Flight: InSystemJump

       Name: InSystemJump                                            [Show more]
       Type: Subroutine
   Category: Flight
    Summary: Perform an in-system (fast-forward) jump
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * FastForwardJump calls InSystemJump
             * WARP calls InSystemJump


 This routine performs an in-system jump by subtracting 64 from z_hi for the
 planet and sun, and removing all other ships from the bubble. This is the same
 as our ship moving forwards in space by z_hi = 64, and leaving all the other
 ships behind.


.InSystemJump

 LDY #32                ; We start by charging the shields and energy banks 32
                        ; times, so set a loop counter in Y

.jump1

 JSR ChargeShields      ; Charge the shields and energy banks

 DEY                    ; Decrement the loop counter

 BNE jump1              ; Loop back to charge the shields until we have done it
                        ; 32 times

                        ; We now move the sun and planet backwards in space and
                        ; remove everything else from the ship slots, to make it
                        ; appear as if we have jumped forward, leaving
                        ; everything else behind

 LDX #0                 ; We are about to loop through the ship slots, moving
                        ; everything backwards so we appear to jump forwards in
                        ; space, so set X = 0 to use as the slot number

 STX GNTMP              ; Set GNTMP = 0 to cool the lasers down completely

.jump2

 STX XSAV               ; Store the slot number in XSAV so we can retrieve it
                        ; below

 LDA FRIN,X             ; Load the ship type for the X-th slot

 BEQ jump4              ; If the slot contains 0 then it is empty and we have
                        ; processed all the slots (as they are always shuffled
                        ; down in the main loop to close up and gaps), so jump
                        ; to jump4 as we are done

 BMI jump3              ; If the slot contains a ship type with bit 7 set, then
                        ; it contains the planet or the sun, so jump down to
                        ; jump3 to move the planet or sun in space

                        ; If we get here then this is not the planet or sun, so
                        ; we now remove this ship from our local bubble of
                        ; universe

 JSR GINF               ; Call GINF to get the address of the data block for
                        ; ship slot X and store it in INF

 JSR RemoveShip         ; Fetch the ship's data block and remove the ship from
                        ; our local bubble of universe

 LDX XSAV               ; Set X to the slot counter that we stored in XSAV above

 JMP jump2              ; Loop back to process the next slot

.jump3

 JSR GINF               ; Call GINF to get the address of the data block for
                        ; ship slot X and store it in INF

 LDA #$80               ; Set (S R) = -64
 STA S                  ;
 LSR A                  ; This is a sign-magnitude number, with bit 7 of S set
 STA R                  ; and R = 128 / 2 = 64

 LDY #7                 ; Set P = z_hi from the ship's data block
 LDA (INF),Y
 STA P

 INY                    ; Set A = z_sign from the ship's data block
 LDA (INF),Y

 JSR ADD                ; Set (A X) = (A P) + (S R)
                        ;           = (z_sign z_hi) - 64

 STA (INF),Y            ; Store the result in (z_sign z_hi) in the ship's data
 DEY                    ; block, so the object moves backwards by a distance of
 TXA                    ; z_hi = 64 (which is the distance of an in-system jump)
 STA (INF),Y

 LDX XSAV               ; Set X to the slot counter that we stored in XSAV above

 INX                    ; Increment X to point to the next ship slot

 BNE jump2              ; Loop back to process the next slot (this BNE is
                        ; effectively a JMP as we will exit the above loop well
                        ; before X wraps around to 0

.jump4

 RTS                    ; Return from the subroutine