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BBC Micro Elite

Universe: KILLSHP

Name: KILLSHP [View in context] Type: Subroutine Category: Universe Summary: Remove a ship from our local bubble of universe
Remove the ship in slot X from our local bubble of universe. This happens when we kill a ship, collide with a ship and destroy it, or when a ship moves outside our local bubble. We also use this routine when we move out of range of the space station, in which case we replace it with the sun. When removing a ship, this creates a gap in the ship slots at FRIN, so we shuffle all the later slots down to close the gap. We also shuffle the ship data blocks at K% and ship line heap at WP, to reclaim all the memory that the removed ship used to occupy. Arguments: X The slot number of the ship to remove XX0 The address of the blueprint for the ship to remove INF The address of the data block for the ship to remove
.KILLSHP STX XX4 \ Store the slot number of the ship to remove in XX4 \ The following two instructions appear in the BASIC \ source file (ELITEF), but in the text source file \ (ELITEF.TXT) they are replaced by: \ \ CPX MSTG \ \ which does the same thing, but saves two bytes of \ memory (as CPX MSTG is a two-byte opcode, while LDA \ MSTG and CMP XX4 take up four bytes between them) LDA MSTG \ Check whether this slot matches the slot number in CMP XX4 \ MSTG, which is the target of our missile lock BNE KS5 \ If our missile is not locked on this ship, jump to KS5 LDY #&EE \ Otherwise we need to remove our missile lock, so call JSR ABORT \ ABORT to disarm the missile and update the missile \ indicators on the dashboard to green/cyan (Y = &EE) LDA #200 \ Print recursive token 40 ("TARGET LOST") as an JSR MESS \ in-flight message .KS5 LDY XX4 \ Restore the slot number of the ship to remove into Y LDX FRIN,Y \ Fetch the contents of the slot, which contains the \ ship type CPX #SST \ If this is the space station, then jump to KS4 to BEQ KS4 \ replace the space station with the sun DEC MANY,X \ Decrease the number of this type of ship in our little \ bubble, which is stored in MANY+X (where X is the ship \ type) LDX XX4 \ Restore the slot number of the ship to remove into X \ We now want to remove this ship and reclaim all the \ memory that it uses. Removing the ship will leave a \ gap in three places, which we need to close up: \ \ * The ship slots in FRIN \ \ * The ship data blocks in K% \ \ * The descending ship line heap at WP down \ \ The rest of this routine closes up these gaps by \ looping through all the occupied ship slots after the \ slot we are removing, one by one, and shuffling each \ ship's slot, data block and line heap down to close \ up the gaps left by the removed ship. As part of this, \ we have to make sure we update any address pointers \ so they point to the newly shuffled data blocks and \ line heaps \ \ In the following, when shuffling a ship's data down \ into the preceding empty slot, we call the ship that \ we are shuffling down the "source", and we call the \ empty slot we are shuffling it into the "destination" \ \ Before we start looping through the ships we need to \ shuffle down, we need to set up some variables to \ point to the source and destination line heaps LDY #5 \ Fetch byte #5 of the removed ship's blueprint into A, LDA (XX0),Y \ which gives the ship's maximum heap size for the ship \ we are removing (i.e. the size of the gap in the heap \ created by the ship removal) \ INF currently contains the ship data for the ship we \ are removing, and INF(34 33) contains the address of \ the bottom of the ship's heap, so we can calculate \ the address of the top of the heap by adding the heap \ size to this address LDY #33 \ First we add A and the address in INF+33, to get the CLC \ low byte of the top of the heap, which we store in P ADC (INF),Y STA P INY \ And next we add A and address in INF+34, with any LDA (INF),Y \ from the previous addition, to get the high byte of ADC #0 \ the top of the heap, which we store in P+1, so P(1 0) STA P+1 \ points to the top of this ship's heap \ Now, we're ready to start looping through the ships \ we want to move, moving the slots, data blocks and \ line heap from the source to the destination. In the \ following, we set up SC to point to the source data, \ and INF (which currently points to the removed ship's \ data that we can now overwrite) points to the \ destination \ \ So P(1 0) now points to the top of the line heap for \ the destination .KSL1 INX \ On entry, X points to the empty slot we want to \ shuffle the next ship into (the destination), so \ this increment points X to the next