Universe: ZINF

       Name: ZINF                                                    [Show more]
       Type: Subroutine
   Category: Universe
    Summary: Reset the INWK workspace and orientation vectors
  Deep dive: Orientation vectors
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * BRIEF calls ZINF
             * DOKEY calls ZINF
             * FRS1 calls ZINF
             * KS4 calls ZINF
             * Main game loop (Part 2 of 6) calls ZINF
             * SOLAR calls ZINF
             * Ze calls ZINF


 Zero-fill the INWK ship workspace and reset the orientation vectors, with
 nosev pointing out of the screen, towards us.


 Returns:

   Y                    Y is set to $FF


.ZINF

 LDY #NI%-1             ; There are NI% bytes in the INWK workspace, so set a
                        ; counter in Y so we can loop through them

 LDA #0                 ; Set A to 0 so we can zero-fill the workspace

.ZI1

 STA INWK,Y             ; Zero the Y-th byte of the INWK workspace

 DEY                    ; Decrement the loop counter

 BPL ZI1                ; Loop back for the next byte, ending when we have
                        ; zero-filled the last byte at INWK, which leaves Y
                        ; with a value of $FF

                        ; Finally, we reset the orientation vectors as follows:
                        ;
                        ;   sidev = (1,  0,  0)
                        ;   roofv = (0,  1,  0)
                        ;   nosev = (0,  0, -1)
                        ;
                        ; 96 * 256 ($6000) represents 1 in the orientation
                        ; vectors, while -96 * 256 ($E000) represents -1. We
                        ; already set the vectors to zero above, so we just
                        ; need to set up the high bytes of the diagonal values
                        ; and we're done. The negative nosev makes the ship
                        ; point towards us, as the z-axis points into the screen

 LDA #96                ; Set A to represent a 1 (in vector terms)

 STA INWK+18            ; Set byte #18 = roofv_y_hi = 96 = 1

 STA INWK+22            ; Set byte #22 = sidev_x_hi = 96 = 1

 ORA #%10000000         ; Flip the sign of A to represent a -1

 STA INWK+14            ; Set byte #14 = nosev_z_hi = -96 = -1

 RTS                    ; Return from the subroutine