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Algorithmic Extensions

Allowing More General Branch Metrics

Viterbi Decoder Implementation

4-5

This code is almost identical to the butterfly code in Section 3, with the following

differences. The first executable line is a long move to register Y. This allows us to grab

two branch metrics at the same time. For this example, we show both branch metrics

being added to the path metrics to update the states. Line 9 in the loop (the assembly line

move l:(r2)+,y) is a long read of two more branch metrics. This is an extra line of code,

but it executes in the same number of cycles as the original code because it takes the

place of a pipeline stall. Finally, note that address register 5 is still incremented, but

without a dummy read into a register.

The loop uses the same branch metrics for both upper and lower path metric updates.

This is what is needed for Example 4-1, where we still have some symmetry because

both encoding polynomials have a tap on the input bit. If this is not the case, the code can

accommodate other conditions. A more general setup is to uncomment the long move

that reads additional branch metrics in line 4 of the loop. This line will also take the place

of a pipeline stall, and so does not increase the number of cycles needed to execute the

butterfly.

;*****************************************************************

;

ACS

macro

;

move

#BRY,r2

;r2 points to branch metrics

move

l:(r2)+,y

;get first branch metric

move

l:(r5)+n5,a

;load 1st metric/path pair

;

#NoAcs,_P_NextStage

;update each state

add

y0,a

l:(r5)-n5,b

;sum pt,br met,get next pair

add

y1,b

;update metric

max

a,b

l:(r5)+n5,a

;pick max, reload 1st pair

;

move

l:(r2)+,y

;load next branch metrics

vsl

b,0,l:(r4)+

;save surivior, end top half

add

y1,a

l:(r5)-n5,b

;sum pt,br,reload next pair

add

y0,b

(r5)+

;sum again,inc pt mt read ptr

max

a,b

l:(r5)+n5,a

;pick max, load next pair

move

l:(r2)+,y

;load next branch metrics

vsl

b,1,l:(r4)+

;write survivor,end 2nd half

_P_NextStage

nop

;needed to separate do loop ends

endm

Example 4-1

Modified Viterbi Butterfly (Continued)

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