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Elite on the BBC Micro

Version analysis of Ze

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

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.

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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.

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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

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