

An elaboration in another direction is to provide a function like the RPG function on the DEC-10. RPG insures that one does not use the relocatable form of a program when the symbolic form has been modified since the compiler or assembler was applied to the program. RPG invokes the compiler or assembler to insure the timeliness of the relocatable file. In Gnosis we postulate two segments, the symbolic and the relocatable. These two segments share the same segment keeper. When the symbolic program is written into, by the editor perhaps, the segment keeper invalidates all of the pages of the relocatable. When the relocatable is referenced for read access the segment keeper changes the symbolic segment back to read-only and invokes the compiler. The effect is to invoke the compiler when RPG would have.

Since node keys may be used in initial slots of segment nodes a synthetic node or sense key must be a segment key. The segment keeper must respond to each of the calls upon nodes as the node would. If the virtual node has initial slots that extend into those slots required in the real segment node, an LSS of 12 may be used in the real segment node and slot 0 of that node may be a segment key to a node that faithfully models the virtual node.

Similar tactics may be used if the synthetic node is asked to produce a segment key corresponding the synthetic node. See (p3,synseg).

There are two aspects of synthetic domains: as seen by the start key holder and as seen by the domain key holder. The Gnosis gate jump has been defined so as to facilitate interposing a program between the caller and callee. This means that given a start key, one can produce a new start key with the same function as the former but with the additional proviso that all calls and returns to the subroutine will pass thru the interpolated code.

A synthetic domain is made of two domains, X and Y. The synthetic domain key is really a start key to X. The domain X remembers the state of the synthetic domain. X holds a domain key to Domain Y. When X is requested to produce a start key for the synthetic domain, X causes Y to be like the synthetic domain except, perhaps, for the trap key. X now provides a start key to Y to serve the function of a start key to the synthetic domain.