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BBC Micro Elite

Universe: KS2

Name: KS2 [View in context] Type: Subroutine Category: Universe Summary: Check the local bubble for missiles with target lock
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 INX \ Increment the counter (so it starts at 0 on the first \ iteration) LDA FRIN,X \ If slot X is empty, loop round again until it isn't, BEQ KS3 \ at which point A contains the ship type in that slot 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) }