add summary of layers

_posts/2017-03-03-layer-summary.md

@@ -0,0 +1,93 @@
+---
+layout: news
+author: Torsten
+---
+
+Going on holiday without a computer was great. Forcing me to recap and write things down on paper.
+
+## Layers
+
+One of the main results was that the current layers are a bit mixed up and that will have to be
+fixed. But first, some of the properties in which i think of the different layers.
+
+### Layer properties
+
+**Structure of the representation** is one of the main distinction of the layers. We know the parser gives us a **tree** and that the produced binary is a **blob**, but what in between. As options we would still have graphs and lists.
+
+A closely related property of the representation is whether it is **abstract or concrete**.
+An abstract representation is represented as a single class in ruby and it's properties are
+accessible through an abstract interface, like a hash. A concrete representation would use
+a class per type, have properties available as ruby attributes and thus allow functions on the
+class.
+
+If we think of the layer as a language, what **Language level** would it be, assembler, c, oo.
+Does it have **control structures**, or **jumps**.
+
+### Ruby Layer
+
+The top ruby layer is a given, since it is provided by the external gem *parser*.
+Parser outputs an abstract syntax tree (AST), so it is a *tree*. Also it is abstract, thus
+represented by a single ruby class, which carries a type as an attribute.
+
+What might sound self-evident that this layer is very close to ruby, this means that inherits
+all of ruby's quirks, and all the redundancy that makes ruby a nice language. By quirks i mean
+things like the integer 0 being true in an if statement. A good example of redundancy is the
+existence of if and until, or the ability to add if after the statement.
+
+### Virtual Language
+
+The next layer down, and the first to be defined in ruby-x, is the virtual language layer.
+By language i mean object oriented language, and by virtual an non existent minimal version of an
+object oriented language. This is like ruby, but without the quirks or redundancy. This is
+meant to be compatible with other oo languages, meaning that it should be possible to transform
+a python or smalltalk program into this layer.
+
+The layer is represented as a concrete tree and derived from the ast by removing:
+- unless, the ternary operator and post conditionals
+- splats and multi-assignment
+- implicit block passing
+- case statement
+- global variables
+
+It should be relatively obvious how these can be replaced by existing constructs (details in code)
+
+### Virtual object Machine
+
+The next down represents what we think of as a machine, more than a language, and an object
+oriented at that.
+
+A differentiating factor is that a machine has no control structures like a language. Only jumps.
+The logical structure is more a stream or array. Something closer to the memory that
+i will map to in lower layers. We still use a tree representation for this level, but with the
+interpretation that neighboring children get implicitly jumped to.
+
+The machine deals in objects, not in memory as a von Neumann machine would. The machine has
+instructions to move data from one object to another. There are no registers, just objects.
+Also basic arithmetic and testing is covered by the instruction set.
+
+### Risc layer
+
+This layer is a minimal abstraction of an arm processor. Ie there are eight registers, instructions
+to and from memory and between registers. Basic integer operations work on registers. So does
+testing, and off course there are jumps. While the layer deals in random access memory, it is
+aware and uses the object machines objects.
+
+The layer is minimal in the sense that it defines only instructions needed to implement ruby.
+Instructions are defined in a concrete manner, ie one class per Instruction, which make the
+set of Instructions extensible by other gems.
+
+The structure is a linked list which is manly interested in three types of Instructions. Namely
+Jumps, jump targets (Labels), and all other. All the other Instructions a linear in the von Neumann
+sense, that the next instruction will be executed implicitly.
+
+### Arm and elf Layer
+
+The mapping of the risc layer to the arm layer is very straightforward, basically one to one with
+the exception of constant loading (which is quirky on the arm 32 bit due to historical reasons).
+Arm instructions (being instructions of a real cpu), have the ability to assemble themselves into
+binary, which apart from the loading are 4 bytes.
+
+The structure of the Arm instruction is the same as the risc layer, a linked list.
+
+There is also code to assemble the objects, and with the instruction stream make a binary elf
+executable. While elf support is minimal, the executable does execute on rasperry pi or qemu.