Maths (Arithmetic): LL123

       Name: LL123                                                   [Show more]
       Type: Subroutine
   Category: Maths (Arithmetic)
    Summary: Calculate (Y X) = (S R) / XX12+2 or (S R) * XX12+2
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * LL118 calls LL123
             * LL120 calls via LL121
             * LL120 calls via LL133


 Calculate the following:

   * If T = 0, this is a shallow slope, so calculate (Y X) = (S R) / XX12+2

   * If T <> 0, this is a steep slope, so calculate (Y X) = (S R) * XX12+2

 giving (Y X) the opposite sign to the slope direction in XX12+3.


 Arguments:

   XX12+2               The line's gradient * 256 (so 1.0 = 256)

   XX12+3               The direction of slope:

                          * Bit 7 clear means top left to bottom right

                          * Bit 7 set means top right to bottom left

   T                    The gradient of slope:

                          * 0 if it's a shallow slope

                          * $FF if it's a steep slope


 Other entry points:

   LL121                Calculate (Y X) = (S R) / Q and set the sign to the
                        opposite of the top byte on the stack

   LL133                Negate (Y X) and return from the subroutine

   LL128                Contains an RTS


.LL123

 JSR LL129              ; Call LL129 to do the following:
                        ;
                        ;   Q = XX12+2
                        ;     = line gradient
                        ;
                        ;   A = S EOR XX12+3
                        ;     = S EOR slope direction
                        ;
                        ;   (S R) = |S R|
                        ;
                        ; So A contains the sign of S * slope direction

 PHA                    ; Store A on the stack so we can use it later

 LDX T                  ; If T is non-zero, then it's a steep slope, so jump up
 BNE LL122              ; to LL122 to calculate this instead:
                        ;
                        ;   (Y X) = (S R) * Q

.LL121

                        ; The following calculates:
                        ;
                        ;   (Y X) = (S R) / Q
                        ;
                        ; using the same shift-and-subtract algorithm that's
                        ; documented in TIS2

 LDA #%11111111         ; Set Y = %11111111
 TAY

 ASL A                  ; Set X = %11111110
 TAX

                        ; This sets (Y X) = %1111111111111110, so we can rotate
                        ; through 15 loop iterations, getting a 1 each time, and
                        ; then getting a 0 on the 16th iteration... and we can
                        ; also use it to catch our result bits into bit 0 each
                        ; time

.LL130

 ASL R                  ; Shift (S R) to the left
 ROL S

 LDA S                  ; Set A = S

 BCS LL131              ; If bit 7 of S was set, then jump straight to the
                        ; subtraction

 CMP Q                  ; If A < Q (i.e. S < Q), skip the following subtractions
 BCC LL132

.LL131

 SBC Q                  ; A >= Q (i.e. S >= Q) so set:
 STA S                  ;
                        ;   S = (A R) - Q
                        ;     = (S R) - Q
                        ;
                        ; starting with the low bytes (we know the C flag is
                        ; set so the subtraction will be correct)

 LDA R                  ; And then doing the high bytes
 SBC #0
 STA R

 SEC                    ; Set the C flag to rotate into the result in (Y X)

.LL132

 TXA                    ; Rotate the counter in (Y X) to the left, and catch the
 ROL A                  ; result bit into bit 0 (which will be a 0 if we didn't
 TAX                    ; do the subtraction, or 1 if we did)
 TYA
 ROL A
 TAY

 BCS LL130              ; If we still have set bits in (Y X), loop back to LL130
                        ; to do the next iteration of 15, until we have done the
                        ; whole division

 PLA                    ; Restore A, which we calculated above, from the stack

 BMI LL128              ; If A is negative jump to LL128 to return from the
                        ; subroutine with (Y X) as is

.LL133

 TXA                    ; Otherwise negate (Y X) using two's complement by first
 EOR #%11111111         ; setting the low byte to ~X + 1
;CLC                    ;
 ADC #1                 ; The CLC instruction is commented out in the original
 TAX                    ; source. It would have no effect as we know the C flag
                        ; is clear from when we passed through the BCS above

 TYA                    ; Then set the high byte to ~Y + C
 EOR #%11111111
 ADC #0
 TAY

.LL128

 RTS                    ; Return from the subroutine