122 lines
5.5 KiB
HTML
122 lines
5.5 KiB
HTML
---
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layout: crystal
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title: Crystal, a simple and minimal oo machine
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---
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<div class="row vspace10">
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<div class="span12 center">
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<h3><span>Crystal layers</span></h3>
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<p>Map pretty much to top level directories.</p>
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</div>
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</div>
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<div class="row vspace20">
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<div class="span11">
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<h5>Machine Code, bare metal</h5>
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<p>
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This is the code in arm directory. It creates binary code according to the arm specs. All about shifting bits in the
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right way.
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<br/>
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As an abstraction it is not far away from assembler. I mapped the memnonics to function calls and the registers
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can be symbols or Values (from vm). But on the whole this is as low level as it gets.
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<br/>
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Different types of instructions are implemented by different classes. To make machine dependant code possible,
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those classes are derived from Vm versions.
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<br/>
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There is an intel directory which contains an expanded version of wilson, but it has yet to be made to fit into
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the architecture. So for now crystal produces arm code.
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</p>
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</div>
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</div>
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<div class="row">
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<div class="span12">
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<h5>Parsing, forever descending</h5>
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<p>
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Parsing is relatively straightforward too. We all know ruby, so it's just a matter of getting the rules right.
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<br/>
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If only! Ruby is full of niceties that actually make parsing it quite difficult. But at the moment that story hasn't
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even started.
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<br/>
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Traditionally, yacc or bison or talk of lr or ll would come in here and all but a few would zone out. But llvm has
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proven that recursive descent parsing is a viable alternative, also for big projects. And Parslet puts that into a nice
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ruby framework for us.
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<br/>
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Parslet lets us use modules for parts of the parser, so those files are pretty self-explanitory. Not all is done, but
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a good start.
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<br/>
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Parslet also has a seperate Transformation pass, and that creates the AST. Those class names are also
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easy, so you can guess what an IfExpression represents.
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<br/>
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</p>
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</div>
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</div>
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<div class="row">
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<div class="span12">
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<h5>Virtual Machine</h5>
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<p>
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The Virtual machine layer is where it gets interesting, but also a little fuzzy.
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<br/>
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After some trying around the virtual machine layer has become a completely self contained layer to describe and
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implement an oo machine. In other words it has no reference to any physical machine, that is the next layer down.
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<br/>
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One can get headaches quite easily while thinking about implementing an oo machine in oo, it's just so difficult to
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find the boundaries. To determine those, i like to talk of types (not classes) for the objects (values) in which the
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vm is implemented. Also it is neccessary to remove ambiguity about what message sending means.
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<br/>
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One way to think of this (helps to keep sane) is to think of the types of the system known at compile time. In the
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simplest case this could be object reference and integer. The whole vm functionality can be made to work with only
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those two types, and it is not specified how the type information is stored. but off course there needs to be a
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way to check it at run-time.
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<br/>
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The vm has an instruction set that, apart from basic integer manipulation, only alows for memory access into an
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object. Instead of an implicit stack, we use activation frames and store all variables explicitly.
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</p>
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</p>
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</div>
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</div>
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<div class="row">
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<div class="span12">
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<h5>Neumann Machine</h5>
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<p>
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The von Neumann machine layer is a relatively close abstraction of hardware.
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<br/>
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Currently still quite simple, we have Classes for things we know, like program and function. Also things we need
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to create the code, like Blocks and Instructions.
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<br/>
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The most interesting thing is maybe the idea of a Value. If you think of Variables, Values are what a variable may
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be assigned, but it may carry a storage place (register). Values are constant, and so to
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change a value, we have to create a new Value (of possibly different basic type). Thus
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all machine instructions are the transformation of values into new ones.
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<br/>
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Also interesting is the slightly unripe Basic Type system. We have a set of machine-word size types and do not
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tag them (like mri or BB), but keep type info seperate. These types include integer (signed/unsigned) object reference
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and function. Most of the oo machine will build on object references. To make that clearer: The (virtual)machine is
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strongly typed (with rtti) and the dynamic ruby behaviour it implemented using that basic type system.
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</p>
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</p>
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</div>
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</div>
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<div class="row">
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<div class="span12">
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<h5>The flux</h5>
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<p>
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This is just a section of things that are unclear, in flux as it were. This does not included undone things, those
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are plenty too.
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<ul>
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<li> the whole type system, more of values, for object types its quite clear</li>
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<li> booting. There is this Dichotomy of writing code, and figuring out what it should do when it executes,
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that works ok, until i try to think of both, like for booting. </li>
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<li> the oo machine abstraction. Currently non-existant i feel like there is a whole layer missing. Possibly
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with it's own intruction set</li>
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<li> where the core ends, parfait starts and what can be external. </li>
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</ul>
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</p>
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</div>
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</div>
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