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Old 05 May 2022, 10:45   #1
koobo
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Loops with two instructions

I did some optimizing recently and shrunk a 7 instruction loop into this:

Code:
.loop  cmp.l (a2)+,d0
       dbhi  d5,.loop
The array in A2 contains magnitude values as an exponential curve. D0 contains the measurement. The result in D5 is a decibel representation of the magnitude minus one. I thought this was pretty neat.

Any other interesting examples of two instruction loops?
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Old 14 May 2022, 12:39   #2
koobo
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Having fun by myself here, bumped into this when looking at some source:

Code:
_FindMSBSet:
        move.b  d1,d0
        beq.s   2$
        moveq   #7,d0
1$:
        asl.b   #1,d1
        dbcs    d0,1$
2$:
        rts
Input is a byte in d1, output in d0 is the bit index of the first bit that is set in input.
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Old 14 May 2022, 13:02   #3
phx
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asl.b #1,d1
could be optimized to
add.b d1,d1
(although the assembler would probably do that).

IIRC, these kind of two-byte DBcc loops have an advantage on the 68010, because they are executed from a small cache.

To give an example as well, I'm frequently using it to determine a string length:
Code:
        moveq   #-1,d1
.1:     tst.b   (a0)+
        dbeq    d1,.1
        not.w   d1
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Old 14 May 2022, 14:06   #4
a/b
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The msbitset example above doesn't work if d1 is 0 (returns 0 if no bit is set or bit0 is set). Alternative (returns -1.w if no bit is set):
Code:
	moveq	#7,d0
Loop	bclr	d0,d1
	dbne	d0,Loop
edit: btst works too ;P.

Last edited by a/b; 14 May 2022 at 14:13. Reason: btst lives matter
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