Version analysis of PLS6 - Elite on the BBC Micro and NES

This code appears in the following versions (click to see it in the source code):

Cassette, Disc (flight), 6502 Second Processor, Master and Electron

Code variations between these versions are shown below.

       Name: PLS6
       Type: Subroutine
   Category: Drawing planets
    Summary: Calculate (X K) = (A P) / (z_sign z_hi z_lo)


 Calculate the following:

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

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


 Arguments:

Code variation 1 of 3A variation in the comments only

Tap on a block to expand it, and tap it again to revert.

Cassette, Flight, 6502SP, Master
Electron
   INWK                 The planet or sun's ship data block
   INWK                 The planet's ship data block



 Returns:

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


 Other entry points:

   PL44                 Clear the C flag and return from the subroutine

Code variation 2 of 3A variation in the comments only

This variation is blank in the Cassette, Disc (flight), 6502 Second Processor and Electron versions.

Master
   PL6                  Contains an RTS


.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 PL21               \ If A is non-zero then the two high bytes of K(3 2 1 0)
                        \ are non-zero, so jump to PL21 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)

Code variation 3 of 3A variation in the comments only

This variation is blank in the Disc (flight) and Electron versions.

Cassette, 6502SP, Master
\CLC                    \ This instruction is commented out in the original
                        \ source. It would have no effect as we know the C flag
                        \ is already clear, as we skipped past the BCS above

 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

.PL44

 CLC                    \ Clear the C flag to indicate success

.PL6

 RTS                    \ Return from the subroutine