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⌂ ∞ ▽ The Sentinel on the BBC Micro

Main game loop: PlayGame

       Name: PlayGame                                                [Show more]
       Type: Subroutine
   Category: Main game loop
    Summary: Start playing the generated landscape
  Deep dive: Program flow of the main game loop

    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * CheckSecretCode (Part 2 of 2) calls PlayGame




 EQUB &B9               \ This byte is never executed, as the stack modification
                        \ in part 2 of the CheckSecretCode routine sets the
                        \ return address on the stack to PlayGame-1, so the RTS
                        \ instruction at the end of CheckSecretCode will
                        \ therefore jump to PlayGame (as that's how the RTS
                        \ instruction works)
                        \
                        \ This byte is the opcode for a Y-indexed absolute LDA
                        \ instruction, so this makes it look like there is a
                        \ PlayGame routine that starts with the following:
                        \
                        \   &B9 &A9 &83     LDA &83A9,Y
                        \
                        \ as the LDA instruction below assembles into &A9 &83
                        \
                        \ This is intended to confuse any crackers who have
                        \ reached this point in their analysis

.PlayGame

 LDA #&83               \ Set the palette to the first set of colours from the
 JSR SetColourPalette   \ colourPalettes table, which the SpawnEnemies routine
                        \ set to the correct palette for playing the game (so
                        \ for landscape 0000 that would be blue, black, white
                        \ and green, for example)
                        \
                        \ So this displays the landscape preview screen that we
                        \ drew in PreviewLandscape with an all-blue palette
                        \ before jumping here

 JSR ReadKeyboard       \ Enable the keyboard, flush the keyboard buffer and
                        \ read a character from it (so this waits for a key
                        \ press before starting the game, following the "PRESS
                        \ ANY KEY" message on the landscape preview screen)

 LSR gameInProgress     \ Clear bit 7 of gameInProgress to indicate that a game
                        \ now in progress and we are no longer in the title and
                        \ preview screens (so the interrupt handler can now
                        \ update the game)