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Path: utzoo!news-server.csri.toronto.edu!cs.utexas.edu!swrinde!elroy.jpl.nasa.gov!decwrl!world!paradigm!gjc
From: gjc@paradigm.com
Newsgroups: comp.lang.scheme,comp.lang.lisp,comp.lang.smalltalk
Subject: Re: implementing an interpreter
Message-ID: <3653@paradigm.com>
Date: 14 Mar 91 17:42:32 GMT
References: <1991Mar8.153120.19836@tripos.com> <50211@apple.Apple.COM>
Distribution: comp
Organization: Paradigm Associates Inc, Cambridge MA
Lines: 33

In article <50211@apple.Apple.COM>, mikel@Apple.COM (Mikel Evins) writes:
> 
> A third approach is the Big Bag of Pages (BiBoP) arrangement, in
> which objects of various types are each allocated only in certain
> ranges of addresses. In this way, an object's type can be
> determined by examining its address. This scheme somewhat complicates
> an already somewhat complicated gc algorithm if you plan to
> use, let us say, generation scavenging, but type determination
> is fairly cheap.

No heaper than the smalltalk technique that the poster mentioned.
(Because of the extra shift. Although you may need a shift anyway).

In fact, as far as types are concerned the smalltalk technique
could be considered BIBOP with a page size of 1 word.

int bibop_typeof(ptr)
 LISP ptr;
{return(type_table[((long)ptr) >> 9].typeof);}

Although specific type checking is usually optimized by having a precooked
set of bits, thereby avoiding:

(defun numberp (x) (memq (type-of x) '(fixnum bignum flonum))

Instead:

LISP numberp(ptr)
 LISP ptr;
{if (type_table[((long)ptr) >> 9].numberp)
  return(A_TRUE_VALUE);
 else
  return(A_FALSE_VALUE);}