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⌂ ∞ ▽ Elite on the 6502

Save and load: UNMUTILATE

[Apple II version]

       Name: UNMUTILATE                                              [Show more]
       Type: Subroutine
   Category: Save and load
    Summary: Decrypt the commander file in the buffer at comfil

    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * LOD calls UNMUTILATE




.UNMUTILATE

 LDY #3                 ; To decrypt the commander file, we need to set the four
                        ; random number seeds at RAND to RAND+3 to the four
                        ; bytes at comfil2 in the encrypted commander file, so
                        ; set a byte counter in Y to copy four bytes

.MUTIL2

 LDA comfil2,Y          ; Copy the Y-th seed from the commander file to the Y-th
 STA RAND,Y             ; random number seed at RAND

 DEY                    ; Decrement the byte counter

 BPL MUTIL2             ; Loop back until we have copied all four bytes

 BMI MUTIL3             ; Jump to MUTIL3 to apply the decryption process to the
                        ; commander file, which is simply a repeat of the
                        ; encryption process with the same seeds (this BMI is
                        ; effectively a JMP as we just passed through a BPL)