This code appears in the following versions (click to see it in the source code):
Code variations between these versions are shown below.
Name: KILLSHP 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.
.KILLSHP STX XX4 \ Store the slot number of the ship to remove in XX4When 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
BNE KS5 \ If our missile is not locked on this ship, jump to KS5
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
In the enhanced versions, the Constrictor is a special ship, and killing it ends the first mission.
This variation is blank in the Cassette and Electron versions.
CPX #CON \ Did we just kill the Constrictor from mission 1? If BNE lll \ not, jump to lll LDA TP \ We just killed the Constrictor from mission 1, so set ORA #%00000010 \ bit 1 of TP to indicate that we have successfully STA TP \ completed mission 1
In the Master version, killing the Constrictor at the end of mission 1 instantly gives you 256 kill points.
This variation is blank in the Cassette, Disc (flight), 6502 Second Processor and Electron versions.
INC TALLY+1 \ Award 256 kill points for killing the Constrictor
There are rock hermits in the advanced versions, and they are classed as junk (along with the escape pod, alloy plate, cargo canister, asteroid, splinter, Shuttle and Transporter).
This variation is blank in the Cassette, Disc (flight) and Electron versions.
CPX #HER \ Did we just kill a rock hermit? If we did, jump to BEQ blacksuspenders \ blacksuspenders to decrease the junk count
In the cassette version, only the escape pod, asteroid and cargo canister are classed as junk. In the enhanced versions, the alloy plate, splinter, Shuttle and Transporter are also junk (and in the advanced versions, rock hermits are also junk). Junk in the vicinity doesn't prevent you from performing an in-system jump - in fact, it gets dragged along for the ride.
See below for more variations related to this code.
This variation is blank in the Cassette and Electron versions.
CPX #JL \ If JL <= X < JH, i.e. the type of ship we killed in X BCC KS7 \ is junk (escape pod, alloy plate, cargo canister, CPX #JH \ asteroid, splinter, Shuttle or Transporter), then keep BCS KS7 \ going, otherwise jump to KS7
This variation is blank in the Cassette, Disc (flight) and Electron versions.
Code variation 11 of 14
See variation 9 above for details.
This variation is blank in the Cassette and Electron versions.
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
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
Ship data blocks have an extra byte in the enhanced versions - the NEWB flags. This gives a total of 37 bytes per ship, compared to 36 bytes in the cassette version.
Tap on a block to expand it, and tap it again to revert.
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)