Version analysis of Ze - 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: Ze
       Type: Subroutine
   Category: Universe
    Summary: Initialise the INWK workspace to a hostile ship
  Deep dive: Fixing ship positions


 Specifically, this routine does the following:

   * Reset the INWK ship workspace

   * Set the ship to a fair distance away in all axes, in front of us but
     randomly up or down, left or right

   * Give the ship a 4% chance of having E.C.M.

   * Set the ship to hostile, with AI enabled

 This routine also sets A, X, T1 and the C flag to random values.

 Note that because this routine uses the value of X returned by DORND, and X
 contains the value of A returned by the previous call to DORND, this routine
 does not necessarily set the new ship to a totally random location. See the
 deep dive on "Fixing ship positions" for details.


.Ze

 JSR ZINF               \ Call ZINF to reset the INWK ship workspace

 JSR DORND              \ Set A and X to random numbers

 STA T1                 \ Store A in T1

 AND #%10000000         \ Extract the sign of A and store in x_sign
 STA INWK+2

 TXA                    \ Extract the sign of X and store in y_sign
 AND #%10000000
 STA INWK+5

Code variation 1 of 2Related to a standard feature

The cassette version spawns new ships at a distance of 32 unit vectors, while the other versions spawn new ships noticeably closer, at a distance of 25 unit vectors.

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

Cassette, Electron
Flight, Docked, 6502SP, Master
 LDA #32                \ Set x_hi = y_hi = z_hi = 32, a fair distance away
 STA INWK+1
 STA INWK+4
 STA INWK+7
 LDA #25                \ Set x_hi = y_hi = z_hi = 25, a fair distance away
 STA INWK+1
 STA INWK+4
 STA INWK+7

Code variation 2 of 2Other (e.g. bug fix, optimisation)

When spawning hostile ships in space, the cassette and 6502SP versions reuse the previous random number in X for the 4% check (i.e. the one used to determine the x and y signs of the new ship), while the disc version generates a new random number instead of reusing, so this change presumably improves the randomness of the spawning in the disc version somehow.

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

Cassette, Docked, 6502SP, Master, Electron
Flight
 TXA                    \ Set the C flag if X >= 245 (4% chance)
 CMP #245
 JSR DORND              \ Set A and X to random numbers

 CMP #245               \ Set the C flag if A >= 245 (4% chance)

 ROL A                  \ Set bit 0 of A to the C flag (i.e. there's a 4%
                        \ chance of this ship having E.C.M.)

 ORA #%11000000         \ Set bits 6 and 7 of A, so the ship is hostile (bit 6
                        \ and has AI (bit 7)

 STA INWK+32            \ Store A in the AI flag of this ship

                        \ Fall through into DORND2 to set A, X and the C flag
                        \ randomly