The TT103 subroutine - Elite on the 6502

       Name: TT103                                                   [Show more]
       Type: Subroutine
   Category: Charts
    Summary: Draw a small set of crosshairs on a chart
    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:
             * hm calls TT103
             * HME2 calls TT103
             * TT102 calls TT103
             * TT16 calls TT103
             * TT23 calls TT103


 Draw a small set of crosshairs on a galactic chart at the coordinates in
 (QQ9, QQ10).


.TT103

 LDA #GREEN             \ Switch to stripe 3-1-3-1, which is white/yellow in the
 STA COL                \ chart view

 LDA QQ11               \ Fetch the current view type into A

 BMI TT105              \ If this is the Short-range Chart screen, jump to TT105

 LDA QQ9                \ Store the crosshairs x-coordinate in QQ19
 JSR SCALEX             \
 STA QQ19               \ The call to SCALEX has no effect as it only contains
                        \ an RTS, but having this call instruction here would
                        \ enable different scaling to be applied by altering
                        \ the SCALE routines
                        \
                        \ This code is left over from the Apple II version,
                        \ where the scale factor is different

 LDA QQ10               \ Halve the crosshairs y-coordinate and store it in QQ19
 JSR SCALEY             \ (we halve it because the Long-range Chart is half as
 STA QQ19+1             \ high as it is wide)
                        \
                        \ The call to SCALEY simply does an LSR A, but having
                        \ this call instruction here would enable different
                        \ scaling to be applied by altering the SCALE routines
                        \
                        \ This code is left over from the Apple II version,
                        \ where the scale factor is different

 LDA #4                 \ Set QQ19+2 to 4 denote crosshairs of size 4
 STA QQ19+2

 JMP TT15               \ Jump to TT15 to draw crosshairs of size 4 at the
                        \ crosshairs coordinates, returning from the subroutine
                        \ using a tail call

[X]

Variable COL in workspace ZP

Temporary storage, used to store colour information when drawing pixels in the dashboard

[X]

Configuration variable GREEN = %10101111

Four mode 1 pixels of colour 3, 1, 3, 1 (cyan/yellow)

[X]

Variable QQ10 in workspace ZP

The galactic y-coordinate of the crosshairs in the galaxy chart (and, most of the time, the selected system's galactic y-coordinate)

[X]

Variable QQ11 in workspace ZP

The type of the current view

[X]

Variable QQ19 in workspace ZP

Temporary storage, used in a number of places

[X]

Variable QQ9 in workspace ZP

The galactic x-coordinate of the crosshairs in the galaxy chart (and, most of the time, the selected system's galactic x-coordinate)

[X]

Subroutine SCALEX (category: Maths (Geometry))

Scale the x-coordinate in A (leave it unchanged)

[X]

Subroutine SCALEY (category: Maths (Geometry))

Scale the y-coordinate in A to 0.5 * A

[X]

Subroutine TT105 (category: Charts)

Draw crosshairs on the Short-range Chart, with clipping

[X]

Subroutine TT15 (category: Drawing lines)

Draw a set of crosshairs

Charts: TT103

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