Path: utzoo!mnetor!uunet!epiwrl!epimass!jbuck
From: jbuck@epimass.EPI.COM (Joe Buck)
Newsgroups: comp.arch
Subject: Re: RISC is a nasty no-no!
Message-ID: <1970@epimass.EPI.COM>
Date: 4 Mar 88 17:34:48 GMT
References: <179@wsccs.UUCP: <696@nuchat.UUCP> <284@scdpyr.UUCP> <11199@duke.cs.duke.edu> <998@PT.CS.CMU.EDU>
Reply-To: jbuck@epimass.EPI.COM (Joe Buck)
Organization: Entropic Processing, Inc., Cupertino, CA
Lines: 23

In article <998@PT.CS.CMU.EDU> edw@IUS1.CS.CMU.EDU (Eddie Wyatt) writes:
>> Multiplication is not necessary to access 2-D arrays if the array is
>> set up like most arrays in C, where each row is a typical vector and
>> the 2-D array is just a vector of pointers to each row vector.
>EXCUSE ME, but when I declare an array to be an n by m matrix in
>C (float foo[n][m]) I get a contiguous block of memory.  The
>representation IS NOT row-vector or column-vector.  So when I access
>index number i,j someone has to perform the calculation 
>base + sizeof(type)*(i*m+j).

No, the compiler doesn't have to do things that way.  It can create
an extra row of pointers to the beginning of each row:  p_foo[n],
where p_foo[i] contains &foo[i][0].  Then foo[i][j] is at *p_foo[i] +
j.  No multiply required, but there might be a shift or two to
accomodate two-byte or four-byte array elements.  The old Fortran
compiler for PDP-11's running RT-11 had an option to do this:  you're
trading off speed for space.  They called it "array vectoring".  The
array itself is still allocated the way you think it is, but there
are extra pointers.

-- 
- Joe Buck  {uunet,ucbvax,sun,<smart-site>}!epimass.epi.com!jbuck
	    Old Internet mailers: jbuck%epimass.epi.com@uunet.uu.net