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⌂ ∞ ▽ Elite on the 6502

Drawing planets: PL2

[6502 Second Processor version]

       Name: PL2                                                     [Show more]
       Type: Subroutine
   Category: Drawing planets
    Summary: Remove the planet or sun from the screen

    Context: See this subroutine in context in the source code
 Variations: See code variations for this subroutine in the different versions
 References: This subroutine is called as follows:
             * PLANET calls PL2
             * PROJ calls via PL2-1




 Other entry points:

   PL2-1                Contains an RTS



.PL2

 LDA TYPE               \ Shift bit 0 of the planet/sun's type into the C flag
 LSR A

 BCS PL57               \ If the planet/sun's type has bit 0 clear, then it's
                        \ either 128 or 130, which is a planet; meanwhile, the
                        \ sun has type 129, which has bit 0 set. So if this is
                        \ the sun, jump to PL57 to skip the following
                        \ instructions

 JSR LS2FL              \ Call LS2FL to send the ball line heap to the I/O
                        \ processor for drawing on-screen, which redraws the
                        \ planet and this removes it from the screen

 STZ LSP                \ Reset the ball line heap by setting the ball line heap
                        \ pointer to 0

 RTS                    \ Return from the subroutine

.PL57

 JMP WPLS               \ This is the sun, so jump to WPLS to remove it from
                        \ screen, returning from the subroutine using a tail
                        \ call