Maths: ScaleDown (Part 4 of 4)

       Name: ScaleDown (Part 4 of 4)                                 [Show more]
       Type: Subroutine
   Category: Maths
    Summary: Scale the y-coordinate
    Context: See this subroutine in context in the source code
 References: No direct references to this subroutine in this source file


.down18

                        \ We now shift the y-coordinate in (RR Q P) by Y
                        \ places in the correct direction, discarding the
                        \ fractional part in P when we are done, but only after
                        \ rounding (RR Q) to the nearest integer

 TYA                    \ If Y < 0, jump to down21 to shift the y-coordinate
 BMI down21             \ right by Y places, with A set to Y

 BNE down19             \ If Y > 0, jump to down19 to shift the y-coordinate
                        \ right by Y places

 ASL P                  \ Y = 0, so shift bit 7 of P into the C flag and jump
 JMP down23             \ to down23 to round the result to the nearest integer,
                        \ without shifting the y-coordinate first

.down19

                        \ We now shift (RR Q P) left by X places

 LDA Q                  \ Set (RR A P) = (RR Q P)

.down20

 ASL P                  \ Set (RR A P) = (RR A P) << 1
 ROL A
 ROL RR

 DEY                    \ Decrement the shift counter

 BNE down20             \ Loop back until we have shifted left by Y places

 STA Q                  \ Set (RR Q P) = (RR A P)

 ASL P                  \ Shift bit 7 of P into the C flag, so it contains bit
                        \ 7 of the fractional part

 JMP down23             \ Jump to down23 to round the result to the nearest
                        \ integer

.down21

                        \ If we get here then we want to shift the result right
                        \ by the number of places in A, where A is negative

 EOR #&FF               \ Set Y = -A so we can use it as a shift counter for the
 CLC                    \ number of places to shift
 ADC #1
 TAY

                        \ We now shift (RR Q P) right by Y places, and because
                        \ RR = 0 and we are going to discard the fractional part
                        \ in P, we only actually need to shift Q

 LDA Q                  \ Set A = Q

.down22

 LSR A                  \ Set A = A >> 1

 DEY                    \ Decrement the shift counter

 BNE down22             \ Loop back until we have shifted right by Y places

 STA Q                  \ Set Q = A, so now we have shifted (RR Q) right by
                        \ Y places, and the C flag is set to the last bit that
                        \ we shifted out of Q, which would be bit 7 of the
                        \ fractional part

.down23

                        \ We now round up the result, if bit 7 of the fractional
                        \ part is set

                        \ The C flag contains the next bit that would have been
                        \ left-shifted out of PP into the result in (RR Q), or
                        \ right-shifted out of the result in (RR Q), so if that
                        \ bit is set, we need to round up the result

 BCC down24             \ If the next bit is clear, then jump to return from the
                        \ subroutine, as we do not need to round up the result

 INC Q                  \ Otherwise increment the low byte of the result in
 BNE down24             \ (RR Q) to round it up, and if we did that without
                        \ overflowing, jump to down24 to return from the
                        \ subroutine as we now have our result

 INC RR                 \ If the increment overflowed the low byte, increment
                        \ the high byte of the result in (RR Q) to round it up

 LDA RR                 \ If the high byte is < &40, jump to down24 to return
 CMP #&40               \ from the subroutine as we now have our result
 BCC down24

 LDA #&3F               \ Otherwise set (RR Q) = &3FFF as the highest value we
 STA RR                 \ can return as the y-coordinate
 LDA #&FF
 STA Q

.down24

 RTS                    \ Return from the subroutine