Hardware and Software Development Tools for the HCS12 - The HCS12/9S12: An Introduction to Software and Hardware Interfacing - page 122

sum ds.w 1

org $1500

ldx #array ; place the starting address of array in X

movw #0,sum ; initialize sum to 0

ldy #N ; initialize loop count to N

loop ldd 1,x 1 ; place one number in D and move array pointer

addd sum ; add to sum

std sum ; update the sum

dbne y,loop ; add all numbers to sum yet?

swi ; return to monitor

end
At the fi rst run of this program, sum ($A61F) is also incorrect.
+ md 1010
1010 22 24 26 28 - A6 1F B9 A9 - 2A CA FA DB - AC DA 18 97 “$&(. . . .*. . . . . . .
.
Again, the error can be found by tracing. This time the error will not be found until the
instruction loop ldd 1,x 1 is traced.
+ pc 1500
PP PC SP X Y D 5 A:B CCR 5 SXHI NZVC

38 1500 3C00 1014 0000 A6:1F 1001 1000

xx: 1500 CE1000 LDX #$1000

+ t
PP PC SP X Y D 5 A:B CCR 5 SXHI NZVC

38 1503 3C00 1000 0000 A6:1F 1001 0000

xx: 1503 180300001014 MOVW #$0000,$1014

+ t
PP PC SP X Y D 5 A:B CCR 5 SXHI NZVC

38 1509 3C00 1000 0000 A6:1F 1001 0000

xx: 1509 CD0014 LDY #$0014

+ t
PP PC

SP X Y D 5 A:B CCR 5 SXHI NZVC

38 150C 3C00 1000 0014 A6:1F 1001 0000

xx: 150C EC30 LDD 1,X 1

+ t
PP PC SP X Y D 5 A:B CCR 5 SXHI NZVC

38 150E 3C00 1001 0014 02:04 1001 0000

xx: 150E F31014 ADDD $1014

.
We expect this instruction to place the first array element ($02) in D. Instead, it places
$0204 in D. This is obviously wrong! The error is due to the fact that the ldd instruction loads
a word (2 bytes) instead of 1 byte into D. At this point, we should be able to figure out the right
instruction to use and fix the problem.

I NAPPROPRIATE U SE OF I NDEX A DDRESSING M ODE
When the index addressing mode is used to step through an array, one needs to increment or
decrement the index register to reach the next element. Depending on the size of the element,