DSP56300 datasheet(87/108 Pages) FREESCALE

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Extended Algorithm Program Listing

Viterbi Decoder Implementation

B-9

;***********************TRACEBACK OUTPUT PATH MACRO*******************

;

FUNCTION: To output the correct data, we begin at the end.

We take

;

the output path of the survivor state (0), and place its

;

associated output path in memory as the last output data

;

byte.

Then we use bits 3-7 of that data as an offset pointer

;

to the correct traceback data of the next previous path data

;

memory.

We continue this until we have traced the data back

;

to the begining.

;

INPUTS:

;

n0 should point to memory where the path bits are to be stored

;

r5 should point to the latest path metric for state 0

;

OUTPUTS:

;

Decoder output data in Y memory

;

REGISTERS USED:

;

a,b,y,r0,n0,r2,r5

;

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

;

Store off path data in bytes to avoid overflow in path reg's

TRACEBACK macro

;

move

N0,R0

;path ptr to r0

move

#0,n0

;prep for traceback

move

#DECOUT+(NUMINPUTS/8/2),r2 ;point to end of output buffer

move

y:(r5),b

;recall last path

move

#-1,m2

;r2 now linear

move

b1,x:(r0)

;save off last path data

move

#$513,y0

;control word for extract

;

;*****************BEGIN TRACEBACK*****************************

IF (EVEN==0)

move

x:(r0+n0),a

;recall last path

extractu y0,a,b

;bits 3-7 of a1 point to next data

lua

(r0-NUMSTATES),r0

;dec r0 to next earlier path set

lsl

#8,a

;move to upper byte

move

b0,n0

;load as offset for traceback

move

a1,y:(r2)-

;save off

ENDIF

;

#NUMINPUTS/8/2,TRCBK

;once for each byte pair

move

x:(r0+n0),a

;recall last path

extractu y0,a,b

;get ptr to next earlier path

lua

(r0-NUMSTATES),r0

;point r0 to next earlier states

move

b0,n0

;save ptr as offset

move

a1,y1

;save out byte in x1

move

x:(r0+n0),a

;do it all again!

extractu y0,a,b

lua

(r0-NUMSTATES),r0

move

b0,n0

lsl

#8,a

;move to upper byte

y1,a

;or in last byte to get 16 bit word

move

a1,y:(r2)-

;store result

TRCBK

endm

Example B-1

Extended Algorithm Program Listing (Continued)

Freescale Semiconductor, Inc.

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