rom/utility/smult32.c

   1 /*
   2     Copyright © 1995-2001, The AROS Development Team. All rights reserved.
   3     $Id$
   4
   5     Desc: Signed 32 bit multiplication function.
   6     Lang: english
   7 */
   8 #include "intern.h"
   9
  10 /*****************************************************************************
  11
  12     NAME */
  13 #include <proto/utility.h>
  14
  15         AROS_LH2(LONG, SMult32,
  16
  17 /*  SYNOPSIS */
  18         AROS_LHA(LONG, arg1, D0),
  19         AROS_LHA(LONG, arg2, D1),
  20
  21 /*  LOCATION */
  22         struct UtilityBase *, UtilityBase, 23, Utility)
  23
  24 /*  FUNCTION
  25         Performs the signed 32-bit multiplication of arg1 * arg2 and
  26         returns a signed 32 bit value.
  27
  28     INPUTS
  29         arg1, arg2  -   32 bit signed longs
  30
  31     RESULT
  32         arg1 * arg2
  33
  34     NOTES
  35         This can perform the multiplication either using the machines
  36         native instructions (if they exist), or in software using a
  37         simple algorithm based on expanding algebraic products.
  38
  39         The utility.library math functions are unlike all other utility
  40         functions in that they don't require the library base to be
  41         loaded in register A6, and they also save the values of the
  42         address registers A0/A1.
  43
  44         This function is mainly to support assembly programers, and is
  45         probably of limited use to higher-level language programmers.
  46
  47     EXAMPLE
  48
  49         LONG a = 352543;
  50         LONG b = -52464;
  51         LONG c = SMult32(a,b);
  52         c == -1315946768
  53
  54     BUGS
  55         Of limited use to C programmers.
  56
  57     SEE ALSO
  58         UMult32(), UMult64(), SMult64()
  59
  60     INTERNALS
  61         May be handled by code in config/$(KERNEL), may not be...
  62         It is for m68k-native.
  63
  64         For emulation we are performing the operation:
  65
  66             (2^16 * a + b) * (2^16 * c + d)
  67           = 2^32 * ab + 2^16 * ad + 2^16 * bc + bd
  68           = 2^32 * ab + 2^16 ( ad + bc ) + bd
  69
  70         Now since the result is a 32-bit number, the 2^32 term will have
  71         no effect. (Since 2^32 > max (32-bit number)), as will the
  72         high part of ad + bc.
  73
  74         Therefore:
  75         product = 2^16( ad + bc ) + bd
  76
  77     HISTORY
  78         29-10-95    digulla automatically created from
  79                             utility_lib.fd and clib/utility_protos.h
  80         18-08-96    iaint   Implemented as described above.
  81
  82 *****************************************************************************/
  83 {
  84     AROS_LIBFUNC_INIT
  85
  86     /* If we have native support for 32 * 32 -> 32, use that. */
  87
  88     return arg1 * arg2;
  89
  90 #if 0
  91     /* This is effectively what the emulation does, see also
  92         config/m68k-native/sumult32.s
  93     */
  94
  95     UWORD a1, b1, a0, b0;
  96
  97     a1 = (arg1 >> 16) & 0xffff;
  98     a0 = arg1 & 0xffff;
  99     b1 = (arg2 >> 16) & 0xffff;
 100     b0 = arg2 & 0xffff;
 101
 102     return ((a0 * b1) + (a1 * b0)) << 16 + (b0 * a0);
 103 #endif
 104
 105     AROS_LIBFUNC_EXIT
 106 } /* SMult32 */