Maths (Arithmetic): TIS1

       Name: TIS1                                                    [Show more]
       Type: Subroutine
   Category: Maths (Arithmetic)
    Summary: Calculate (A ?) = (-X * A + (S R)) / 96
  Deep dive: Shift-and-subtract division
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * TIDY calls TIS1


 Calculate the following expression between sign-magnitude numbers, ignoring
 the low byte of the result:

   (A ?) = (-X * A + (S R)) / 96

 This uses the same shift-and-subtract algorithm as TIS2, just with the
 quotient A hard-coded to 96.


 Returns:

   Q                    Gets set to the value of argument X


.TIS1

 STX Q                  ; Set Q = X

 EOR #%10000000         ; Flip the sign bit in A

 JSR MAD                ; Set (A X) = Q * A + (S R)
                        ;           = X * -A + (S R)

.DVID96

 TAX                    ; Set T to the sign bit of the result
 AND #%10000000
 STA T

 TXA                    ; Set A to the high byte of the result with the sign bit
 AND #%01111111         ; cleared, so (A ?) = |X * A + (S R)|

                        ; The following is identical to TIS2, except Q is
                        ; hard-coded to 96, so this does A = A / 96

 LDX #254               ; Set T1 to have bits 1-7 set, so we can rotate through
 STX T1                 ; 7 loop iterations, getting a 1 each time, and then
                        ; getting a 0 on the 8th iteration... and we can also
                        ; use T1 to catch our result bits into bit 0 each time

.DVL3

 ASL A                  ; Shift A to the left

 CMP #96                ; If A < 96 skip the following subtraction
 BCC DV4

 SBC #96                ; Set A = A - 96
                        ;
                        ; Going into this subtraction we know the C flag is
                        ; set as we passed through the BCC above, and we also
                        ; know that A >= 96, so the C flag will still be set
                        ; once we are done

.DV4

 ROL T1                 ; Rotate the counter in T1 to the left, and catch the
                        ; result bit into bit 0 (which will be a 0 if we didn't
                        ; do the subtraction, or 1 if we did)

 BCS DVL3               ; If we still have set bits in T1, loop back to DVL3 to
                        ; do the next iteration of 7

 LDA T1                 ; Fetch the result from T1 into A

 ORA T                  ; Give A the sign of the result that we stored above

 RTS                    ; Return from the subroutine