Maths: ScaleDown (Part 3 of 4)

       Name: ScaleDown (Part 3 of 4)                                 [Show more]
       Type: Subroutine
   Category: Maths
    Summary: Scale the x-coordinate
    Context: See this subroutine in context in the source code
 References: No direct references to this subroutine in this source file


.down12

                        \ We now shift the x-coordinate in (SS QQ PP) by X
                        \ places in the correct direction, discarding the
                        \ fractional part in PP when we are done, but only after
                        \ rounding (SS QQ) to the nearest integer

 TXA                    \ If X < 0, jump to down15 to shift the x-coordinate
 BMI down15             \ right by X places, with A set to X

 BNE down13             \ If X > 0, jump to down13 to shift the x-coordinate
                        \ left by X places

 ASL PP                 \ X = 0, so shift bit 7 of PP into the C flag and jump
 JMP down17             \ to down17 to round the result to the nearest integer,
                        \ without shifting the x-coordinate first

.down13

                        \ We now shift (SS QQ PP) left by X places

 LDA QQ                 \ Set (SS A PP) = (SS QQ PP)

.down14

 ASL PP                 \ Set (SS A PP) = (SS A PP) << 1
 ROL A
 ROL SS

 DEX                    \ Decrement the shift counter

 BNE down14             \ Loop back until we have shifted left by X places

 STA QQ                 \ Set (SS QQ PP) = (SS A PP)

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

 JMP down17             \ Jump to down17 to round the result to the nearest
                        \ integer

.down15

                        \ 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 X = -A so we can use it as a shift counter for the
 CLC                    \ number of places to shift
 ADC #1
 TAX

                        \ We now shift (SS QQ PP) right by X places, and because
                        \ SS = 0 and we are going to discard the fractional part
                        \ in PP, we only actually need to shift QQ

 LDA QQ                 \ Set A = QQ

.down16

 LSR A                  \ Set A = A >> 1

 DEX                    \ Decrement the shift counter

 BNE down16             \ Loop back until we have shifted right by X places

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

.down17

                        \ 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 (SS QQ), or
                        \ right-shifted out of the result in (SS QQ), so if that
                        \ bit is set, we need to round up the result

 BCC down18             \ If the next bit is clear, then jump to down18 to move
                        \ on to scaling the y-coordinate, as we do not need to
                        \ round up the result

 INC QQ                 \ Otherwise increment the low byte of the result in
 BNE down18             \ (SS QQ) to round it up, and if we did that without
                        \ overflowing, jump to down18 to move on to scaling the
                        \ y-coordinate

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

 LDA SS                 \ If the high byte is < &40, jump to down18 to move on
 CMP #&40               \ to scaling the y-coordinate
 BCC down18

 LDA #&3F               \ Otherwise set (SS QQ) = &3FFF as the highest value we
 STA SS                 \ can return as the x-coordinate
 LDA #&FF
 STA QQ