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⌂ ∞ Elite on the BBC Micro and NES

Maths (Arithmetic): MAS3

[NES version, Bank 0]

       Name: MAS3                                                    [Show more]
       Type: Subroutine
   Category: Maths (Arithmetic)
    Summary: Calculate A = x_hi^2 + y_hi^2 + z_hi^2 in the K% block

    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * CheckAltitude calls MAS3
             * Main flight loop (Part 15 of 16) calls MAS3




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

   A = x_hi^2 + y_hi^2 + z_hi^2

 returning A = $FF if the calculation overflows a one-byte 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).



 Arguments:

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

 Returns

   A                    A = x_hi^2 + y_hi^2 + z_hi^2

                        A = $FF if the calculation overflows a one-byte result



.MAS3

 LDA K%+1,Y             ; Set (A P) = x_hi * x_hi
 JSR SQUA2

 STA R                  ; Store A (high byte of result) in R

 LDA K%+4,Y             ; Set (A P) = y_hi * y_hi
 JSR SQUA2

 ADC R                  ; Add A (high byte of second result) to R

 BCS MA30               ; If the addition of the two high bytes caused a carry
                        ; (i.e. they overflowed), jump to MA30 to return A = $FF

 STA R                  ; Store A (sum of the two high bytes) in R

 SETUP_PPU_FOR_ICON_BAR ; If the PPU has started drawing the icon bar, configure
                        ; the PPU to use nametable 0 and pattern table 0

 LDA K%+7,Y             ; Set (A P) = z_hi * z_hi
 JSR SQUA2

 ADC R                  ; Add A (high byte of third result) to R, so R now
                        ; contains the sum of x_hi^2 + y_hi^2 + z_hi^2

 BCC P%+4               ; If there is no carry, skip the following instruction
                        ; to return straight from the subroutine

.MA30

 LDA #$FF               ; The calculation has overflowed, so set A = $FF

 RTS                    ; Return from the subroutine