Universe: KS2

       Name: KS2                                                     [Show more]
       Type: Subroutine
   Category: Universe
    Summary: Check the local bubble for missiles with target lock
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * KILLSHP calls KS2


 Check the local bubble of universe to see if there are any missiles with
 target lock in the vicinity. If there are, then check their targets; if we
 just removed their target in the KILLSHP routine, then switch off their AI so
 they just drift in space, otherwise update their targets to reflect the newly
 shuffled slot numbers.

 This is called from KILLSHP once the slots have been shuffled down, following
 the removal of a ship.


 Arguments:

   XX4                  The slot number of the ship we removed just before
                        calling this routine


.KS2

 LDX #$FF               ; We want to go through the ships in our local bubble
                        ; and pick out all the missiles, so set X to $FF to
                        ; use as a counter

.KSL4

 SETUP_PPU_FOR_ICON_BAR ; If the PPU has started drawing the icon bar, configure
                        ; the PPU to use nametable 0 and pattern table 0

 INX                    ; Increment the counter (so it starts at 0 on the first
                        ; iteration)

 LDA FRIN,X             ; If slot X is empty then we have worked our way through
 BEQ KS3                ; all the slots, so jump to KS3 to stop looking

 CMP #MSL               ; If the slot does not contain a missile, loop back to
 BNE KSL4               ; KSL4 to check the next slot

                        ; We have found a slot containing a missile, so now we
                        ; want to check whether it has target lock

 TXA                    ; Set Y = X * 2 and fetch the Y-th address from UNIV
 ASL A                  ; and store it in SC and SC+1 - in other words, set
 TAY                    ; SC(1 0) to point to the missile's ship data block
 LDA UNIV,Y
 STA SC
 LDA UNIV+1,Y
 STA SC+1

 LDY #32                ; Fetch byte #32 from the missile's ship data (AI)
 LDA (SC),Y

 BPL KSL4               ; If bit 7 of byte #32 is clear, then the missile is
                        ; dumb and has no AI, so loop back to KSL4 to move on
                        ; to the next slot

 AND #%01111111         ; Otherwise this missile has AI, so clear bit 7 and
 LSR A                  ; shift right to set the C flag to the missile's "is
                        ; locked" flag, and A to the target's slot number

 CMP XX4                ; If this missile's target is less than XX4, then the
 BCC KSL4               ; target's slot isn't being shuffled down, so jump to
                        ; KSL4 to move on to the next slot

 BEQ KS6                ; If this missile was locked onto the ship that we just
                        ; removed in KILLSHP, jump to KS6 to stop the missile
                        ; from continuing to hunt it down

 SBC #1                 ; Otherwise this missile is locked and has AI enabled,
                        ; and its target will have moved down a slot, so
                        ; subtract 1 from the target number (we know C is set
                        ; from the BCC above)

 ASL A                  ; Shift the target number left by 1, so it's in bits
                        ; 1-6 once again, and also set bit 0 to 1, as the C
                        ; flag is still set, so this makes sure the missile is
                        ; still set to being locked

 ORA #%10000000         ; Set bit 7, so the missile's AI is enabled

 STA (SC),Y             ; Update the missile's AI flag to the value in A

 BNE KSL4               ; Loop back to KSL4 to move on to the next slot (this
                        ; BNE is effectively a JMP as A will never be zero)

.KS6

 LDA #0                 ; The missile's target lock just got removed, so set the
 STA (SC),Y             ; AI flag to 0 to make it dumb and not locked

 BEQ KSL4               ; Loop back to KSL4 to move on to the next slot (this
                        ; BEQ is effectively a JMP as A is always zero)