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Section
04 Part 08 – The MULU & MULS Instructions |
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“But I was thinking of a way
To multiply by ten, And always, in the answer, get The question back again.” ~Lewis
Carroll |
Introduction
These
two instructions are for multiplication, and there are two of them because one
is for unsigned and the other is for signed.
The MULU
Instruction
MULU – Unsigned MULtiply
This
instruction will multiply a word
of the destination
operand by the word of the source operand; the result is saved in long-word in the destination operand.
Examples
This
is pretty much simple multiplication right here. So, we’ll start with this:
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mulu.w #$0010,d0 |
We’ll
pretend that d0 contains 00200040 for this example.
What
happens is the word of d0 is read (0040) and is
multiplied by 0010.
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0040 x 0010 = 00000400 |
The
result is then saved into d0,
and now d0 contains 00000400. You’ll notice the 0020 that was in the upper
word of d0 was replaced and ignored entirely, be sure to keep that in mind.
We’ll
have another example, this time d0 contains FFFFF800:
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mulu.w #$0087,d0 |
The word of d0 (F800) is multiplied by 0087.
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F800 x 0087 = 0082C800 |
The
result is then saved into d0,
and now d0 contains 0082C800. Remember, MULU is unsigned multiplication, so
both F800 and 0087 are treated as unsigned numbers (positive).
The destination operand must be a data register; the source operand however, can be an immediate number, a data
register, a memory address, or even a memory address using an address
register. A few examples:
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mulu.w d1,d0 mulu.w $00000010,d0 mulu.w (a0),d0 |
In all
of these examples above, the word of the d0, is multiplied by a word of
the source
operands. And the result is always a long-word saved back into d0.
The
only thing you cannot use for
the source
operand is an
address register directly:
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mulu.w a0,d0 |
And
one final note, the size can only be word,
you cannot use byte or long-word.
The MULS
Instruction
MULS – Signed MULtiply
This
is exactly the same as the MULU instruction (including syntax rules). Except of course, the source and destination are both treated as “signed”
instead of “unsigned”. So numbers that
are from 8000 to FFFF are negative.
Examples
We’ll
pretend that d0 contains 0000FFC0 in this example.
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muls.w #$0010,d0 |
So,
the word FFC0 from d0 is multiplied by the word 0010.
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FFC0 x 0010 = FFFFFC00 |
Notice
how the FFC0 is treated a negative, while 0010 is positive. What you effectively have is:
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-0040 x +0010 = -00000400 |
And
so, d0 now contains FFFFFC00 (-400).
That is
the only difference between MULU and MULS.