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⌂ ∞ Elite on the BBC Micro and NES

Charts: TT16

[NES version, Bank 0]

       Name: TT16                                                    [Show more]
       Type: Subroutine
   Category: Charts
    Summary: Move the crosshairs on a chart

    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * TT102 calls TT16




 Move the chart crosshairs by the amount in X and Y.



 Arguments:

   X                    The amount to move the crosshairs in the x-axis

   Y                    The amount to move the crosshairs in the y-axis



.cros1

 JMP SetSelectedSystem  ; Jump to SetSelectedSystem to update the message on
                        ; the chart that shows the current system, returning
                        ; from the subroutine using a tail call

.TT16

 LDA controller1B       ; If the B button is being pressed, jump to cros1 to
 BMI cros1              ; return from the subroutine, as the arrow buttons have
                        ; a different meaning when the B button is held down

 LDA controller1Left03  ; If none of the directional buttons have been held down
 ORA controller1Right03 ; in the last four VBlanks, jump to cros1 to return from
 ORA controller1Up      ; the subroutine, as we don't need to move the
 ORA controller1Down    ; crosshairs
 AND #%11110000
 BEQ cros1

 TXA                    ; Push the change in X onto the stack (let's call this
 PHA                    ; the x-delta)

 BNE mvcr1              ; If the x-delta is non-zero, jump to mvcr1 to update
                        ; the icon bar if required

 TYA                    ; If the y-delta is zero, then the crosshairs are not
 BEQ mvcr2              ; moving, so jump to mvcr2 to skip the following,
                        ; otherwise keep going to update the icon bar if
                        ; required

.mvcr1

                        ; If we get here then the crosshairs are on the move

 LDX #%00000000         ; Set X to use as the new value of selectedSystemFlag
                        ; below

 LDA selectedSystemFlag ; Set A to the value of selectedSystemFlag

 STX selectedSystemFlag ; Clear bits 6 and 7 of selectedSystemFlag to indicate
                        ; that there is no currently selected system

 ASL A                  ; If bit 6 of selectedSystemFlag was previously clear,
 BPL mvcr2              ; then before the crosshairs moved we weren't locked
                        ; onto a system that we could hyperspace to, so the
                        ; Hyperspace button wouldn't have been showing on the
                        ; icon bar, so jump to mvcr2 to skip the following as
                        ; we don't need to update the icon bar

 TYA                    ; Update the icon bar to remove the hyperspace button
 PHA                    ; now that the crosshairs are no longer pointing to
 JSR UpdateIconBar_b3   ; that system, preserving Y across the subroutine call
 PLA
 TAY

.mvcr2

 DEY                    ; Negate the change in Y and push it onto the stack
 TYA                    ; (let's call this the y-delta)
 EOR #$FF
 PHA

 LDA QQ11               ; If the view type in QQ11 is $9C (Short-range Chart),
 CMP #$9C               ; jump to hair1 to skip the following
 BEQ hair1

                        ; This is not the Short-range Chart, so it must be the
                        ; Long-range Chart, so we double the x-delta and y-delta
                        ; so the crosshairs move twice as fast

 PLA                    ; Set X to the y-delta from the top of the stack
 TAX

 PLA                    ; Fetch the x-delta from the top of the stack, double it
 ASL A                  ; and put it back
 PHA

 TXA                    ; Double the y-delta value in X and put it back on the
 ASL A                  ; stack
 PHA

.hair1

 JSR WaitForNMI         ; Wait until the next NMI interrupt has passed (i.e. the
                        ; next VBlank)

 PLA                    ; Store the y-delta in QQ19+3 and fetch the current
 STA QQ19+3             ; y-coordinate of the crosshairs from QQ10 into A, ready
 LDA QQ10               ; for the call to TT123

 JSR TT123              ; Call TT123 to move the selected system's galactic
                        ; y-coordinate by the y-delta, putting the new value in
                        ; QQ19+4

 LDA QQ19+4             ; Store the updated y-coordinate in QQ10 (the current
 STA QQ10               ; y-coordinate of the crosshairs)

 STA QQ19+1             ; This instruction has no effect, as QQ19+1 is
                        ; overwritten below, both in TT103 and TT105

 PLA                    ; Store the x-delta in QQ19+3 and fetch the current
 STA QQ19+3             ; x-coordinate of the crosshairs from QQ10 into A, ready
 LDA QQ9                ; for the call to TT123

 JSR TT123              ; Call TT123 to move the selected system's galactic
                        ; x-coordinate by the x-delta, putting the new value in
                        ; QQ19+4

 LDA QQ19+4             ; Store the updated x-coordinate in QQ9 (the current
 STA QQ9                ; x-coordinate of the crosshairs)

 STA QQ19               ; This instruction has no effect, as QQ19 is overwritten
                        ; below, both in TT103 and TT105

                        ; Now we've updated the coordinates of the crosshairs,
                        ; fall through into TT103 to redraw them at their new
                        ; location