Skip to navigation

⌂ ∞ ▽ Elite on the BBC Micro and NES

Universe: SPIN

[NES version, Bank 0]

       Name: SPIN                                                    [Show more]
       Type: Subroutine
   Category: Universe
    Summary: Randomly spawn cargo from a destroyed ship

    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * Main flight loop (Part 11 of 16) calls SPIN
             * Main flight loop (Part 11 of 16) calls via SPIN2




 Arguments:

   Y                    The type of cargo to consider spawning (typically #PLT
                        or #OIL)



 Other entry points:

   SPIN2                Remove any randomness: spawn cargo of a specific type
                        (given in X), and always spawn the number given in A



.SPIN

 JSR DORND              ; Fetch a random number, and jump to oh if it is
 BPL oh                 ; positive (50% chance)

 TYA                    ; Copy the cargo type from Y into A and X
 TAX

 LDY #0                 ; Set Y = 0 to use as an index into the ship's blueprint
                        ; in the call to GetShipBlueprint

 STA CNT                ; Store the random number in CNT

 JSR GetShipBlueprint   ; Fetch the first byte of the hit ship's blueprint,
                        ; which determines the maximum number of bits of
                        ; debris shown when the ship is destroyed

 AND CNT                ; AND with the random number we fetched above

 AND #15                ; Reduce the random number in A to the range 0-15

.SPIN2

 STA CNT                ; Store the result in CNT, so CNT contains a random
                        ; number between 0 and the maximum number of bits of
                        ; debris that this ship will release when destroyed
                        ; (to a maximum of 15 bits of debris)

.spl

 DEC CNT                ; Decrease the loop counter

 BMI oh                 ; We're going to go round a loop using CNT as a counter
                        ; so this checks whether the counter was zero and jumps
                        ; to oh when it gets there (which might be straight
                        ; away)

 LDA #0                 ; Call SFS1 to spawn the specified cargo from the now
 JSR SFS1               ; deceased parent ship, giving the spawned canister an
                        ; AI flag of 0 (no AI, no E.C.M., non-hostile)

 JMP spl                ; Loop back to spawn the next bit of random cargo