The Theme: UpdateBullets

       Name: UpdateBullets                                           [Show more]
       Type: Subroutine
   Category: The Theme
    Summary: Move the bullets through the air
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * MainLoop (Part 3 of 15) calls UpdateBullets


 This is called UBUL in the original source code.


.UpdateBullets

 LDY #15                \ The bullets are made up of objects 12 to 15, so set a
 STY objectId           \ counter in objectId to count through all four

 LDA #98                \ Objects 12 to 15 are made up of points 95 to 98, so
 STA GG                 \ set a counter in GG to count through all four

.ubul1

 TYA                    \ Set X = Y + 216
 CLC                    \
 ADC #216               \ so X will be 228 to 231 for objects 12 to 15, which is
 TAX                    \ the ID of each bullet object's velocity vector, as set
                        \ up in the FireGuns routine

 JSR AddPointToObject   \ Add the vector in point X to the object coordinates
                        \ for object Y (in other words, add the velocity vector
                        \ to object Y's coordinates, which moves the bullet in
                        \ space by the correct amount)

 LDA #0                 \ Set showLine = 0 as a starting point for the line's
 STA showLine           \ visibility (so we start out by assuming the line is
                        \ visible, and change this in the following call to
                        \ SetObjectCoords if we find that it isn't)

 JSR SetObjectCoords    \ Calculate the object's coordinates and visibilty,
                        \ updating the object's status byte with the results

 BPL ubul2              \ If bit 7 of the object's updated status byte is clear,
                        \ then the bullets have hit the ground, so jump to ubul2
                        \ to remove the bullets

 LDY GG                 \ Set Y to the point ID that we're checking

 LDX #60                \ Set X to line ID 60 for the call to CheckLineDistance

 JSR CheckLineDistance  \ Check whether point GG on line 60 is within the
                        \ visible distance for the line

 BEQ ubul3              \ If the result is 0, then the point is visible, so
                        \ jump to ubul3 to move onto the next bullet point

.ubul2

                        \ If we get here then this bullet point has either hit
                        \ the ground, or it is too far away to be seen any more,
                        \ so it's time to remove the bullets

 LDA #0                 \ Set firingStatus = 0 to indicate that there are no
 STA firingStatus       \ bullets are in the air

.ubul3

 DEC GG                 \ Decrement the point counter to move on to the next
                        \ point from 98 down to 95

 DEC objectId           \ Decrement the object counter to move on to the next
                        \ object from 15 down to 12

 LDY objectId           \ Loop back until we have processed all four objects
 CPY #12                \ and points
 BCS ubul1

 RTS                    \ Return from the subroutine