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BBC Micro Elite

The competition code

All the information that's hidden in the Elite competition code

References: COK, SVE, CHK, CHK2, STATUS

Bundled with Elite, and easy to miss amongst the razzmatazz of the manual, novel, function key strip and control summary card, was the competition entry form. Unfortunately the entry deadline of March 1985 has long since passed, but back in the day, budding players could fill out the postcard in hope of joining the Order of Elite, with the six best players standing a chance of being invited to the "national Elite tournament" in April 1985. Accordingly, piles of postcards poured into Acornsoft's offices and sat in tottering stacks around the office, waiting for some lucky soul to process them all.

My postcard was in there somewhere.

Apart from your name and address, the postcard asked for two things: your credit balance, and the competition code that the game displayed on-screen during the saving process. These two bits of information enabled Acornsoft to work out who was a legitimately amazing pilot, and who had cheated, using a decoding algorithm that was a closely guarded secret.

This algorithm can be seen in the BBC BASIC program called UNPACK that's included on the Elite source disc. Some lucky Acornsoft employee presumably had to sit there and enter these two bits of information into UNPACK, which would then tell them whether the cash levels on the postcard were correct, and most importantly, what the player's combat rank was. UNPACK could also tell them whether that person had tampered with their save file, and it also revealed which version of the game this code was from, which could then be cross-checked against the colour of the postcard (blue for the BBC Micro cassette version, brown for the BBC Micro disc version, and green for the Acorn Electron version).

And then... onto the next postcard. I bet that was a fun job.

People would have sold their grandmothers to get hold of UNPACK, so let's see how it all works, starting with the competition flags.

The competition flags
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The competition flags at location COK track three vital bits of information that get encoded into the final competition code. They are as follows:

  • Bit 0 is set in routine ptg if we hold CTRL during hyperspace to force a mis-jump into witchspace (having first paused the game and toggled on the author credits with X). UNPACK does not report the status of this bit, so presumably manually mis-jumping was not regarded as cheating by Acornsoft, but perhaps it was recorded to see if anyone found this way of hunting Thargoids
  • Bit 1 is set if this commander file has ever been saved by the cassette version of BBC Micro Elite. Note that commander files can be loaded into and saved from any BBC Micro or Electron version of the game, so this flag indicates that it was, at some point, saved from the cassette version, and it can be set alongside the other platform bits, if applicable
  • Bit 2 is set if this commander file has ever been saved by the disc version of BBC Micro Elite. It is also set by the 6502 Second Processor version, but this version was launched after the competition expired, so presumably the authors just reused the disc version's save routine code
  • Bit 3 is set if this commander file has ever been saved by the Acorn Electron version of Elite
  • Bits 4-6 are not set by any of the above versions, but if they are set in the commander file, the platform is reported as "Something else??" by UNPACK
  • Bit 7 is set if the CHK and CHK2 checksums in the commander file do not match, which indicates that the commander data has been tampered with. CHK2 is set to CHK EOR &A9 when the file is saved, so anyone tampering with the file would not only need to update the CHK checksum accordingly, they would also need to update CHK2 as well. The game hangs if you try to load a commander file with an incorrect CHK value, but it lets an incorrect CHK2 through, so the chances are a hacker wouldn't know that CHK2 needed to be correct for a valid competition entry

The competition flag is buried within the competition code that players had to copy onto the competition postcard, so now let's look at the competition code.

The competition code
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The competition code is calculated and shown on screen in the SVE routine when the commander file is saved. It's a four-byte number with a maximum value of 4,294,967,295, which fits nicely into the ten-box slot on the competition postcard.

It is calculated into K(0 1 2 3), which is a big-endian number with the most significant byte in K and the least significant in K+3 (so it's stored in the same way that the player's cash is stored in location CASH, and it's printed out by the same BPRNT routine that displays the credit balance). The calculation is done in this order:

  K   = CHK OR %10000000
  K+2 = K EOR COK
  K+1 = K+2 EOR CASH+2
  K+3 = K+1 EOR &5A EOR TALLY+1

The result is then printed on-screen using BPRNT, and the file is saved.

Extracting data from the competition code
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So to extract the various bits of information encoded in the competition code, we can apply the algorithm in UNPACK, described here using the same variable names as used UNPACK, so you can follow along if you want. The following makes use of the following facts about EOR:

  • EOR is commutative:
  •       A EOR B = B EOR A
    
  • EOR is transitive:
  •       (A EOR B) EOR C = A EOR (B EOR C)
    
  • EOR is its own self-inverse:
  •       A EOR A = 0
    
  • EOR 0 is the identity:
  •       A EOR 0 = 0 EOR A = A
    

Given this, let's see how UNPACK extracts the data from the competition code.

  • Split the code into the four bytes by AND'ing as follows (UNPACK has to do some division to avoid BBC BASIC overflow errors, but this is effectively what it does):
  •       B1% = code AND &000000FF (i.e. K+3)
          B2% = code AND &0000FF00 (i.e. K+2)
          B3% = code AND &00FF0000 (i.e. K+1)
          B4% = code AND &FF000000 (i.e. K)
    
  • Extract COK:
  •       B% = B4% EOR B2%
             = K EOR K+2
             = K EOR (K EOR COK)
             = (K EOR K) EOR COK
             = COK
    
  • Extract CASH+2:
  •       B% = B2% EOR B3%
             = K+2 EOR K+1
             = (K EOR COK) EOR (K+2 EOR CASH+2)
             = (K EOR COK) EOR ((K EOR COK) EOR CASH+2)
             = ((K EOR COK) EOR (K EOR COK)) EOR CASH+2
             = CASH+2
    
  • Extract TALLY+1:
  •       B% = B3% EOR B1% EOR &5A
             = K+1 EOR K+3 EOR &5A
             = (K+2 EOR CASH+2) EOR (K+1 EOR &5A EOR TALLY+1) EOR &5A
             = (K+2 EOR CASH+2) EOR ((K+2 EOR CASH+2) EOR &5A EOR TALLY+1) EOR &5A
             = ((K+2 EOR CASH+2) EOR (K+2 EOR CASH+2)) EOR &5A EOR TALLY+1 EOR &5A
             = &5A EOR TALLY+1 EOR &5A
             = (&5A EOR &5A) EOR TALLY+1
             = TALLY+1
    

So in this way we can extract the competition flags (COK), one byte of the cash amount (CASH+2) and the high byte of the combat rank (TALLY+1). We can check the extracted cash byte against the cash amount entered on the postcard, as follows:

  CASH+2 = (cash total from postcard * 10) AND &FF00

and we can work out the combat rank from TALLY+1 using the same algorithm as the STATUS routine:

  Competent     TALLY+1 = 1 to 2
  Dangerous     TALLY+1 = 2 to 9
  Deadly        TALLY+1 = 10 to 24
  Elite         TALLY+1 = 25 and up

and from the value COK we know whether the file has been tampered with, which platforms the game has been played on, and whether the player has done any manual mis-jumps.

That's pretty clever stuff for a few digits on a postcard...