ruby-x.github.io/salama/layers.html

---
layout: salama
title: Salama architectural layers
---


<div class="row span10">
    <h4>Main Layers</h4>
    <p>
      To implement an object system to execute object oriented languages takes a large system.
      The parts or abstraction layers are detailed below.</br>
      It is important to undrstand the approach first though, as it differs from the normal
      interpretation. The idea is to compile (eg) ruby. It may be easiest to compare to a static
      object oriented language like c++. When c++ was created c++ code was translated into c, which
      then gets translated into assembler, which gets translated to binary code, which is linked
      and executed. Compiling to binaries is what gives these languages speed, and is one reason
      to compile ruby.       </br>
      In a similar way to the c++ example, we need language between ruby and assembler, as it is too
      big a mental step from ruby to assembler. Off course course one could try to compile to c, but
      since c is not object oriented that would mean dealing with all off c's non oo heritance, like
      linking model, memory model, calling convention etc. (more on this in the book) <br/>
      The layers are:
      <ul>
        <li> <b> Binary and cpu specific assembler.</b>  This includes arm assembly and elf support
              to produce a binary that can then read in ruby programs</li>
        <li> <b> Risc register machine abstraction </b> provides a level of machine abstraction, but
                  as the name says, quite a simle one.</li>
        <li> <b> Soml, Salama object machine language, </b> which is like our object c. Statically
                typed object oriented with object oriented call sematics. </li>
        <li> <b> Salama </b> , which is the layer compiling ruby code into soml and includes
            bootstraping code</li>
      </ul>
    </p>
</div>

<div class="row span10">
    <h5>Binary , Arm and Elf</h5>
    <p>
      A physical machine will run binaries containing intructions that the cpu understands. With arm
      being our main target, this means we need code to produce binary, which is contained in a
      seperate module <a href="https://github.com/salama/salama-arm"> salama-arm </a>. <br/>
      To be able to run code on a unix based operating system, binaries need to be packaged in a
      way that the os understands, so minimal elf support is included in the package. <br/>
      Arm is a risc architecture, but anyone who knows it will attest, with it's own quirks.
      For example any instruction may be executed conditionally in arm. Or there is no 32bit
      register load instruction. It is possible to create very dense code using all the arm
      special features, but this is not implemented yet.
    </p>
</div>

<div class="row span10">
    <h5>Register Machine</h5>
    <p>
      The Register machine layer is a relatively close abstraction of risc hardware, but without the
      quirks.
      <br/>
      The register machine has registers, indexed addressing, operators, branches and everything
      needed for the next layer. It doesn not try to abstract every possible machine leature
      (like llvm), but rather "objectifies" the risc view to provide what is needed for soml, the
      next layer up.
      <br/>
      The machine has it's own (abstract) instruction set, and the mapping to arm is quite
      straightforward. Since the instruction set is implemented as derived classes, additional
      instructions may be defined and used later, as long as translation is provided for them too.
      In other words the instruction set is extensible (unlike cpu instruction sets).
    </p>
    <p>
      Basic object oriented concepts are needed already at this level, to be able to generate a whole
      self contained system. Ie what an object is, a class, a method etc. This minimal runtime is called
      parfait and will be coded in soml eventually. But since it is the same objects at runtime and
      compile time, it will then be translated back to ruby for use at compile time. Currenty there
      are two versions of the code, in ruby and soml, being hand synchronized. More about parfait below.
    </p>
    <p>
      Since working with at this low machine level (essentially assembler) is not easy to follow for
      everyone, an interpreter was created. Later a graphical interface, a kind of
      <a href="https://github.com/salama/salama-debugger"> visual debugger </a> was added.
      Visualizing the control flow and being able to see values updated immediately helped
      tremendously in creating this layer. And the interpreter helps in testing, ie keeping it
      working in the face of developer change.
    </p>
</div>

<div class="row span10">
    <h5>Soml, Salama object machine language</h5>
    <p>
      Soml is probably the larest single part of the system and much more information can be found
      <a href="/soml/soml.html"> here </a>.
      <br/>
      Before soml, a more traditional virtual machine approach was taken and abandoned. The language
      is easy to understand and provides a good abstraction, both in terms of object orienteation,
      and in terms of how this is expressed in the register model. <br/>
      It is like ruby with out the dynamic aspects, but typed. <br/>
      In broad strokes it consists off:
      <ul>
        <li> <b> Parser:</b> Currently a peg parser, though a hand coded one is planned.
                  The result of which is an AST</li>
        <li> <b> Compiler:</b>  compiles the ast into a sequence of Register instructions.
                  and runtime objects (classes, methods etc)</li>
        <li> <b> Parfait: </b> Is the runtime, ie the minimal set of objects needed to
                  create a binary with the required information to be dynamic</li>
        <li> <b> Builtin: </b>  A very small set of primitives that are impossible to express
                  in soml (remembering that parfait will be expressed in soml eventually)</li>
      </ul>
    </p>
    <p>
    Just to summarize a few of soml features that are maybe unusual:
    <ul>
      <li> <b> Mesage based calling:</b> Calling is completely object oriented (not stack based)
              and uses Message and Frame objects.</li>
      <li> <b> Return addresses:</b>  A soml method call may return to several addresses, according
              to type, and in case of exception</li>
      <li> <b> Overloaded arguments </b> A method is defined by name, but may have several
                  implementations for different types of the arguments (statically matched)</li>
    </ul>
  </p>
</div>

<div class="row span10">
    <h5>Salama</h5>
    <p>
      To compile and run ruby, we need to parse and compile ruby code. To compile ruby to soml a clear
      mapping has to be achieved. Particularly the dynamic aspects, and typing need to be addressed.
      <br/>
      While parsing ruby is quite a difficult task, it has already been implemented in pure ruby
      <a href="https://github.com/whitequark/parser"> here </a>. The output of the parser is again
      an ast, which needs to be compiled to soml. <br/>
      The dynamic aspects of ruby are actually realtively easy to handle, once the whole system is
      in place, because the whole system is written in ruby without external dependencies.
      Since (when finished) it can compile ruby, it can do so to produce a binary. This binary can
      then contain the whole of the system, and so the resulting binary will be able to produce
      binary code when it runs. With small changes to the linking process (easy in ruby!) it can
      then extend itself.
    </p>
    <p>
      The type aspect is more tricky: Ruby is not typed and soml is after all. And if everything
      were objects (as we like to pretend in ruby) we could just do a lot of dynamic checking,
      possibly later introduce some caching. But everything is not an object, minimally integers
      are not, but maybe also floats and other values. The destinction between what is an integer
      and what an object has sprouted an elaborate type system, which is (by necessity) present in
      soml (see there).
    </p>
    <p>
      The idea (because it hasn't been implemented yet) is to have different functions for different
      types. The soml layer defines the Type class and BasicTypes and also lets us return to different
      places from a function (in effect a soml function call is like an if). By using this, we can
      compile a single ruby method into several soml functtions. Each such function is typed, ie all
      arguments and variables are of known type. According to these types we can call functions according
      to their signatures. Also we can autognerate error methods for unhandled types, and predict
      that only a fraction of the possible combinations will actually be needed.
    </p>
</div>