An Introduction to Ten15
          - A personal retrospective.

Martin C. Atkins, Mission Critical Applications Limited.

The Ten15 system was developed by Michael Foster, Ian Currie, Philip Core, et al, at the Royal Signals and Radar Establishment (RSRE, then DERA, now QinetiQ) at Malvern in the U.K. The project was active from around 1987 to 1992, and made many breakthroughs in the areas of virtual machines, polymorphic type systems, persistence, and distributed system design that are still not generally known, and in some cases have not, to my knowledge, even been re-discovered! With the increasing popularity of virtual machine architectures, such as the JVM and .NET, these ideas are more relevant than ever.

These pages are an attempt to publicise this work (better late than never!). Although I did work on the project at the University of York from 1988 until 1992, none of the ideas described here are my own (unless explicitly stated otherwise), and there might be inaccuracies caused by my never having been on the core team, and my poor memory. I have tried to be as precise as I can, but I apologise in advance for any inaccuracies that might have crept in.

The name "Ten15" is (somewhat confusingly) used at different times to refer to the Ten15 system, the Ten15 language (usually called Ten15 Notation), and the Ten15 intermediate representation for programs. Terms which I introduce below for clarity, but are not the usual Ten15 (or Flex) nomenclature, will be "in quotes".

Background

Ten15 started out as an attempt to make something like the RSRE Flex environment available on modern systems. This has no relationship with the Unix-like embedded operating system, also called Flex, but rather the capability system that RSRE built by replacing the microcode in the Three Rivers PERQ (see here, and here). This in turn was an upgrade of the Flex mainframe, also designed and built by the same group at RSRE, but about which I know very little.

Flex

Flex was a capability architecture and operating system with a graphical user interface, unlike any other computer system I've ever heard of! The capability architecture meant that programs did not run in a flat memory space, as we take for granted these days. Instead a program consisted of a number of memory objects, which in Flex could range in size from one integer, to the whole of physical memory. The memory objects are referred to by "pointers", and it is these pointers that are called capabilities. However capabilities were much more than just pointers. It is impossible to create (or forge) a capability without asking the system for a new object, and it is impossible to access memory that was not pointed to by one of the capabilities currently 'owned' by the program. Thus capabilities formed the basis of the protection mechanism that allowed concurrent programs, possibly owned by different users, to execute without interferring with each other. Permissions were also associated with capabilities, so that a memory object could be defined to contain executable code, and another could contain an array, say. Some systems also used capabilities to refer to data stored on the disk.

Capability architectures are described in much more detail in this book by Henry M. Levy, which is out-of-print, and hence can be downloaded in full! Other more modern systems using capability architectures are Key Logic's KeyKOS, and the Extremely Reliable Operating System, Eros.

In Flex, capabilities were visible in the user interface, and could be mixed freely with normal text stored in a "text object", or "Edfile". In fact, a "text object" was simply a capability to an array of lines; each line was an array of "glyphs"; and each "glyph" was either an array of characters, or a capability (defining glyphs simply as "a character or a capability" would have been more logical and simpler, but the overheads were prohibitive). Capabilities, including "text objects", could be stored as capabilities in other text objects, or assigned to a string name, and stored in a "directory".

The user interface was essentially just a text editor for "text objects", but it also allowed embedded capabilities to be manipulated by cut/copy/paste, and other special operations. A capability was displayed as as a cartouche, a box with text inside. Evaluating a command line would replace it with the capability containing the value, and the cartouche for the capability would contain the evaluated text. Since this usually included other cartouches, the display could quickly start to look something like this:

Flex example - 1

The exclaimation mark, '!', means 'apply', and expressions are entered postfix, so this applies '+' to two parameters, the results of evaluating '1', and '2'. Presumeably the enclosing cartouche's value is 3! Actually the cartouches' values are capabilities to the integer values '1, '2', and '3', rather than the values themselves, and so the type of '+' is something like:

'+': ptr int × ptr int -> ptr int
rather than
'+': int × int -> int

as one might expect, but this user interface was not really designed to be a pocket calculator!

The compilers were (loosely speaking) just functions from "text objects" to module values, and a module value was just a record containing the interface specification, and compiled data and function values. There were no such things as include files; if a program needed to link with a library, then (a capability to) the library's module value was simply inserted into the persistent "text object" holding the program's source. The compiler would pick up the library's interface specification from the module, and the module value itself would be included in the code output by the compiler. At run-time, the program (or the linker) could link to the functions in the library's executable, also taken from the same module value. Module values were updateable, so that a module could be re-compiled, and as long as the interface specification had not changed, all programs with references to the module value would transparently start to use the new version.

Flex was usually programmed in Algol-68 (Ian Currie was on one of the Algol-68 committees - I'm not sure of the details), with the extension that procedures could (safely) escape from their scope - in modern language, this meant that functions were first-class values - indeed, in Flex the result of a program could be a function! Not the text for a function, or even the binary implementing the function, but a capability to the function closure value, complete with bindings to all the variables in its non-local scope. I think there was also an experimental Ada compiler.

