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⌂ ∞ ▽ Aviator on the BBC Micro

Drawing lines: DrawIndicatorBar

       Name: DrawIndicatorBar                                        [Show more]
       Type: Subroutine
   Category: Drawing lines
    Summary: Draw a vertical bar on indicator 9 (rudder) or 11 (thrust)

    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * UpdateIndicator (Part 13 of 15) calls DrawIndicatorBar




 Arguments:

   A                    The height of the vertical bar in pixels

   S                    Defines the starting coordinate for the line:

                          * 0 = (joyCoord, H + W)

                          * 128 = (H + W, joyCoord)

                        As all the vertical bar indicators are horizontal in
                        Aviator, only the second option is used

   X                    The offset from yJoyCoord where we store the indicator
                        value, so it can be erased when the bar needs to move:

                          * 1 = rudder indicator (indicator 9)

                          * 3 = thrust indicator (indicator 11)

   W                    The x-coordinate of the centre point of the indicator

   Y                    The y-coordinate of the top of the bar



.DrawIndicatorBar

 STA U                  \ Set U = A, so the line is A pixels tall

 LDA #1                 \ Set T = 1, so the line is 1 pixel wide
 STA T

 STY joyCoord           \ Set joyCoord = Y, the y-coordinate of the top of the
                        \ bar

 LDA yJoyCoord,X        \ Set G = the X-th byte of yJoyCoord, which contains the
 STA G                  \ x-coordinate of the current bar, so we can erase it

 LDA H                  \ Store H in the X-th byte of yJoyCoord, so the next
 STA yJoyCoord,X        \ time we call this routine, we can use this to erase
                        \ the bar we are about to draw

 JSR EraseOrthoLine     \ Erase the current vertical bar at x-coordinate G

 LDA #0                 \ Set N = 0 to switch the drawing mode to OR logic, so
 STA N                  \ the bar gets drawn on-screen

 JSR DrawOrthoLine      \ Draw a new vertical bar at x-coordinate H

 RTS                    \ Return from the subroutine