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⌂ ∞ ▽ Elite on the 6502

Version analysis of cntr

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

Code variations between these versions are shown below.

       Name: cntr
       Type: Subroutine
   Category: Dashboard
    Summary: Apply damping to the pitch or roll dashboard indicator



 Apply damping to the value in X, where X ranges from 1 to 255 with 128 as the
 centre point (so X represents a position on a centre-based dashboard slider,
 such as pitch or roll). If the value is in the left-hand side of the slider
 (1-127) then it bumps the value up by 1 so it moves towards the centre, and
 if it's in the right-hand side, it reduces it by 1, also moving it towards the
 centre.



.cntr

Code variation 1 of 3Related to an enhanced feature

When the docking computer takes over the controls in the enhanced versions, keyboard damping is disabled, so it can make finer course corrections during docking.

This variation is blank in the Cassette, Disc (docked) and Electron versions.

Flight, 6502SP, Master
 LDA auto               \ If the docking computer is currently activated, jump
 BNE cnt2               \ to cnt2 to skip the following as we always want to
                        \ enable damping for the docking computer

 LDA DAMP               \ If DAMP is non-zero, then keyboard damping is not
 BNE RE1                \ enabled, so jump to RE1 to return from the subroutine

Code variation 2 of 3A variation in the labels only

This variation is blank in the Cassette, Disc (docked) and Electron versions.

Flight, 6502SP, Master
.cnt2

 TXA                    \ If X < 128, then it's in the left-hand side of the
 BPL BUMP               \ dashboard slider, so jump to BUMP to bump it up by 1,
                        \ to move it closer to the centre

Code variation 3 of 3A variation in the labels only

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

Cassette, Flight, Docked, 6502SP, Electron
Master
 DEX                    \ Otherwise X >= 128, so it's in the right-hand side
 BMI RE1                \ of the dashboard slider, so decrement X by 1, and if
                        \ it's still >= 128, jump to RE1 to return from the
                        \ subroutine, otherwise fall through to BUMP to undo
                        \ the bump and then return

 DEX                    \ Otherwise X >= 128, so it's in the right-hand side
 BMI ARSR1              \ of the dashboard slider, so decrement X by 1, and if
                        \ it's still >= 128, jump to ARSR1 to return from the
                        \ subroutine, otherwise fall through to BUMP to undo
                        \ the bump and then return

.BUMP

 INX                    \ Bump X up by 1, and if it hasn't overshot the end of
 BNE RE1                \ the dashboard slider, jump to RE1 to return from the
                        \ subroutine, otherwise fall through to REDU to drop
                        \ it down by 1 again

.REDU

 DEX                    \ Reduce X by 1, and if we have reached 0 jump up to
 BEQ BUMP               \ BUMP to add 1, because we need the value to be in the
                        \ range 1 to 255

.RE1

 RTS                    \ Return from the subroutine