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+* 5.3 Storage Allocation & Garbage Collection
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+we'll assume that our register machine is equipped with a
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+ list-structured memory
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+
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+this is useful but not realistic
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+
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+we will look at how it's implemented. how do we represent the
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+"box-and-pointer" structure of lisp pairs using the storage and
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+addressing of typical computing machines.
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+
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+there are two considerations:
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+ - how to represent it
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+ - how to manage memory as computation continues
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+
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+lisp will be constantly creating new data objects, not all computers
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+have infinite memory. therefore lisp has automatic storage
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+allocation. in this case, /garbage collection/.
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+
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+** 5.3.1 memory as vectors
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+conventional computer memory can be thought of as an array of
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+cubbyholes. each cubby has a unique name--its address.
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+memory systems offer two functions:
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+ - one that fetches data from some location
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+ - one that assigns new data to some location
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+
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+there must be some way to treat memory itself as data. the list
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+structure is one technique of address arithmetic.
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+
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+we will model memory as a VECTOR
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+
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+a vector is a compound data object with elements that can be accessed
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+by an integer index in constant time
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+
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+: (vector-ref vector n)
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+: (vector-set! vector n value)
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+
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+we will imagine computer memory is divided up into two vectors,
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+ the-cars and the-cdrs
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+
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+there also must be some way to deal with primitive data, and a way to
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+identify its type. for this, we tag pointers with the datum's type.
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+
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+two objects are (eq?) if they have the same pointer
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+
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+character strings are stored in the obarray, where each string is
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+stored only once.
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+
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+** 5.3.2 maintaining the illusion of infinite memory
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+5.3.1 solves the implementation of list structure...
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+ if we have infinite memory!
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+
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+the technique of garbage collection used here is "stop-and-copy"
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+
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+it shifts memory back and forth between working memory and free
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+memory, copying the useful stuff to free memory and leaving the crap
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+behind in what is now the old working memory
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+
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+abandoned objects are tagged with a "broken heart"
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