Drawing planets: PLS6

       Name: PLS6                                                    [Show more]
       Type: Subroutine
   Category: Drawing planets
    Summary: Calculate (X K) = (A P+1 P) / (z_sign z_hi z_lo)
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * PROJ calls PLS6


 Calculate the following:

   (X K) = (A P+1 P) / (z_sign z_hi z_lo)

 returning an overflow in the C flag if the result is >= 1024.


 Arguments:

   INWK                 The planet or sun's ship data block


 Returns:

   C flag               Set if the result >= 1024, clear otherwise


.PLS6

 JSR DVID3B2            \ Call DVID3B2 to calculate:
                        \
                        \   K(3 2 1 0) = (A P+1 P) / (z_sign z_hi z_lo)

 LDA K+3                \ Set A = |K+3| OR K+2
 AND #%01111111
 ORA K+2

 BNE PL21S              \ If A is non-zero then the two high bytes of K(3 2 1 0)
                        \ are non-zero, so jump to PL21S to set the C flag and
                        \ return from the subroutine

                        \ We can now just consider K(1 0), as we know the top
                        \ two bytes of K(3 2 1 0) are both 0

 LDX K+1                \ Set X = K+1, so now (X K) contains the result in
                        \ K(1 0), which is the format we want to return the
                        \ result in

 CPX #4                 \ If the high byte of K(1 0) >= 4 then the result is
 BCS PL6                \ >= 1024, so return from the subroutine with the C flag
                        \ set to indicate an overflow (as PL6 contains an RTS)

 LDA K+3                \ Fetch the sign of the result from K+3 (which we know
                        \ has zeroes in bits 0-6, so this just fetches the sign)

 BPL PL6                \ If the sign bit is clear and the result is positive,
                        \ then the result is already correct, so return from
                        \ the subroutine with the C flag clear to indicate
                        \ success (as PL6 contains an RTS)

 LDA K                  \ Otherwise we need to negate the result, which we do
 EOR #%11111111         \ using two's complement, starting with the low byte:
 ADC #1                 \
 STA K                  \   K = ~K + 1

 TXA                    \ And then the high byte:
 EOR #%11111111         \
 ADC #0                 \   X = ~X
 TAX

 CLC                    \ Clear the C flag to indicate success

.PL6

 RTS                    \ Return from the subroutine