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Extended Algorithm Program Listing

Viterbi Decoder Implementation

B-3

B.1

16-BIT ENHANCED VITERBI DECODER PROGRAM LISTING

This appendix contains the complete 16-bit program listing for Section 4. The code in

this section introduces several modifications to the code discussed in Section 3.

Generalized branch metrics are allowed, as well as the optimization of the code that

reduces the computations when data occurs in blocks.

Example B-1

Extended Algorithm Program Listing

OPT

mex

;****************56600**VITERBI DECODER***********************************

; THIS ROUTINE IMPLEMENTS A CONVOLUTIONAL DECODER USING THE VITERBI ALG. IT IS

; OPTIMIZED FOR SPEED, WHICH MEANS THAT EVERY ALU REGISTER AND ALL OF THE R REGISTERS

; ARE USED.

A SIGNIFICANT AMOUNT OF THESE CAN BE FREED BY STORING AND REUSING

; REGISTERS BETWEEN THE FINDMETRICS ROUTINES AND THE ACS,ACSFlush ROUTINES.

; INPUT is at INDATA: real in x imag in y.

OUTPUT begins at y:DECOUT

; GLOBAL REGISTER USE: a,b,x,y,r012345,n025,m245(are set to a modulo mode)

;*****************BRANCH METRIC MACRO*************************************

;

FUNCTION: Input data and generate branch metrics.

This function

;

subtracts the path metric for state 0 from all branch metrics

;

to provide autonormaliztion. For this decoder, the metric is a scaled

;

sum or difference of the real and imag inputs.

;

INPUTS:

;

r2 should point to the beginning of the branch metric table

;

r5 should point to the latest path metric for state 0

;

r1 should point to the next input XY data pair

;

OUTPUTS:

;

Branch metrics are stored at BRX in XY memory

;

REGISTERS USED:

;

a,b,x01,y01,r1,r2,r5, r5 unchanged,r2 unchanged (modulo req'd)

;*****************FINDMETRICS MACRO******************************************

FindMetrics macro

move

x:(r5),a

;st. metric scaling, read last st. 0

neg

l:(r1)+,y ;negate metric|grab dec input

move

#-16,x1

;sign for real component, upper br.

mac

x1,y1,a

a,b

;comp 0x partial br. path mt. to

move

y1,x0

;mv real input to x0

mac

x1,y0,a

a,y1

;a gets 00 br. y1 gets 0x partial br

subl

a,b

b,x0

;a has 00 br, b has 11 br, save path

tfr

y1,a

a,y1

;swap 0x and 00 to compute 01 branch

mac

-x1,y0,a

b,x1

;a gets 01 br

tfr

x0,b

b,y0

;swap path metric,11 branch to y0

subl

a,b

y1,x0

;b gets 10 br, x0 has copy of 00 br.

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