\ ******************************************************************************
\
\ 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, RISC OS 2 and RISC OS 3.00 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 contains decryption code for the RISC OS version of Lander.
\ It decrypts the main game code as part of the !RunImage binary.
\
\ ------------------------------------------------------------------------------
\
\ This source file produces the following binary file:
\
\   * !RunImage.unprot.bin
\
\ after reading in the following file:
\
\   * GameCode.bin
\
\ ******************************************************************************

 CODE = &00008000                   \ The build address for the Absolute file

 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 into the reserved block
                                    \ of memory at CODE%

 P% = CODE                          \ Assemble the code so it runs at the
                                    \ address in CODE

[                                   \ Switch from BASIC into assembly language

 OPT     pass%                      \ Set the assembly option for this pass

\ ******************************************************************************
\
\       Name: RunImageEntry
\       Type: Subroutine
\   Category: Copy protection
\    Summary: Entry point for the !RunImage Absolute 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
\       Type: Variable
\   Category: Copy protection
\    Summary: The unencrypted game code
\
\ ******************************************************************************

.gameCode

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

.gameCodeEnd

\ ******************************************************************************
\
\       Name: DecryptGameBinary
\       Type: Subroutine
\   Category: Copy protection
\    Summary: Placeholder routine to decrypt game code to &8000 so it can be run
\
\ ******************************************************************************

.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
\       Type: Variable
\   Category: Copy protection
\    Summary: The address of the start of the Absolute file
\
\ ******************************************************************************

.absoluteAddr

 EQUD    CODE

\ ******************************************************************************
\
\       Name: gameCodeAddr
\       Type: Variable
\   Category: Copy protection
\    Summary: The address of the game code
\
\ ******************************************************************************

.gameCodeAddr

 EQUD    gameCode

\ ******************************************************************************
\
\       Name: gameCodeEndAddr
\       Type: Variable
\   Category: Copy protection
\    Summary: The address of the end of the game code
\
\ ******************************************************************************

.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