Maths (Geometry): MAS1

       Name: MAS1                                                    [Show more]
       Type: Subroutine
   Category: Maths (Geometry)
    Summary: Add an orientation vector coordinate to an INWK coordinate
  Deep dive: The space station safe zone
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * Main flight loop (Part 14 of 16) calls MAS1


 Add a doubled nosev vector coordinate, e.g. (nosev_y_hi nosev_y_lo) * 2, to
 an INWK coordinate, e.g. (x_sign x_hi x_lo), storing the result in the INWK
 coordinate. The axes used in each side of the addition are specified by the
 arguments X and Y.

 In the comments below, we document the routine as if we are doing the
 following, i.e. if X = 0 and Y = 11:

   (x_sign x_hi x_lo) = (x_sign x_hi x_lo) + (nosev_y_hi nosev_y_lo) * 2

 as that way the variable names in the comments contain "x" and "y" to match
 the registers that specify the vector axis to use.


 Arguments:

   X                    The coordinate to add, as follows:

                          * If X = 0, add (x_sign x_hi x_lo)
                          * If X = 3, add (y_sign y_hi y_lo)
                          * If X = 6, add (z_sign z_hi z_lo)

   Y                    The vector to add, as follows:

                          * If Y = 9,  add (nosev_x_hi nosev_x_lo)
                          * If Y = 11, add (nosev_y_hi nosev_y_lo)
                          * If Y = 13, add (nosev_z_hi nosev_z_lo)


 Returns:

   A                    The highest byte of the result with the sign cleared
                        (e.g. |x_sign| when X = 0, etc.)


 Other entry points:

   MA9                  Contains an RTS


.MAS1

 LDA INWK,Y             ; Set K(2 1) = (nosev_y_hi nosev_y_lo) * 2
 ASL A
 STA K+1
 LDA INWK+1,Y
 ROL A
 STA K+2

 LDA #0                 ; Set K+3 bit 7 to the C flag, so the sign bit of the
 ROR A                  ; above result goes into K+3
 STA K+3

 JSR MVT3               ; Add (x_sign x_hi x_lo) to K(3 2 1)

 STA INWK+2,X           ; Store the sign of the result in x_sign

 LDY K+1                ; Store K(2 1) in (x_hi x_lo)
 STY INWK,X
 LDY K+2
 STY INWK+1,X

 AND #%01111111         ; Set A to the sign byte with the sign cleared,
                        ; i.e. |x_sign| when X = 0

.MA9

 RTS                    ; Return from the subroutine