Lander !RunImage source

 LANDER !RunImage SOURCE

 Lander was written by David Braben and is copyright D.J.Braben 1987

 The code on this site has been reconstructed from a disassembly of the game on
 the Arthur and RISC OS 2 application discs

 The commentary is copyright Mark Moxon, and any misunderstandings or mistakes
 in the documentation are entirely my fault

 The terminology and notations used in this commentary are explained at
 https://lander.bbcelite.com/terminology

 The deep dive articles referred to in this commentary can be found at
 https://lander.bbcelite.com/deep_dives


 This source file produces the following binary file:

   * !RunImage.unprot.bin


 CODE = &00008000                   \ The build address for the Absolute file
                                    \
                                    \ [Show more]
                                    \
                                    \ This variable is used by the following:
                                    \
                                    \   * absoluteAddr
                                    \
                                    \ This list only includes code that refers
                                    \ to the variable by name; there may be
                                    \ other references to this memory location
                                    \ that don't use this label, and these will
                                    \ not be mentioned above

 INCLUDE "3-assembled-output/exports.arm"

 LANDER !RunImage CODE

 Produces the binary file !RunImage.unprot.bin.


 DIM CODE% &A000                    \ Reserve a block in memory for the
                                    \ assembled code

 FOR pass% = 4 TO 6 STEP 2          \ Perform a two-pass assembly, using both
                                    \ P% and O%, with errors enabled on the
                                    \ second pass only

 O% = CODE%                         \ Assemble the code for deployment to
                                    \ address O%

 P% = CODE                          \ Assemble the code into the block at P%

 [                                  \ Switch from BASIC into assembly language

 OPT pass%                          \ Set the assembly option for this pass


       Name: RunImageEntry                                           [Show more]
       Type: Subroutine
   Category: Copy protection
    Summary: Entry point for the !RunImage Absolute file
    Context: See this subroutine on its own page
 References: No direct references to this subroutine in this source file


 B       DecryptGameBinary          \ RISC OS runs !RunImage as an Absolute file
                                    \ which executes from the first instruction
                                    \ rather than having an execute address, so
                                    \ this jumps to the decryption routine to
                                    \ decrypt the game binary before jumping to
                                    \ Entry to start the game


       Name: gameCode                                                [Show more]
       Type: Variable
   Category: Copy protection
    Summary: The unencrypted game code
    Context: See this variable on its own page
 References: This variable is used as follows:
             * gameCodeAddr uses gameCode


.gameCode

 INCBIN "3-assembled-output/GameCode.bin"

.gameCodeEnd


       Name: DecryptGameBinary                                       [Show more]
       Type: Subroutine
   Category: Copy protection
    Summary: Placeholder routine to decrypt game code to &8000 so it can be run
    Context: See this subroutine on its own page
 References: This subroutine is called as follows:
             * RunImageEntry calls DecryptGameBinary


.DecryptGameBinary

 LDR     R0, gameCodeAddr           \ Set R0 to the address of the game code

 LDR     R1, absoluteAddr           \ Set R1 to the address of the start of the
                                    \ Absolute file

 LDR     R2, gameCodeEndAddr        \ Set R2 to the address of the end of the
                                    \ game code

.decr1

 LDR     R12, [R0], #4              \ Copy a word from R0 to R1, updating the
 STR     R12, [R1], #4              \ addresses as we go (this is where the
                                    \ decryption process would work, but
                                    \ currently it is just a copy)

 CMP     R0, R2                     \ Loop back until we have copied the whole
 BNE     decr1                      \ game

 B       Entry                      \ Start the game by jumping to the Entry
                                    \ routine


       Name: absoluteAddr                                            [Show more]
       Type: Variable
   Category: Copy protection
    Summary: The address of the start of the Absolute file
    Context: See this variable on its own page
 References: This variable is used as follows:
             * DecryptGameBinary uses absoluteAddr


.absoluteAddr

 EQUD    CODE


       Name: gameCodeAddr                                            [Show more]
       Type: Variable
   Category: Copy protection
    Summary: The address of the game code
    Context: See this variable on its own page
 References: This variable is used as follows:
             * DecryptGameBinary uses gameCodeAddr


.gameCodeAddr

 EQUD    gameCode


       Name: gameCodeEndAddr                                         [Show more]
       Type: Variable
   Category: Copy protection
    Summary: The address of the end of the game code
    Context: See this variable on its own page
 References: This variable is used as follows:
             * DecryptGameBinary uses gameCodeEndAddr


.gameCodeEndAddr

 EQUD    gameCodeEnd

 Two-pass assembly loop


 ]

 NEXT pass%                         \ Loop back for the second pass

 Save !RunImage.unprot.bin


 OSCLI "SAVE !RunImage "+STR$~CODE%+" "+STR$~O%+" "+STR$~CODE+" "+STR$~CODE