Universe: SetSelectionFlags

       Name: SetSelectionFlags                                       [Show more]
       Type: Subroutine
   Category: Universe
    Summary: Set the selected system flags for the currently selected system
             and update the icon bar if required
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * Main flight loop (Part 15 of 16) calls SetSelectionFlags
             * SelectNearbySystem calls SetSelectionFlags


.SetSelectionFlags

 LDA QQ8+1              ; If the high byte of the distance to the selected
 BNE ssel3              ; system in QQ18(1 0) is non-zero, then it is a long way
                        ; from us, so jump to ssel3 to set the selected system
                        ; flags to indicate we can't hyperspace there

 LDA QQ8                ; If the low byte of the distance to the selected
 BNE ssel1              ; system in QQ18(1 0) is non-zero, then jump to ssel1
                        ; to check whether we have enough fuel to hyperspace
                        ; there

                        ; If we get here then QQ18(1 0) is zero, so the selected
                        ; system is the same as the current system

 LDA MJ                 ; If MJ is zero then we are not in witchspace, so jump
 BEQ ssel3              ; to ssel3 to set the selected system flags to indicate
                        ; we can't hyperspace to the selected system (as it is
                        ; the same as the current system)

 BNE ssel2              ; MJ is non-zero so we are in witchspace, so jump to
                        ; ssel2 to set the selected system flags to indicate we
                        ; can hyperspace to the selected system (as we are in
                        ; the middle of nowhere without a current system)

.ssel1

 CMP QQ14               ; If the distance to the selected system is equal to the
 BEQ ssel2              ; fuel level in QQ14, jump to ssel2 to set the selected
                        ; system flags to indicate we can hyperspace to the
                        ; selected system

 BCS ssel3              ; If the distance to the selected system is greater than
                        ; the fuel level in QQ14, jump to ssel3 to set the
                        ; selected system flags to indicate we can't hyperspace
                        ; to the selected system (as we don't have enough fuel)

                        ; If we get here then the distance to the selected
                        ; system is less than the fuel level in QQ14, so fall
                        ; through into ssel2 to set the selected system flags to
                        ; indicate we can hyperspace to the selected system

.ssel2

 LDA #%11000000         ; Set A so we set bits 6 and 7 of the selected system
                        ; flags below to indicate that a system is selected and
                        ; we can hyperspace to it

 BNE ssel4              ; Jump to ssel4 to skip the following instruction (this
                        ; BNE is effectively a JMP as A is never zero)

.ssel3

 LDA #%10000000         ; Set A so we set bit 7 and clear bit 6 of the selected
                        ; system flags below to indicate that a system is
                        ; selected but we can't hyperspace to it

.ssel4

 TAX                    ; Copy A into X so we can set selectedSystemFlag to this
                        ; value below

 EOR selectedSystemFlag ; Flip bit 7 and possibly bit 6 in selectedSystemFlag
                        ; and keep the result in A

 STX selectedSystemFlag ; Set selectedSystemFlag to X

 ASL A                  ; If bit 6 of the EOR result in A is clear, then the
 BPL RTS6               ; state of bit 6 did not changed in the update above, so
                        ; we don't need to update the icon bar to show or hide
                        ; the hyperspace button, so return from the subroutine
                        ; (as RTS6 contains an RTS)

 JMP UpdateIconBar_b3   ; Otherwise the newly selected system has a different
                        ; "can we hyperspace here?" status to the previous
                        ; selected system, so we need to update the icon bar to
                        ; either hide or show the Hyperspace button, returning
                        ; from the subroutine using a tail call