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Elite on the BBC Micro

Universe: NWSHP [6502SP version]

Name: NWSHP [Show more] Type: Subroutine Category: Universe Summary: Add a new ship to our local bubble of universe
Context: See this subroutine in context in the source code Variations: See code variations for this subroutine in the different versions References: This subroutine is called as follows: * BRIEF calls NWSHP * DEMON calls NWSHP * FRS1 calls NWSHP * GTHG calls NWSHP * KS4 calls NWSHP * Main game loop (Part 1 of 6) calls NWSHP * Main game loop (Part 2 of 6) calls NWSHP * Main game loop (Part 3 of 6) calls NWSHP * Main game loop (Part 4 of 6) calls NWSHP * SFS1 calls NWSHP * SOLAR calls NWSHP * SOS1 calls NWSHP * TITLE calls NWSHP

This creates a new block of ship data in the K% workspace, allocates a new block in the ship line heap at WP, adds the new ship's type into the first empty slot in FRIN, and adds a pointer to the ship data into UNIV. If there isn't enough free memory for the new ship, it isn't added. Arguments: A The type of the ship to add (see variable XX21 for a list of ship types) Returns: C flag Set if the ship was successfully added, clear if it wasn't (as there wasn't enough free memory) INF Points to the new ship's data block in K%
.NWSHP STA T \ Store the ship type in location T LDX #0 \ Before we can add a new ship, we need to check \ whether we have an empty slot we can put it in. To do \ this, we need to loop through all the slots to look \ for an empty one, so set a counter in X that starts \ from the first slot at 0. When ships are killed, then \ the slots are shuffled down by the KILLSHP routine, so \ the first empty slot will always come after the last \ filled slot. This allows us to tack the new ship's \ data block and ship line heap onto the end of the \ existing ship data and heap, as shown in the memory \ map below .NWL1 LDA FRIN,X \ Load the ship type for the X-th slot BEQ NW1 \ If it is zero, then this slot is empty and we can use \ it for our new ship, so jump down to NW1 INX \ Otherwise increment X to point to the next slot CPX #NOSH \ If we haven't reached the last slot yet, loop back up BCC NWL1 \ to NWL1 to check the next slot (note that this means \ only slots from 0 to #NOSH - 1 are populated by this \ routine, but there is one more slot reserved in FRIN, \ which is used to identify the end of the slot list \ when shuffling the slots down in the KILLSHP routine) .NW3 CLC \ Otherwise we don't have an empty slot, so we can't RTS \ add a new ship, so clear the C flag to indicate that \ we have not managed to create the new ship, and return \ from the subroutine .NW1 \ If we get here, then we have found an empty slot at \ index X, so we can go ahead and create our new ship. \ We do that by creating a ship data block at INWK and, \ when we are done, copying the block from INWK into \ the K% workspace (specifically, to INF) JSR GINF \ Get the address of the data block for ship slot X \ (which is in workspace K%) and store it in INF LDA T \ If the type of ship that we want to create is BMI NW2 \ negative, then this indicates a planet or sun, so \ jump down to NW2, as the next section sets up a ship \ data block, which doesn't apply to planets and suns, \ as they don't have things like shields, missiles, \ vertices and edges \ This is a ship, so first we need to set up various \ pointers to the ship blueprint we will need. The \ blueprints for each ship type in Elite are stored \ in a table at location XX21, so refer to the comments \ on that variable for more details on the data we're \ about to access ASL A \ Set Y = ship type * 2 TAY LDA XX21-1,Y \ The ship blueprints at XX21 start with a lookup \ table that points to the individual ship blueprints, \ so this fetches the high byte of this particular ship \ type's blueprint BEQ NW3 \ If the high byte is 0 then this is not a valid ship \ type, so jump to NW3 to clear the C flag and return \ from the subroutine STA XX0+1 \ This is a valid ship type, so store the high byte in \ XX0+1 LDA XX21-2,Y \ Fetch the low byte of this particular ship type's STA XX0 \ blueprint and store it in XX0, so XX0(1 0) now \ contains the address of this ship's blueprint CPY #2*SST \ If the ship type is a space station (SST), then jump BEQ NW6 \ to NW6, skipping the heap space steps below, as the \ space station has its own line heap at LSO (which it \ shares with the sun) \ We now want to allocate space for a heap that we can \ use to store the lines we draw for our new ship (so it \ can easily be erased from the screen again). SLSP \ points to the start of the current heap space, and we \ can extend it downwards with the heap for our new ship \ (as the heap space always ends just before the ship \ blueprints at D%) LDY #5 \ Fetch ship blueprint byte #5, which contains the LDA (XX0),Y \ maximum heap size required for plotting the new ship, STA T1 \ and store it in T1 LDA SLSP \ Take the 16-bit address in SLSP and subtract T1, SEC \ storing the 16-bit result in INWK(34 33), so this now SBC T1 \ points to the start of the line heap for our new ship STA INWK+33 LDA SLSP+1 SBC #0 STA INWK+34 \ We now need to check that there is enough free space \ for both this new line heap and the new data block \ for our ship. In memory, this is the layout of the \ ship data blocks and ship line heaps: \ \ +-----------------------------------+ \ | | \ | Ship blueprints | \ | | \ +-----------------------------------+ &D000 = D% \ | | \ | Current ship line heap | \ | | \ +-----------------------------------+ SLSP \ | | \ | Proposed heap for new ship | \ | | \ +-----------------------------------+ INWK(34 33) \ | | \ . . \ . . \ . . \ . . \ . . \ | | \ +-----------------------------------+ INF + NI% \ | | \ | Proposed data block for new ship | \ | | \ +-----------------------------------+ INF \ | | \ | Existing ship data blocks | \ | | \ +-----------------------------------+ &8200 = K% \ \ So, to work out if we have enough space, we have to \ make sure there is room between the end of our new \ ship data block at INF + NI%, and the start of the \ proposed heap for our new ship at the address we \ stored in INWK(34 33). Or, to put it another way, we \ and to make sure that: \ \ INWK(34 33) > INF + NI% \ \ which is the same as saying: \ \ INWK+33 - INF > NI% \ \ because INWK is in zero page, so INWK+34 = 0 LDA INWK+33 \ Calculate INWK+33 - INF, again using 16-bit SBC INF \ arithmetic, and put the result in (A Y), so the high TAY \ byte is in A and the low byte in Y. The subtraction LDA INWK+34 \ works because the previous subtraction will never SBC INF+1 \ underflow, so we know the C flag is set BCC NW3+1 \ If we have an underflow from the subtraction, then \ INF > INWK+33 and we definitely don't have enough \ room for this ship, so jump to NW3+1, which returns \ from the subroutine (with the C flag already cleared) BNE NW4 \ If the subtraction of the high bytes in A is not \ zero, and we don't have underflow, then we definitely \ have enough space, so jump to NW4 to continue setting \ up the new ship CPY #NI% \ Otherwise the high bytes are the same in our BCC NW3+1 \ subtraction, so now we compare the low byte of the \ result (which is in Y) with NI%. This is the same as \ doing INWK+33 - INF > NI% (see above). If this isn't \ true, the C flag will be clear and we don't have \ enough space, so we jump to NW3+1, which returns \ from the subroutine (with the C flag already cleared) .NW4 LDA INWK+33 \ If we get here then we do have enough space for our STA SLSP \ new ship, so store the new bottom of the ship line LDA INWK+34 \ heap (i.e. INWK+33) in SLSP, doing both the high and STA SLSP+1 \ low bytes .NW6 LDY #14 \ Fetch ship blueprint byte #14, which contains the LDA (XX0),Y \ ship's energy, and store it in byte #35 STA INWK+35 LDY #19 \ Fetch ship blueprint byte #19, which contains the LDA (XX0),Y \ number of missiles and laser power, and AND with %111 AND #%00000111 \ to extract the number of missiles before storing in STA INWK+31 \ byte #31 LDA T \ Restore the ship type we stored above .NW2 STA FRIN,X \ Store the ship type in the X-th byte of FRIN, so the \ this slot is now shown as occupied in the index table TAX \ Copy the ship type into X BMI NW8 \ If the ship type is negative (planet or sun), then \ jump to NW8 to skip the following instructions CPX #HER \ If the ship type is a rock hermit, jump to gangbang BEQ gangbang \ to increase the junk count CPX #JL \ If JL <= X < JH, i.e. the type of ship we killed in X BCC NW7 \ is junk (escape pod, alloy plate, cargo canister, CPX #JH \ asteroid, splinter, Shuttle or Transporter), then keep BCS NW7 \ going, otherwise jump to NW7 .gangbang INC JUNK \ We're adding junk, so increase the junk counter .NW7 INC MANY,X \ Increment the total number of ships of type X .NW8 LDY T \ Restore the ship type we stored above LDA E%-1,Y \ Fetch the E% byte for this ship to get the default \ settings for the ship's NEWB flags AND #%01101111 \ Zero bits 4 and 7 (so the new ship is not docking, has \ has not been scooped, and has not just docked) ORA NEWB \ Apply the result to the ship's NEWB flags, which sets STA NEWB \ bits 0-3 and 5-6 in NEWB if they are set in the E% \ byte LDY #(NI%-1) \ The final step is to copy the new ship's data block \ from INWK to INF, so set up a counter for NI% bytes \ in Y .NWL3 LDA INWK,Y \ Load the Y-th byte of INWK and store in the Y-th byte STA (INF),Y \ of the workspace pointed to by INF DEY \ Decrement the loop counter BPL NWL3 \ Loop back for the next byte until we have copied them \ all over SEC \ We have successfully created our new ship, so set the \ C flag to indicate success RTS \ Return from the subroutine