Topic       : Pure Assembler Documentation
Author      : John Kormylo
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Subject     : Documentation/Pure Assembler
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View Ref-File[ multiply_by_constant ]

/* ******************************************************************
MACRO multiply_by_constant ea,constant

Multiplying by powers of two or other constants can often be
implemented faster using shift instructions rather than by MULS.
Output of this macro will be in register D0.
Uses labels bit and shift.

ea = long effective address (anything except D0)
constant >= 0
****************************************************************** */

MACRO multiply_by_constant ea,constant

IFLT constant
  ERROR "multiply_by_constant macro: constant must be >= 0"
ENDIF

IFEQ constant
  clr.l  D0
  EXITM
ENDIF

bit = $4000_0000  ;check bits of constant from left to right
shift = -1        ;number of shifts needed (-1 = nothing in D0)
  REPT 31

  IFGE shift
    shift = shift + 1
    IF shift > 8  ;maximum shift = 8
      asl.l #8,D0
      shift = 1
    ENDIF
  ENDIF

  IFNE bit & constant
    IFLT shift
      move.l ea,D0  ;leftmost non-zero bit in constant
    ELSE
      asl.l #shift,D0
      add.l ea,D0   ;any other non-zero bits in constant
    ENDIF
    shift = 0
  ENDIF

  bit = bit >> 1
  ENDM           ;end of REPT block

IFGT shift
  asl.l #shift,D0
ENDIF
ENDM             ;end of MACRO multiply_by_constant