Maths (Geometry): MAS2

       Name: MAS2                                                    [Show more]
       Type: Subroutine
   Category: Maths (Geometry)
    Summary: Calculate a cap on the maximum distance to the planet or sun
    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 MAS2
             * Main flight loop (Part 15 of 16) calls MAS2
             * CheckAltitude calls via m


 Given a value in Y that points to the start of a ship data block as an offset
 from K%, calculate the following:

   A = A OR x_sign OR y_sign OR z_sign

 and clear the sign bit of the result. The K% workspace contains the ship data
 blocks, so the offset in Y must be 0 or a multiple of NI% (as each block in
 K% contains NI% bytes).

 The result effectively contains a maximum cap of the three values (though it
 might not be one of the three input values - it's just guaranteed to be
 larger than all of them).

 If Y = 0 and A = 0, then this calculates the maximum cap of the highest byte
 containing the distance to the planet, as K%+2 = x_sign, K%+5 = y_sign and
 K%+8 = z_sign (the first slot in the K% workspace represents the planet).


 Arguments:

   Y                    The offset from K% for the start of the ship data block
                        to use


 Returns:

   A                    A OR K%+2+Y OR K%+5+Y OR K%+8+Y, with bit 7 cleared


 Other entry points:

   m                    Do not include A in the calculation


.m

 LDA #0                 ; Set A = 0 and fall through into MAS2 to calculate the
                        ; OR of the three bytes at K%+2+Y, K%+5+Y and K%+8+Y

.MAS2

 ORA K%+2,Y             ; Set A = A OR x_sign OR y_sign OR z_sign
 ORA K%+5,Y
 ORA K%+8,Y

 AND #%01111111         ; Clear bit 7 in A

 RTS                    ; Return from the subroutine