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COMMENT* Change ROR -> ROL in the TWO marked places to produce UNLOCK.ASM *
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CODE_SEG SEGMENT
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ASSUME CS:CODE_SEG
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ORG 100H
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HERE: JMP THERE
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COPY_RIGHT DB '(C)1985 Steven Holzner'
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PROMPT DB 'Phrase: $' ;All the messages & prompts
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DEFAULT DB 'FILE.LOC',0
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NOTSEEN DB 13,10,'File Not Found$'
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FULL: DB 13,10,'Disk Full$'
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FILETWO DW 0 ;Address of 2nd File name
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FILEND DW 0 ;End of read-in files in memory
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THERE PROC NEAR ;Our procedure
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MOV BX,81H ;Make the filenames into ASCIIZ
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UP: INC BX ;Scan type-in for space, <cr>
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CMP BYTE PTR [BX],' ' ;Space?
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JNE NOSPACE
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MOV BYTE PTR [BX],0 ;Put the Z in ASCIIZ
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MOV FILETWO,BX
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INC FILETWO ;Store filename starting location
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NOSPACE:CMP BYTE PTR [BX],13 ;If not a space, a <cr>?
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JNE UP
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MOV BYTE PTR [BX],0 ;If yes, replace with a 0
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CMP FILETWO,0
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JNZ OVER
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MOV FILETWO,OFFSET DEFAULT ;If no second file given, use default
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OVER: LEA DX,PROMPT ;Type out the prompt with string print
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MOV AH,9
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INT 21H
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MOV BX,80H+40H-2 ;Prepare 40H (64) buffer for key phrase
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MOV BYTE PTR [BX],40H
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PUSH BX ;Set up buffer address
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POP DX
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MOV AH,0AH ;Buffered input
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INT 21H
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MOV BX,80H+40H ;Start of key phrase
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PUSH BX
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JUMP: CMP BYTE PTR [BX],13 ;Set up key phrase's ASCII values
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JE READY ;Scan until <cr>
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OR BYTE PTR [BX],1 ;Make it odd
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AND BYTE PTR [BX],0FH ;Use only lower four bits
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INC BX
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JMP JUMP ;Keep going until <cr>
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READY: POP BX
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MOV AX,3D00H ;Open the file to encrypt
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MOV DX,82H ;Point to its name
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INT 21H
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JNC OKFILE ;Carry Flag --> some problem, assume
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LEA DX,NOTSEEN ; file doesn't exist, say so
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MOV AH,9
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INT 21H
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JMP OUT ;Exit
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OKFILE: PUSH BX ;Store location in key phrase
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MOV BX,AX ;Put handle into BX
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MOV CX,62*1024 ;Ask for 62K bytes to be read from file
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LEA DX,THEBOTTOM ;And put at end of program
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MOV AH,3FH ;Read
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INT 21H
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ADD AX,OFFSET THEBOTTOM ;Actually read AX bytes
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MOV FILEND,AX
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DEC FILEND ;Find how far the file extends in mem.
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MOV AH,3EH ;Close file, thank you very much.
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INT 21H
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POP BX
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LEA CX,THEBOTTOM ;Save current location in file in CX
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SCRMBLE:MOV SI,CX ;Will scramble from [SI] to [DI]
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CMP SI,FILEND ;Past end?
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JAE DONE ;If yes, exit
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MOV DI,CX
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XOR AX,AX
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MOV AL,[BX] ;How many to scramble? (from key phrase)
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ADD DI,AX
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MOV CX,DI
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INC CX ;Store new starting location for next time
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INC BX ;Also, get next character for next scramble
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CMP BYTE PTR [BX],13 ;If at end of key phrase, wrap
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JNE TWIST
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MOV BX,80H+40H
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TWIST: CMP DI,FILEND ;Is DI past end?
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JBE GRAB
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MOV DI,FILEND ;If yes, only scramble to file end
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PUSH DI
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SUB DI,SI ;What about last byte?
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TEST DI,1
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POP DI
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JNZ GRAB ;If left over, rotate it once
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ROR BYTE PTR [DI],1 ;<--- ROL FOR UNLOCK!!!
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DEC DI
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CMP SI,DI
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JAE DONE
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GRAB: MOV DH,[SI] ;Get byte 1
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MOV DL,[DI] ;Get byte 2
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PUSH CX
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MOV CL,[BX] ;Get number of times to rotate
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INC BX ;Set up key phrase char for next time
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CMP BYTE PTR [BX],13
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JNE TWISTER
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MOV BX,80H+40H
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;Rotate the hybrid word
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TWISTER:ROR DX,CL ;<--- ROL FOR UNLOCK!!!
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POP CX
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MOV [SI],DH ;And replace the parts
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MOV [DI],DL
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INC SI ;Point to next part to scramble
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CMP SI,DI ;Have SI and DI met yet?
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JE SCRMBLE ;If yes, move on to next part to scramble
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DEC DI
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JMP GRAB ;Go back until done
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DONE: MOV AH,3CH ;Done
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MOV CX,0 ;Prepare to write out scrambled version
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MOV DX,FILETWO
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INT 21H ;Create the file
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JC ERROR
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MOV BX,AX
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MOV AH,40H
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LEA DX,THEBOTTOM
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MOV CX,FILEND ;File size to write
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SUB CX,OFFSET THEBOTTOM
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INC CX
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INT 21H ;Write it out
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CMP AX,CX ;If error, (returned)AX .NE. (orig.)CX
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JE CLOSE
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ERROR: LEA DX,FULL ;Assume disk is full, say so, leave
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MOV AH,9
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INT 21H
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JMP OUT
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CLOSE: MOV AH,3EH ;Otherwise, close the file and exit
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INT 21H
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OUT: INT 20H
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THERE ENDP
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THEBOTTOM: ;Read-in file starts here.
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CODE_SEG ENDS
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END HERE
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