Motorola M68000 Betriebsanweisung PDF herunterladen (Seite 149)

Logical, Shift and Rotate Instructions 155

will shift DO left by 10 bits. This will shift in 10 zeros, and the value

in DO after the shift will be C00016. Note that we are performing word

operations on the register. The bits shifted out of the low-order word are

not shifted into the high-order word.

Even though the 68000 has a number of multiply and divided instruc

tions, it is sometimes easier and faster to use a shift for these operations.

This only works for multiplication or division by a power of two. A left

shift will multiply by 2n, where n is the number of bits to shift. A right

shift (be sure to use ASR for a signed divide) will divide by 2n. Since

the bits shifted off the end are lost, there will not be a remainder. Also,

be aware that no rounding of the result is performed. This means that

positive numbers are truncated towards zero, and negative numbers are

truncated towards negative infinity. In other words, 5/2 will result in 2,

while —5/2 will result in —3. You can verify this for yourself by writing

the binary values for 5 and —5 and then shifting.

An interesting application of this use of the shift instructions is a mul

tiplication which is not a power of two. A simple unsigned multiplication

can be performed using the left shift, LSL. Simply check each bit position

in the multiplier to determine if it is a 1 or 0. If it is a 1, add the multipli

cand shifted left by the number of bits corresponding to the bit position

in the multiplier. Sum these partial products as we go along. Here is a

simple routine to perform this multiplication:

* UNSIGNED MULTIPLY

* DO = MULTIPLIER

Dl = MULTIPLICAND

D2 = PRODUCT

D3 = TEMP FOR MULTIPLICAND

D4 = SHIFT COUNT

CLR.L D2

CLEAR PRODUCT

MOVEQ.L

#-l,D4

SET UP SHIFT COUNT

TST.L

DO FINISHED?

BEQ

FINI

YES

ADDQ.L

#1,D4

SET UP COUNT FOR NEXT SHIFT

LSR.L

#1 / DO GET NEXT BIT OF MULTIPLIER

BCC NEXT

ZERO, CONTINUE

MOVE.L

D1/D3

COPY MULTIPLICAND

LSL. L

D4/D3

GET PARTIAL PRODUCT

ADD.L

D3/D2

SUM TO PRODUCT

BRA NEXT

FINI:

Notice that the shift count is initialized to —1. This is to ensure that the

first time it is used it will be zero. We drop out of the loop as soon as

the multiplier is zero. This allows the algorithm to operate faster than if

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