Page 125 - Introduction to Microcontrollers Architecture, Programming, and Interfacing of The Motorola 68HC12

102 Chapter 4 Assembly Language Programming

SRCH: CLRB initialize result in case first branch is taken
CMPA #' L' compare to a character of string, move pointer
BEQ EXIT if it is 'L', exit the loop
INCB increase result to 1 in case next branch is taken
CMPA #' A' compare to a character of string, move pointer
BEQ EXIT if it is 'A', exit the loop
INCB increase result to 2 in case next branch is taken
CMPA #' S' compare to a character of string, move pointer
BEQ EXIT if it is 'S', exit the loop. Otherwise it is S
INCB increase result to 3 since all other cases tested
EXIT: SWI return to the debugger
a) Using immediate operands
ORG $800
Cmprd: DC . b "DSAL" ; comparand string (downloaded)

CMPR: LDX #Cmprd ; get address of comparand string
LDAB #3 ; loop counter, and also the return value
NEXT: CMPA 1, X+ ; compare to a character of string, move pointer
BEQ EXIT ; if it is a matching character, exit the loop
DBNE B, NEXT ; decrement B, count out four loop executions
EXIT: SWI ; return to the debugger
b) Using a character string of comparison values
Figure 4.11. Character Search in a String

ORG $800
Diet: Ds. b 32 ; storage for the dictionary
Ptr: DC . w Diet ; address beyond the end of the dictionary
Cntr: DC . b 0 ; size of the dictionary

INSRT: LDX Ptr ; get the pointer
STD 2, X+ ; store letter that is in A and value that is in B
STX Ptr ; save the pointer
INC Cntr ; increment the count of the dictionary size
a) Build

CMPR: LDX #Dict ; get address of beginning of the dictionary
LDAB Cntr ; loop counter, and also the return value
NEXT: CMPA 2, X+ ; compare to a character of string, move pointer by 2
BEQ EXIT ; if it is NULL, exit the loop
DBNE B,NEXT ; decrement B, count out four loop executions
EXIT: LDAB -1, X ; get the numerical value of the letter into B
b) Search
Figure 4.12. Dictionary Program Segments