* The Explicit-Control Evaluator
this is how to implement /scheme itself/ on bare metal. cool as hell.

the register machine contains a stack and seven registers:
  - exp
  - env
  - val
  - continue
  - proc
  - argl
  - unev

** 5.4.1 the core of the explicit-control evaluator
the central element of the evaluator is the sequence of instructions
beginning at eval-dispatch

after eval, the controller goes the the entry point stored in continue
and the val register holds the value of the expression

eval-dispatch is a case analysis on the syntactic type of expression
to be evaluated

** 5.4.2 sequence evaluation and tail recursion
this handles sequences of expressions in procedure bodies or in begin
expressions. 

TAIL RECURSION
a procedure such as
#+BEGIN_SRC scheme
  (define (sqrt-iter guess x)
    (if (good-enuff? guess x)
	guess
	(sqrt-iter (improve guess x)
		   x)))
#+END_SRC
is syntactically recursive but operationally iterative. but only with
tail recursion.

  - tail-recursive evaluator :: an evaluator that can execute a
       procedure such as sqrt-iter without requiring increasing
       storage as the procedure continues to call itself

implementing tail recursion is a simple, universal, and profound thing

** 5.4.3 conditionals, assignment, and definitions
special forms are handles by selectively evaluating fragments of the
expression. assignments and definitions are handled similarly with
set-variable-value!

** 5.4.4 running the evaluator
with the implementation of the evaluator we have come to the
conclusion of a development started back in chapter 1, exploring more
precise models of evaluation:
  - substitution
    - relatively informal
  - environment
    - deal with state and change
  - metacircular evaluator
    - using scheme itself
  - register machines
    - a close look at the mechanisms for storage management, argument
      passing, and control
each layer raised issues and ambiguities that were not apparent at the
previous levels.

now we can implement and monitor the performance of our machine at its
basest levels.

the simulation will of course be slow (it's a simulation running in a
simulation running in a simulation...) but we can do cool things like
monitor and evaluate its performance.