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⌂ ∞ Elite on the BBC Micro and NES

Maths (Arithmetic): FMLTU2

[NES version, Bank 7]

       Name: FMLTU2                                                  [Show more]
       Type: Subroutine
   Category: Maths (Arithmetic)
    Summary: Calculate A = K * sin(A)
  Deep dive: The sine, cosine and arctan tables

    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * CIRCLE2 calls FMLTU2




 Calculate the following:

   A = K * sin(A)

 Because this routine uses the sine lookup table SNE, we can also call this
 routine to calculate cosine multiplication. To calculate the following:

   A = K * cos(B)

 call this routine with B + 16 in the accumulator, as sin(B + 16) = cos(B).



.FMLTU2

 AND #%00011111         ; Restrict A to bits 0-5 (so it's in the range 0-31)

 TAX                    ; Set Q = sin(A) * 256
 LDA SNE,X
 STA Q

 LDA K                  ; Set A to the radius in K

                        ; Fall through into FMLTU to do the following:
                        ;
                        ;   (A ?) = A * Q
                        ;         = K * sin(A) * 256
                        ;
                        ; which is equivalent to:
                        ;
                        ;   A = K * sin(A)