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From: jallen@csserv1.ic.sunysb.edu (Joseph Allen)
Newsgroups: comp.arch
Subject: Re: Segmented Architectures ( formerly Re: 48-bit computers)
Message-ID: <1991Apr5.200219.6451@sbcs.sunysb.edu>
Date: 5 Apr 91 20:02:19 GMT
References: <23615@as0c.sei.cmu.edu> <4919@lib.tmc.edu> <1991Apr04.234928.8637@iecc.cambridge.ma.us>
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Organization: State University of New York at Stony Brook
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In article <1991Apr04.234928.8637@iecc.cambridge.ma.us> johnl@iecc.cambridge.ma.us (John R. Levine) writes:
>A segmented address space need not be a disaster for large arrays, though
>the much reviled Intel implementation is for two reasons:

>  -- Segment arithmetic is very complicated due to Intel's inexplicable
>     decision to make the low three bits of the segment number magic.

>  -- Loading a segment register is so slow on existing implementations (on
>     a 486, a segment load takes 6 cycles, a regular load takes 1) that
>     you have to handle intrasegment addresses differently from

Though this doesn't have as much to do with large arrays, here's another intel
(386) segement gripe:

  -- The segment bounds register is only 20 bits.  This means you're limited
     to 1MB segments or 4GB segments with 4K pages.  This is a big problem
     when you want to impliment mapped files with intel segemnts:  you can't
     make the file grow a byte at a time (automatically) unless they're less
     than 1MB.  (You could kludge it by switching between 1MB and 4GB modes and
     by changing the base address- but that's stupid and inefficient).

--
#define h 23 /* Height */         /* jallen@ic.sunysb.edu (129.49.12.74) */
#define w 79 /* Width */                       /* Amazing */
int i,r,b[]={-w,w,1,-1},d,a[w*h];m(p){a[p]=1;while(d=(p>2*w?!a[p-w-w]?1:0:0)|(
p<w*(h-2)?!a[p+w+w]?2:0:0)|(p%w!=w-2?!a[p+2]?4:0:0)|(p%w!=1?!a[p-2]?8:0:0)){do
i=3&(r=(r*57+1))/d;while(!(d&(1<<i)));a[p+b[i]]=1;m(p+2*b[i]);}}main(){r=time(
0L);m(w+1);for(i=0;i%w?0:printf("\n"),i!=w*h;i++)printf(a[i]?" ":"#");}