MOV WF CNT0 ; INNER LOOP 48 TIMES
DECRYPT_INNER
CLRWDT ; RESET WATCHDOG TIMER
MOV FW CNT1 ; LAST 48 LOOPS RESTORE THE KEY
XORLW 1 ; LAST 48 LOOPS RESTORE THE KEY
SKPNZ ; LAST 48 LOOPS RESTORE THE KEY
GOTO ROTATE_KEY ; LAST 48 LOOPS RESTORE THE KEY
; THE LOOKUP TABLE IS COMPRESSED INTO IN 4 BYTES TO SAVE SPACE
; USE THE 3 LOW INDEX BITS TO MAKE UP AN 8-BIT BIT MASK
; USE THE 2 HIGH INDEX BITS TO LOOK UP THE VALUE IN THE TABLE
; USE THE BIT MASK TO ISOLATE THE CORRECT BIT IN THE BYTE
; PART OF THE REASON FOR THIS SCHEME IS BECAUSE NORMAL TABLE LOOKUP
; REQUIRES AN ADDITIONAL STACK LEVEL
CLRC ; CLEAR CARRY (FOR THE LEFT SHIFT)
MOV LW 1 ; INITIALISE MASK = 1
BTFSC HOP3,3 ; SHIFT MASK 4X IF BIT 2 SET
MOV LW 10000B ; SHIFT MASK 4X IF BIT 2 SET
MOV WF MASK ; INITIALISE MASK = 1
BTFSS HOP2,0 ; SHIFT MASK ANOTHER 2X IF BIT 1 SET
GOTO $+3
RLF MASK,F
RLF MASK,F
BTFSC HOP1,0 ; SHIFT MASK ANOTHER 1X IF BIT 0 SET
RLF MASK,F
; MASK HAS NOW BEEN SHIFTED 0-7 TIMES ACCORDING TO BITS 2:1:0
MOV LW 0 ; TABLE INDEX = 0
BTFSC HOP4,1
IORLW 2 ; IF BIT 3 SET ADD 2 TO THE TABLE INDEX
BTFSC HOP4,6
IORLW 4 ; IF BIT 4 SET ADD 4 TO THE TABLE INDEX
ADDWF PC,F ; ADD THE INDEX TO THE PROGRAM COUNTER
; [ MUST BE IN LOWER HALF OF PAGE ]
TABLE
MOV LW 02EH ; BITS 4:3 WERE 00
GOTO TABLE_END ; END OF LOOKUP
MOV LW 074H ; BITS 4:3 WERE 01
GOTO TABLE_END ; END OF LOOKUP
MOV LW 05CH ; BITS 4:3 WERE 10
GOTO TABLE_END ; END OF LOOKUP
MOV LW 03AH ; BITS 4:3 WERE 11
TABLE_END
ANDWF MASK,F ; ISOLATE THE CORRECT BIT BY ANDING WITH MASK
MOV LW 0 ; COPY THE BIT TO BIT 7
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本文关键字：源程序 PIC单片机，单片机-工控设备 - PIC单片机
