Motorola DSP56305 Manuel d'utilisateur télécharger pdf (Page 76)

Pipeline Interlocks

Data ALU Pipeline Interlocks

MOTOROLA Optimizing DSP56300/DSP56600 Applications 6-3

and Status Interlock may be avoided by adding some useful

instructions in between the instructions. The following paragraph

will demonstrate few ways to overcome the Arithmetic Interlock.

6.1.2 Avoiding Data ALU Pipeline Interlocks

There are few common ways to avoid the Arithmetic Interlock. The

first is to change the order of the instructions such that a sequence

that caused the interlocks will not be part of the re-ordered code.

The second is to unroll the loop and use two accumulators in an

interleaving manner.

6.1.2.1 Code Reorder

The following DSP56000 program code is part of Real input FFT

based on Glenn Bergland algorithm. It is taken from Appendix B of

the

DSP56000 Family Manual

(Figure B-5, sheet 7):

move x:(r0)+,x1y:(r4)-,y1 ;x1=b, y1=d, r4 ptr back to c

mpy x1,y0,B x:(r3)+,r7 ;A=bWr,

mac x0,y1,B x:(r3)+,r1 ;B=bWr+dWi=T1, get first index

sub B,A ;A=a-T1=c’, get second index

addl A,B

A,x:(r1)

;B=a+T1=a’, PUT c’ to x:b

mpy y1,y0,A

B,x:(r7)

;B=dWr, B=c PUT a’

mac -x1,x0,Ay:(r4)+n4,B ;A=dWi-bWr=T2, B=c, r4 ptr to

;next c

sub B,A x:(r2)+,x0y:(r6)+,y0 ;A=T2-c=d’,x0=next Wi,y0=next

;Wr

addl A,B

A,y:(r1)

;B=T2+c=b’,update r4,A=next a,

;PUT d’

move x:(r0)+,A

B,y:(r7)

;PUT b’, A=next a

move y:(r4)+,B ;B=next c

This code consists of four arithmetic stalls that are caused by the

accumulator read operation in the fifth, sixth, ninth, and tenth

instructions. By reordering the parallel moves we can write a similar

code that consists of no pipeline interlocks at all (notice that pointers

r4 and r0 were switched).

move x:(r4)+,x1y:(r0)-,y1 ;x1=b, y1=d, r0 ptr back to c

mpy x1,y0,B x:(r3)+,r7 ;A=bWr,

mac x0,y1,B x:(r3)+,r1 ;B=bWr+dWi=T1, get first index

sub B,A ;A=a-T1=c’, get second index

addl A,B ;B=a+T1=a’, PUT c’ to x:b

mpy y1,y0,A A,x:(r1) ;B=dWr, B=c PUT a’

mac -x1,x0,AB,x:(r7)y:(r0)+n0,B ;A=dWi-bWr=T2, B=c, r0 ptr to

;next c

sub B,A x:(r2)+,x0y :(r6)+,y0;A=T2-c=d’,x0=next Wi,

;y0=next Wr

addl A,B ;B=T2+c=b’,update r0,

1 2 ... 71 72 73 74 75 76 77 78 79 80 81 ... 111 112

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Motorola DSP56305 Manuel d'utilisateur Page 76

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