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⌂ ∞ ▽ The Sentinel on the BBC Micro

3D objects: SpawnObject

       Name: SpawnObject                                             [Show more]
       Type: Subroutine
   Category: 3D objects
    Summary: Add a new object of the specified type to the objectTypes table

    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * AddEnemiesToTiles calls SpawnObject
             * ExpendEnemyEnergy calls SpawnObject
             * PerformHyperspace calls SpawnObject
             * SpawnPlayer calls SpawnObject
             * SpawnTrees calls SpawnObject
             * ProcessActionKeys (Part 2 of 2) calls via SpawnObject+3




 This routine spawns a new object by searching the objectFlags table for a free
 number to allocate. If there is no free number then the routine returns with
 the C flag set, otherwise the C flag is clear, X and currentObject are set to
 the number of the new object, and the object type is added to the objectTypes
 table.

 Note that this routine only adds the object to the objectTypes table; it
 doesn't update the flags or add any other information about the object.



 Arguments:

   A                    The type of object to spawn:

                          * 0 = Robot (one of which is the player)

                          * 1 = Sentry

                          * 2 = Tree

                          * 3 = Boulder

                          * 4 = Meanie

                          * 5 = The Sentinel

                          * 6 = The Sentinel's tower



 Returns:

   X                    The number of the new object (if successful)

   currentObject        The number of the new object (if successful)

   C flag               Status flag:

                          * Clear if the object was successfully spawned

                          * Set if there is no free number for the new object



 Other entry points:

   SpawnObject+3        Spawn an object of the type specified in keyPress

                        The keyPress and objectType variables share the same
                        memory location, so this lets us store object types in
                        the key press codes in keyLoggerConfig, so that pressing
                        one of the "create" keys will automatically spawn that
                        type of object



.SpawnObject

 STA objectType         \ Store the object type in objectType for future
                        \ reference

 LDX #63                \ In order to be able to create a new object, we need to
                        \ find an unallocated object number in the objectFlags
                        \ table
                        \
                        \ The game can support up to 64 objects, each with its
                        \ own number (0 to 63), so set a counter in X to work
                        \ through the object numbers until we find one that is
                        \ not allocated to an object

.sobj1

 LDA objectFlags,X      \ Set A to the object flags for object #X, which are
                        \ stored in the X-th entry in the objectFlags table

 BMI sobj2              \ If bit 7 of object flags for object #X is set then
                        \ this object number is not yet allocated to an object,
                        \ so jump to sobj2 use this number for our new object

 DEX                    \ Otherwise decrement the counter in X to move on to the
                        \ next object number

 BPL sobj1              \ Loop back to sobj1 to check the next object number

 SEC                    \ If we get here then we have checked all 64 object
                        \ numbers and none of them are free, so set the C flag
                        \ to indicate that we have failed to spawn the object

 RTS                    \ Return from the subroutine

.sobj2

                        \ If we get here then we have found an unallocated
                        \ object number in the objectFlags table at index X

 STX currentObject      \ Set currentObject to the object number in X

 LDA objectType         \ Set the corresponding entry in the objectTypes table
 STA objectTypes,X      \ to the object type that we are spawning, which we
                        \ stored in objectType above

 CLC                    \ Clear the C flag to indicate that we have successfully
                        \ added a new object to the objectFlags table

 RTS                    \ Return from the subroutine