Loader: S%

       Name: S%                                                      [Show more]
       Type: Subroutine
   Category: Loader
    Summary: Checksum, decrypt and unscramble the main game code, and start the
             game
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * Checksum calls S%


 RTS                    ; The checksum byte goes here, at S%-1. In the original
                        ; source this byte is set by the first call to ZP in the
                        ; Big Code File, though in the BeebAsm version this is
                        ; populated by elite-checksum.py

.S%

 CLD                    ; Clear the D flag to make sure we are in binary mode

 LDX #2                 ; We now copy the contents of zero page between $0002
                        ; and $00FF to the page at $CE00, so set an index in X
                        ; to start from byte $0002
                        ;
                        ; We do this so we can use the SWAPPZERO routine to swap
                        ; zero page with the page at $CE00 when saving or
                        ; loading commander files

.ZONKPZERO

 LDA $0000,X            ; Copy the X-th byte of zero page to the X-th byte of
 STA $CE00,X            ; $CE00

 INX                    ; Increment the loop counter

 BNE ZONKPZERO          ; Loop back until we have copied all of zero page

 JSR DEEOR              ; Decrypt the main game code between $1300 and $9FFF

 JSR COLD               ; Configure memory, set up interrupt handlers and
                        ; configure the VIC-II, SID and CIA chips

;JSR Checksum           ; This instruction is commented out in the original
                        ; source

 JMP BEGIN              ; Jump to BEGIN to start the game