Sunday, 12 April 2009

Protel I

Protel is the PRocedure Oriented Type Enforcing Language used for most DMS software. Currently there's not much information available about it online. An early paper describing the language is referenced here, but is hidden behind a subscription-only portal. Wikipedia offers a very minimal definition of the term but little else. I think there's some good stuff that should be recorded so I'll attempt to describe what I found interesting.

So what is Protel like? I'm told that it's similar to Modula-2 and even Modula-3 and it's true that it shares explicit BEGIN / END block syntax with Pascal, and all code is divided into modules.
One of the most basic differences between Protel and these languages is its use of the composite symbol '->' or 'Gozinta' (Goes into) for assignment. This eliminates any confusion between assignment and equality testing. This 'removal of ambiguity' is a key pattern in the design of Protel. Similar manifestations of the pattern are the rules :
  • No operator precedence rules
    Unlike C, Protel assigns no built-in relative precedence to various operators. All expressions are evaluated left-to-right, and the programmer must use brackets explicitly to specify non l2r evaluation.
  • No preprocessor
    There is no standard preprocessor and therefore no macro language. The compiler supports some limited compile time expression evaluation including sizeof() for types etc. This avoids context specific semantics for source code and non-visible code expansion
  • No support for pointer arithmetic
    Where a pointer is to be treated as referring to some element of an array, the descriptior mechanism, which includes bounds checking, is to be used rather than pointer arithmetic.
  • Control structure specific end-of-block keywords
    Rather than a single END keyword, Protel employs ENDBLOCK, ENDPROC, ENDWHILE, etc. to aid code readability.
  • No 'keystroke reduction mechanisms'
    C's ++, += etc.
These rules can make life harder when writing code and increase verbosity, but can aid readability and reduce the amount of non-local knowledge needed to understand code.

Protel modules contain one or more source code files which can export definitions for use by other modules. Different source files within a module can be arranged into trees which control compile order within a module. Modules often have multiple levels of interface source files - most public and general APIs in the top level, more private and specific APIs in the lower levels. Access to definitions in each interface file can be controlled independently if required.

Basic Protel supports built-in and user defined types, pointers, arrays, an 'array slice' descriptor mechanism, and a novel extensible fixed-size type called an Area as well as some type-reflection capabilities.

A Protel DESCriptor is used to refer to a range of elements in an array of . It is used in the same way an array - with a subsript as an lvalue or rvalue to an expression (though the usual meaning of those terms is confused by the Gozinta operator!). The compiler is aware of the of the slice being DESCribed, and by inference the size of the elements. In the storage allocated to the DESC itself, it stores a pointer to the zeroth element and an upperbound in terms of elements. In this way it can provide bounds checking on accesses through the DESC. When an out-of-bounds exception is hit, the actual upperbound and the supplied subscript are available in the exception report, often allowing debugging straight from the trace. The array slice abstraction can be a nice way to deal with zero-copy in a protocol stack.

Protel offers the BIND keyword which can be used to define a local-scope alias to some variable instance. It's use is encouraged to reduce keystrokes, and it is also useful for indicating to the reader and the compiler that some dereferencing operations need only be performed once even though the referenced value is used multiple times. A side effect of its use is that it reduces the tension between short, easily typed and long, descriptive variable names, allowing long descriptive names to be shortened in use when necessary. Of course this can add to ambiguity and confusion.

Protel supports typed procedure pointers with an explicit type classifier - PROCVAR. I suspect that this type classifier exists to improve code readability rather than for any syntactic necessity as PTR to PROC can be used similarly. PROCVARs are used heavily in SOS code to allow applications to override behaviours and extend OS behaviour. SOS has unique terms for the use of procedure pointers :
  • GATE
    SOS name for a Procedure Variable that is expected to be set by some other module. It is a 'gate' to some other implementation. Usually gates are defined in lower-level modules.
  • TARGET
    SOS name for a procedure implementation referenced from a GATE. This is the 'target' of a call to a 'gate'. Usually targets are defined in higher-level modules
  • ASPECT
    SOS name for a structure containing a number of procedure variables. This can be thought of as an 'interface' in the Java sense - a set of method signatures. Often a lower-level module would provide an API for a higher-level API to add some functionality including some data and perhaps an 'aspect' of procedure variables.

As well as supporting standard composite structures similar to a C struct, Protel supports the concept of an Area which can be used to implement a form of type-inheritance. An AREA is similar to a struct, but contains as part of its definition a storage size in bits, as well as zero or more members. At compile time, it is checked that the declared member's types fit within the size given, and instances will be created with the size given. Other modules can declare further areas which REFINE this area, and add definitions to the original AREA. The compiler will check that the complete set of member definitions continue to fit in the bit size of the original AREA. This mechanism can be used to create trees of hierarchically related data types which is very useful for code modularity and extensibility as well as more basic optionality similar to a C union. Putting procedure pointers into the Area gives a rather rough extensible virtual-method mechanism in-language. However, most DMS software designers used AREA refinements for hierarchically varying data rather than allowing control flow overrides.

Protel offers some reflection capability via the TYPEDESC operator. It is applied to a type and returns a structure which can be used to determine type names, bit offsets etc. SOS supports an online extensible data dictionary which uses TYPEDESC to track types and their relationships. It is version aware and is used by Table Control and for data reformatting between software releases.

Combining PROCVARs and REFINEd AREAs allowed extensible systems to be built fairly easily without OO techniques built into the language. However, the explicit nature of the PROCVARs, the requirement to define up-front bitsizes for refinable areas and the general micromanagement required to define, initialise and use 'object' hierarchies made from these components discouraged most designers from using them in this way. Providing tools at this atomic level encouraged each designer to try their own combination of hard coding, ProcVars, extensible areas, pointers-to-extension-structs, pointers-to-data-structs-with-procvars etc. More manual visualisation effort was required to grasp these mechanisms than would be required for an equivalent language with OO extensions.

I think PROTEL was fairly state-of-the-art when it was introduced for DMS software. Especially considering its planned use for telecoms switching equipment, it is a very general purpose language, not visibly oriented towards telecoms. It has a fairly clean split between language features and runtime libraries. Perhaps if it had been more widely known of beyond the confines of Nortel/BNR then it could have enjoyed some life of its own? I believe it is still being actively used - these days SOS images run in virtualised environments with code written by outsourced employees paid by a broken company, but I imagine there must still be patches getting written. However the outlook looks bleak. With Nortel on the rocks and apparently no interesting information about Protel available on the web (except this blog of course :) ), it looks like it could vanish after 30 years.

In the mid to late nineties, Nortel added object orientation to Protel, I'll talk briefly about that in a future post.

1 comment:

Honeyman said...

Losing such a language would be a real pity. I still miss a lot of its features from the current mainstream languages. The world would probably be a bit safer place if Protel had gained a popularity of C.