slot - i.e. the \ source slot we want to shuffle down LDA FRIN,X \ Copy the contents of the source slot into the STA FRIN-1,X \ destination slot BEQ KS2 \ If the slot we just shuffled down contains 0, then \ the source slot is empty and we are done shuffling, \ so jump to KS2 to move on to processing missiles ASL A \ Otherwise we have a source ship to shuffle down into TAY \ the destination, so set Y = A * 2 so it can act as an \ index into the two-byte ship blueprint lookup table \ at XX21 for the source ship LDA XX21-2,Y \ Set SC(0 1) to point to the blueprint data for the STA SC \ source ship LDA XX21-1,Y STA SC+1 LDY #5 \ Fetch blueprint byte #5 for the source ship, which LDA (SC),Y \ gives us its maximum heap size, and store it in T STA T \ We now subtract T from P(1 0), so P(1 0) will point to \ the bottom of the line heap for the destination \ (which we will use later when closing up the gap in \ the heap space) LDA P \ First, we subtract the low bytes SEC SBC T STA P LDA P+1 \ And then we do the high bytes, for which we subtract SBC #0 \ 0 to include any carry, so this is effectively doing STA P+1 \ P(1 0) = P(1 0) - (0 T) \ Next, we want to set SC(1 0) to point to the source \ ship's data block TXA \ Set Y = X * 2 so it can act as an index into the ASL A \ two-byte lookup table at UNIV, which contains the TAY \ addresses of the ship data blocks. In this case we are \ multiplying X by 2, and X contains the source ship's \ slot number so Y is now an index for the source ship's \ entry in UNIV LDA UNIV,Y \ Set SC(1 0) to the address of the data block for the STA SC \ source ship LDA UNIV+1,Y STA SC+1 \ We have now set up our variables as follows: \ \ SC(1 0) points to the source's ship data block \ \ INF(1 0) points to the destination's ship data block \ \ P(1 0) points to the destination's line heap \ \ so let's start copying data from the source to the \ destination LDY #35 \ We are going to be using Y as a counter for the 36 \ bytes of ship data we want to copy from the source \ to the destination, so we set it to 35 to start things \ off, and will decrement Y for each byte we copy LDA (SC),Y \ Fetch byte #35 of the source's ship data block at SC, STA (INF),Y \ and store it in byte #35 of the destination's block \ at INF, so that's the ship's energy copied from the \ source to the destination. One down, quite a few to \ go... DEY \ Fetch byte #34 of the source ship, which is the LDA (SC),Y \ high byte of the source ship's line heap, and store STA K+1 \ in K+1 LDA P+1 \ Set the low byte of the destination's heap pointer STA (INF),Y \ to P+1 DEY \ Fetch byte #33 of the source ship, which is the LDA (SC),Y \ low byte of the source ship's heap, and store in K STA K \ so now we have the following: \ \ K(1 0) points to the source's line heap LDA P \ Set the low byte of the destination's heap pointer STA (INF),Y \ to P, so now the destination's heap pointer is to \ P(1 0), so that's the heap pointer in bytes #33 and \ #34 done DEY \ Luckily, we can just copy the rest of the source's \ ship data block into the destination, as there are no \ more address pointers, so first we decrement our \ counter in Y to point to the next byte (the AI flag) \ in byte #32) and then start looping .KSL2 LDA (SC),Y \ Copy the Y-th byte of the source to the Y-th byte of STA (INF),Y \ the destination DEY \ Decrement the counter BPL KSL2 \ Loop back to KSL2 to copy the next byte until we have \ copied the whole block \ We have now shuffled the ship's slot and the ship's \ data block, so we only have the heap data itself to do LDA SC \ First, we copy SC into INF, so when we loop round STA INF \ again, INF will correctly point to the destination for LDA SC+1 \ the next iteration STA INF+1 LDY T \ Now we want to move the contents of the heap, as all \ we did above was to update the pointers, so first \ we set a counter in Y that is initially set to T \ (which we set above to the maximum heap size for the \ source ship) \ \ As a reminder, we have already set the following: \ \ K(1 0) points to the source's line heap \ \ P(1 0) points to the destination's line heap \ \ so we can move the heap data by simply copying the \ correct number of bytes from K(1 0) to P(1 0) .KSL3 DEY \ Decrement the counter LDA (K),Y \ Copy the Y-th byte of the source heap at K(1 0) to STA (P),Y \ the destination heap at P(1 0) TYA \ Loop back to KSL3 to copy the next byte, until we BNE KSL3 \ have done them all BEQ KSL1 \ We have now shuffled everything down one slot, so \ jump back up to KSL1 to see if there is another slot \ that needs shuffling down (this BEQ is effectively a \ JMP as A will always be zero)