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4-4

Viterbi Decoder Implementation

Algorithmic Extensions

Allowing More General Branch Metrics

4.2.1

Modify Viterbi Butterfly

Begin modification with the butterfly loop. The code from Section 3 reads in the branch

metric from y memory near the loop end (the y:(r0)+,y1 that appears two lines

from _P_NextStage). The easiest way to get two branch metrics is to do long reads on the

branch metrics. For the code in Section 3, however, finding the space to do the extra

branch read is harder. Most of the data movement is tightly controlled and cannot be

moved in the code without disrupting the data flow. Instead, we can make use of the

pipeline stall in the Viterbi butterfly loop. The modified code appears in Example 4-1.

Example 4-1

Modified Viterbi Butterfly

;*******************viterbi add, compare, select butterfly macro***

;

FUNCTION: Update path metrics/paths for the Viterbi algorithm by

;

doing an add,compare,select update for state pairs.

;

INPUTS:

;

r2 should point to the beginning of the branch metric table

;

r2 should point to the beginning of the branch metric table

;

r5 should point to the latest path metric for state 0

;

r4 should point to the storage location for updated state 0

;

n5 should offset addresses by NUMSTATES/2

;

OUTPUTS:

;

Updated path metrics/paths stored in XY memory

;

REGISTERS USED:

;

a,b,y01,r2,r4,r5,n5 r2 unchanged (modulo req'd)

;

Registers:

;

r5, pointer to the path metric/path table, arranged as

;

x: path metric, y: path,states ordered assuming

;

bits shift right to left.

;

r4, pointer to the output path metric/path table

;

r0, pointer to the branch metric table, arranged

;

as x:C, y:D, CD,CD,CD, etc.

;

SA------NSA

;

C /

;

D \

;

D /

;

SB------NSB

;

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