In order to support this, Flex's main memory had to be garbage-collected.  Flex also supported capabilities to objects on the disk and to remote objects accessible over the network. In fact, the Flex disk was also garbage collected - an off-line operation that used to take a couple of hours for the 20Mbyte(?) PERQ disks. This was, by the way, ample space for the complete Flex system, including all the source!

Flex was designed to be an integrated program development environment, but a variant was deployed for at least one embedded application.

The motivation for Ten15

With modern microprocessor-based systems (here we are talking about Micro VAX, Sun 3, Sun 4, 386-based PCs, etc., but the conclusion has not changed!) microcoding a capability architecture was no longer possible. So it began to look increasingly like the Flex system would die with the PERQ. In order to avoid this a radical new approach was needed, and Ten15 was invented. Folklore has it that the idea for Ten15 was proposed at 10:15 in the morning, and hence the name! Perhaps it's a good thing that the designers didn't work for an advertising company ;-)

Ten15 took a virtual machine approach to portability, but this did not automatically solve the problem of enforcing the capability architecture, without making every access to a capability go though a permission check - an approach that would have had horrible performance implications. Instead it was decided to make everything in the virtual machine strongly-typed, and to use the type-checking to enforce the permission constraints.

For example, in Flex each word in memory was tagged so that capabilities could be distinguished (by the hardware/microcode) from other data. These tags were not directly accessible to normal instructions, and so it was not possible to forge a capability to access memory that the program had not been explicitly given the right to access. In Ten15, the compiler knew the type of every word of memory directly accessible to the program at each point in its execution. Thus, if the program currently held the base address of a record, and attempted to use the 5th member, say, as a pointer to an integer, then the compiler would know if this field really did contain a valid pointer to an integer, or whether it was some other data, and the programmer was (intentionally, or unintentionally) trying to access something he had no right to access.

This is no different from any strongly-typed language, however in Ten15 the type system was enforced uniformly across every data item in the system - there was no way around the type system visible to the user - and thus the type system (together with run-time bounds checking, and garbage collection) actually became the equivalent, as far as security and safety was concerned, of the virtual memory and dynamic address translation mechanisms in conventional systems.

In retrospect, writing an emulator for the Flex/PERQ capability machine would probably have achieved the project's goals more quickly, and certainly by today the performance would have been adequate - but such is the benefit of hindsight, and perhaps the performance would not have been good enough, quickly enough to be really useful!

Ten15's virtual machine

Of course, virtual machines were already old-hat in 1987 - Smalltalk-80 had been around since, well, 1980, but at that time processors were still so slow that these kinds of virtual machines had a very bad reputation for performance. Thus Ten15 also took a different route here - using something more like Just-In-Time compilation.

Like Microsoft's .NET framework, Ten15's virtual machine was intended to support the interworking of a number of high-level languages. However, unlike .NET, the virtual machine wasn't designed from the union of all the features in all the languages that were of interest, rather it was an evaluator for a strongly-typed higher-order eager-evaluation lambda calculus, represented by a parse tree, which by its generality could automatically support any typed imperative language up to, and well beyond, Standard ML, and anything that could be mapped into any of these. Another way of looking at this is that Ten15 was an idealized abstraction of high-level languages, whereas other virtual machines have usually been idealized abstractions of assembly-level languages.

As we have seen, the really unique design decision in Ten15 was for all the operations in the virtual machine to be strongly-typed, and to use strong-typing to provide the security that Flex had got by using micro-code enforced capabilities. This meant that the type system had to be very powerful, otherwise it would limit the languages that could be supported. Ten15's type system included first-class function types, explicit bounded universal and existential polymorphism, with subtypes and range types. The only other languages (known to the author) that have approached this in expressive power are Cardelli's F<:, and Quest, and some unimplemented 'paper' languages, such as 'Fun' used in this excellent computer surveys paper on type systems: On understanding types, data abstraction, and polymorphism, by Cardelli and Wegner. Ten15 also had a type Type, the type of values that represent a type at runtime, but this was usually only needed for the interactive shell, for debugging, and for implementing run-time typed languages, such as LISP.

Some novel features of the type system, such as unique types, and a solution to the problem of typing updateable values never, to my knowledge, even got published (until now that is, if you are interested, see my articles below).

Rather than using just-in-time compilation as we now know it, to speed up the execution of this code, Ten15 did the compilation as part of the loading process. Thus by the time you came to run something, it had already been converted to optimised binary instructions for the actual hardware you were running on.

Non-main memory types

As we have said, every aspect of Ten15 was strongly-typed. This included objects stored in the persistent object store (on disk), and references to objects on remote Ten15 machines, accessed over the network.

