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Sun Apr 19 11:27:37 CEST 2009

what is an assembly instruction?


SYNTAX:

   [asm] -> [asm]

      An abstract data type to be manipulated by coma macros.  This
      type has expansion-time information that checks constraints on
      the syntax: name and arity. (and later possibly type).

SEMANTICS:

   (asm,here,operands) -> [bin]

      Assembly instructions can be _compiled_ to binary machine code,
      relative to the location at which they are supposed to be
      placed.  Note that operands need to be bit vectors.  This
      function is called a primitive assembler.  The (partial) inverse
      of this is called a primitive disassembler.

   (asm,here,operands) -> (m -> m)

      The primitive simulator is merely a different representation of
      the semantics, accessible from the syntax manipulation phase.
      It translates a fully specified instruction into a machine state
      update function, enabling the instruction to be _interpreted_.

GRAPH:

   ([asm],dict,start) -> [bin]

      Instances of asm structures can contain symbolic information
      which eventually depends on the size of the [bin] output of
      primitive assemblers.  This circularity is broken by sucessive
      approximation using a relaxation algorithm.



Anything that translates to [bin] looses information: in this stage,
all symbolic/parameterized information is gone.

The interesting parts are the coma [asm] -> [asm] translation and the
interpretation steps (asm,here,operands) -> (m -> m).  If the latter
can be made to perform abstract interpretation, a lot of checks can be
constructed.

It looks like the important conclusions are:

   - flexibility of representation of instruction semantics (m -> m)
     will determine how the information encoded in these functions can
     be fed back to [asm] -> [asm] transformers (i.e. to generate
     peephole optimizations).  however, this can be added later on, on
     top of basic static structure.

   - practically: information about the instruction's type needs a
     compile-time binding.



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