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3-14

Viterbi Decoder Implementation

Expanding the Viterbi Algorithm

Traceback: Obtaining the Decoder Output

To store the paths, begin by recalling the path storage address pointer. In the example

code, this is stored in n0. We place copies into r3 (to use as the address pointer for data

writes) and r0 (to update the pointer to the next page at the end of the routine). We then

mask off the 5 LSBs, and proceed to store the paths. We have designed this macro to

process a variable number of states, controlled by the macro argument LPCNT. This

variability will be used in Section 4.

Example 3-3

Partial Path Storage Code Listing

;***********************STORE PARTIAL PATH METRICS MACRO***************

;

FUNCTION: The storage is somewhat twisted.

The stored paths are current

;

up to the most recent input bit, which is NOT convenient for

;

traceback.

I process the data as follows:

I pre

;

loaded the path with 6 bits.

Thereafter, I process 8 path bits

;

so the path has 14 bits.

The most significant 8, I save.

;

the remaining bits are the current encoder values for that path.

;

The 5 lsb's are the current state.

;

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:

;

Updated paths storage X memory, n0 points to next path storage

;

REGISTERS USED:

;

a,b,x1,r0,n0,r3,r5

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

;

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

;

STOREPATHS macro LPCNT

;

move

n0,r3

;n0 stores path data pointer

move

n0,r0

move

#>$1f,x1

;mask for 5 lsb's(NUMENCBITS of 1's)

LPCNT,_PSTORE1

;store paths for each state

move

y:(r5),b

;grab path

asr

#5,b,a

;align bits 5-12 with a1

;

(the 8 bits beyond enc)

and

x1,b

;mask off 5 lsb's to return to path

move

a1,x:(r3)+

;store 8 ms path bits

move

b1,y:(r5)+

;return 5 ls path bits

_PSTORE1

lua

(R4+NUMSTATES),R5

;r5 points to latest states

lua

(r0+NUMSTATES),n0

;update path data pointer

endm

Freescale Semiconductor, Inc.

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