The Ten15 filestore was modelled on that of Flex, with a mostly non-overwriting structure, and object roots stored in presistent variables updated using two-phase commit algorithms from database theory, so that the filestore was never in an inconsistent state. It was also possible to commit groups of persistent variables simultaniously, thus providing transactions, and this was also more efficient (used fewer writes to disk) than a sequence of single-variable commits. A three-phase commit algorithm was designed (and, I believe, implemented), that allowed distributed transactions modifying multiple filestores accessed over the network. The filestore was garbage collected, like the Flex filestore.

Of course, there was nothing to stop Ten15 systems from communicating with non-Ten15 systems using, say TCP/IP, or from reading non-Ten15 files. The results would still have had a good, strong type, probably "array of Byte", and would have to have been parsed into higher-level types. This is no different from the way that current systems usually operate. The difference when accessing native Ten15 data was that this parsing and type recovery was carried out transparently by the underlying system, and the application only had to deal with the high-level, parsed representation.

Languages on Ten15

The main language for programming the Ten15 system was the Ten15 language. This was the closest thing to an assembler language for Ten15, but was more like an extended Standard ML. It took very much the role of C# in Microsoft's .NET, being the language that most readily mapped onto the virtual machine, and the only language with full access to all the virtual machine's facilities. Algol-68 was also very important, since most of the Flex code was written in Algol-68, and an Ada compiler was being developed. Of course, any language could interwork with any other, so long as the data being transferred didn't use facilities (or types) beyond either language's capabilities.

Unfortunately, the one language Ten15 could not support was C, since C is not adequately typed, and this - in the era of C's rising dominance - was a big reason for Ten15's downfall. Ten15 was seen as being increasingly marginalized by everyone's desire to run C, and increasingly C++,  programs. This is somewhat ironic given the recent proliferation and popularity of perl, python, ruby, Java, C#, etc, all of which would have mapped nicely onto Ten15.

Low-Level Ten15 and TDF

In order to make the porting of Ten15 easier, I suggested (I was attempting to port Ten15 to the 680X0 at the time!) that a lower-level, intermediate language be defined. This would be the un-typed output of the Ten15 compiler, after type-checking had succeeded, and all the target-independent optimisations had been applied. In other words, it would be the input to the target-dependent part of the Ten15 load-time compiler, which would become the only part of the compiler that would need to be re-targeted to run Ten15 on a new computer architecture. In other respects this new intermediate language was much like normal Ten15 code, being a parse tree of a lambda-calculus-like language. It became called "low-level Ten15".

It was realised that low-level Ten15, since it was untyped, could actually be the target of weakly-typed languages, such as C and C++, and when the Open Software Foundation (OSF) sent out their request for technologies for ANDF, the Architecturally-Neutral Distribution Format, low-level Ten15 was renamed TDF (standing for "Ten15 Distribution Format"), and proposed to the OSF. This bid was eventually successful, with TDF winning the OSF competition for ANDF, but in the process of making the application, work on Ten15 proper was halted, and as far as I know, never resumed.

Since then, TDF has itself had a checkered history. Having met all the OSF requirements, and winning the ANDF competition, the take-up of TDF was disappointing. After some years, the whole toolkit, including an optimizing C/C++ compiler and installation tools for a number of target platforms, was renamed "TenDRA", and released as open-source. It is even available as a Debian package! Very recently there seems to be a re-newal of interest with the setting up of TenDRA.org, and new developments appear to be happening. See TenDRA.org for more information about the history and past and future development of TDF.

It appears that the TDF acronym was retrospectively re-defined to mean "TenDRA Disribution Format", despite the fact that TDF pre-dates TenDRA by at least 9 years. Thus pushing references to Ten15 further into the mists of history....

Specific Topics

The Ten15 Type system

The Ten15 run-time system

References

  1. The Algebraic Specification of a Target Machine: Ten15, J M Foster, Chapter 9 in ??(pages 198-225). Computer Systems Series. Published by Pitman, London, 1989.
  2. Ten15: An Abstract Machine for Portable Environments, Ian F. Currie, J. M. Foster, P. W. Core, in Proceedings of ESEC '87, 1st European Software Engineering Conference, edited by Howard K. Nichols and Dan Simpson, Strasbourg, France, September 9-11, 1987. No 289 in the Lecture Notes in Computer Science series, published by Springer Verlag, 1987. ISBN 3-540-18712-X.
  3. Remote Capabilities, J. M. Foster, Ian F. Currie. The Computer Journal 30(5): 451-457 (1987)
  4. Validating Microcode Algebraically, J. M. Foster. The Computer Journal 29(5): 416-422 (1986)
  5. Some IPSE aspects of the FLEX project, I.F. Currie, Chapter 6 in Integrated project support environments (pages 76-85), edited by John McDermid. Published by Peter Peregrinus Ltd, 1985 (ISBN 0 86341 050 2). This book is the proceedings of "The Conference on Integrated Project Support Environments" held at the University of York (U.K.), 10-12 April, 1985.

To be added:

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