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+## Copyright (C) 1993, 1994, 1995 Auburn University.  All rights reserved.
+##
+## This file is part of Octave.
+##
+## Octave is free software; you can redistribute it and/or modify it
+## under the terms of the GNU General Public License as published by the
+## Free Software Foundation; either version 2, or (at your option) any
+## later version.
+##
+## Octave is distributed in the hope that it will be useful, but WITHOUT
+## ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+## FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+## for more details.
+##
+## You should have received a copy of the GNU General Public License
+## along with Octave; see the file COPYING.  If not, write to the Free
+## Software Foundation, 59 Temple Place, Suite 330, Boston, MA 02111 USA.
+
+## -*- texinfo -*-
+## @deftypefn {Function File} {[@var{n}, @var{m}, @var{p}] =} abcddim (@var{a}, @var{b}, @var{c}, @var{d})
+## Check for compatibility of the dimensions of the matrices defining
+## the linear system
+## @iftex
+## @tex
+## $[A, B, C, D]$ corresponding to
+## $$
+## \eqalign{
+##  {dx\over dt} &= A x + B u\cr
+##             y &= C x + D u}
+## $$
+## @end tex
+## @end iftex
+## @ifinfo
+## [A, B, C, D] corresponding to
+##
+## @example
+## dx/dt = a x + b u
+## y = c x + d u
+## @end example
+##
+## @end ifinfo
+## or a similar discrete-time system.
+##
+## If the matrices are compatibly dimensioned, then @code{abcddim} returns
+##
+## @table @var
+## @item n
+## The number of system states.
+##
+## @item m
+## The number of system inputs.
+##
+## @item p
+## The number of system outputs.
+## @end table
+##
+## Otherwise @code{abcddim} returns @var{n} = @var{m} = @var{p} = @minus{}1.
+##
+## Note: n = 0 (pure gain block) is returned without warning.
+##
+## @end deftypefn
+## @seealso{is_abcd}
+
+## Author: A. S. Hodel <a.s.hodel@eng.auburn.edu>
+## Created: August 1993.
+## a s hodel: modified to accept pure-gain systems aug 1996
+
+function [n, m, p] = abcddim (a, b, c, d)
+
+  if (nargin != 4)
+    error ("abcddim: four arguments required");
+  endif
+
+  n = m = p = -1;
+
+  [a,an,am] = abcddims(a);
+  [b,bn,bm] = abcddims(b);
+  [c,cn,cm] = abcddims(c);
+  [d,dn,dm] = abcddims(d);
+
+  if ( (!is_square(a)) & (!isempty(a)) )
+    warning (["abcddim: a is not square (",num2str(an),"x",num2str(am),")"]);
+    return
+  endif
+
+  if( (bm == 0) & (dm == 0) )
+    warning("abcddim: no inputs");
+  elseif (bn != am)
+    warning (["abcddim: a(",num2str(an),"x",num2str(am), ...
+      " and b(",num2str(bn),"x",num2str(bm),") are not compatible"]);
+    return
+  endif
+
+  if( (cn == 0) & (dn == 0 ) )
+    warning("abcddim: no outputs");
+  elseif (cm != an)
+    warning (["abcddim: a(",num2str(an),"x",num2str(am), ...
+        " and c(",num2str(cn),"x",num2str(cm),") are not compatible"]);
+    return
+  endif
+
+  have_connections = (bn*cn != 0);
+
+  if( (dn == 0) & have_connections)
+    warning("abcddim: empty d matrix passed; setting compatibly with b, c");
+    [d,dn,dm] = abcddims(zeros(cn,bm));
+  endif
+
+  if(an > 0)
+    [dn, dm] = size(d);
+    if ( (cn != dn) & have_connections )
+      warning (["abcddim: c(",num2str(cn),"x",num2str(cm), ...
+        " and d(",num2str(dn),"x",num2str(dm),") are not compatible"]);
+      return
+    endif
+
+    if ( (bm != dm) & have_connections )
+      warning (["abcddim: b(",num2str(bn),"x",num2str(bm), ...
+          " and d(",num2str(dn),"x",num2str(dm),") are not compatible"]);
+      return
+    endif
+
+    m = bm;
+    p = cn;
+  else
+    [p,m] = size(d);
+  endif
+  n = an;
+endfunction