Version analysis of PL2 - Elite on the 6502

This code appears in the following versions (click to see it in the source code):

Cassette, Disc (flight), 6502 Second Processor, Master and Electron

Code variations between these versions are shown below.

       Name: PL2
       Type: Subroutine
   Category: Drawing planets

Code variation 1 of 4A variation in the comments only

Tap on a block to expand it, and tap it again to revert.

Cassette, Flight, 6502SP, Master
Electron
    Summary: Remove the planet or sun from the screen
    Summary: Remove the planet from the screen



 Other entry points:

   PL2-1                Contains an RTS


.PL2

Code variation 2 of 4Specific to an individual platform

This variation is blank in the Electron version.

Cassette, Flight, 6502SP, Master
 LDA TYPE               \ Shift bit 0 of the planet/sun's type into the C flag
 LSR A

Code variation 3 of 4Related to Elite's use of the Tube

Tap on a block to expand it, and tap it again to revert.

Cassette, Flight, Master
6502SP
Electron
 BCS P%+5               \ 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, skip the following instruction

 JMP WPLS2              \ This is the planet, so jump to WPLS2 to remove it from
                        \ screen, returning from the subroutine using a tail
                        \ call
 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 WPLS2              \ This is the planet, so jump to WPLS2 to remove it from
                        \ screen, returning from the subroutine using a tail
                        \ call

Code variation 4 of 4Specific to an individual platform

This variation is blank in the Electron version.

Cassette, Flight, 6502SP, Master
 JMP WPLS               \ This is the sun, so jump to WPLS to remove it from
                        \ screen, returning from the subroutine using a tail
                        \ call