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dissident · 21 November 2018, 10:20

Quote:

Originally Posted by Gorf

Multiplication by a constant can be done very efficiently. So if you know by what factor you are going to multiply, you could use one of these tricks:

multiply d0 by 29:

Code:

move.l  d0, d1
lsl.l   #4, d0
sub.l   d1, d0
add.l   d0, d0
sub.l   d1, d0

multiply d0 by 625:

Code:

move.l  d0, d1
lsl.l   #2, d0
add.l   d1, d0
lsl.l   #3, d0
sub.l   d1, d0
lsl.l   #4, d0
add.l   d1, d0

multiply d0 by 156:

Code:

move.l  d0, d1
lsl.l   #2, d1
add.l   d1, d0
lsl.l   #5, d0
sub.l   d1, d0

I've attached my collection of these multiply tricks based on the MC68000 execution times (including these three cases above) as macros, so they can be easily used in every sourcecode.