Version analysis of EXNO2 - 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), Disc (docked), 6502 Second Processor, Master and Electron

Code variations between these versions are shown below.

       Name: EXNO2
       Type: Subroutine
   Category: Status
    Summary: Process us making a kill
  Deep dive: Combat rank


 We have killed a ship, so increase the kill tally, displaying an iconic
 message of encouragement if the kill total is a multiple of 256, and then
 make a nearby explosion sound.

Code variation 1 of 3A variation in the comments only

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

Master
 Arguments:

   X                    The type of the ship that was killed


.EXNO2

Code variation 2 of 3Related to the Master version

The Master version incorporates a fractional kill tally when calculating the combat rank, so each ship type has a different number of kill points, while all the other versions count one point for each kill.

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

Cassette, Flight, Docked, 6502SP, Electron
Master
 INC TALLY              \ Increment the low byte of the kill count in TALLY

 BNE EXNO-2             \ If there is no carry, jump to the LDX #7 below (at
                        \ EXNO-2)
 LDA TALLYL             \ We now add the fractional kill count to our tally,
 CLC                    \ starting with the fractional bytes:
 ADC KWL%-1,X           \
 STA TALLYL             \   TALLYL = TALLYL + fractional kill count
                        \
                        \ where the fractional kill count is taken from the
                        \ KWL% table, according to the ship's type (we look up
                        \ the X-1-th value from KWL% because ship types start
                        \ at 1 rather than 0)

 LDA TALLY              \ And then we add the low byte of TALLY(1 0):
 ADC KWH%-1,X           \
 STA TALLY              \   TALLY = TALLY + carry + integer kill count
                        \
                        \ where the integer kill count is taken from the KWH%
                        \ table in the same way

 BCC davidscockup       \ If there is no carry, jump straight to EXNO3 to skip
                        \ the following three instructions

 INC TALLY+1            \ Increment the high byte of the kill count in TALLY

 LDA #101               \ The kill total is a multiple of 256, so it's time
 JSR MESS               \ for a pat on the back, so print recursive token 101
                        \ ("RIGHT ON COMMANDER!") as an in-flight message

Code variation 3 of 3Specific to an individual platform

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

Cassette, Flight, Docked, 6502SP, Electron
Master
 LDX #7                 \ Set X = 7 and fall through into EXNO to make the
                        \ sound of a ship exploding
.davidscockup

                        \ Fall through into EXNO3 to make the sound of a
                        \ ship exploding