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Elite on the BBC Micro

Version analysis of Main game loop (Part 1 of 6)

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

Code variations between these versions are shown below.

Name: Main game loop (Part 1 of 6) Type: Subroutine Category: Main loop

Code variation 1 of 8A variation in the comments only

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Cassette, Electron

Flight, 6502SP, Master

Summary: Spawn a trader (a peaceful Cobra Mk III)
Summary: Spawn a trader (a Cobra Mk III, Python, Boa or Anaconda)
Deep dive: Program flow of the main game loop Ship data blocks
This is part of the main game loop. This is where the core loop of the game lives, and it's in two parts. The shorter loop (just parts 5 and 6) is iterated when we are docked, while the entire loop from part 1 to 6 iterates if we are in space. This section covers the following:

Code variation 2 of 8A variation in the comments only

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Cassette, Electron

Flight, 6502SP, Master

* Spawn a trader, i.e. a Cobra Mk III that isn't hostile, with a 50% chance of it having a missile, a 50% chance of it having an E.C.M., a speed between 16 and 31, and a gentle clockwise roll We call this from within the main loop, with A set to a random number.
* Spawn a trader, i.e. a Cobra Mk III, Python, Boa or Anaconda, with a 50% chance of it having a missile, a 50% chance of it having an E.C.M., a 50% chance of it docking and being aggressive if attacked, a speed between 16 and 31, and a gentle clockwise roll We call this from within the main loop.
.MTT4

Code variation 3 of 8Specific to an individual platform

This variation is blank in the Cassette and Electron versions.

Flight, 6502SP, Master

JSR DORND \ Set A and X to random numbers
 LSR A                  \ Clear bit 7 of our random number in A and set the C
                        \ flag to bit 0 of A, which os random

 STA INWK+32            \ Store this in the ship's AI flag, so this ship does
                        \ not have AI

 STA INWK+29            \ Store A in the ship's roll counter, giving it a
                        \ clockwise roll (as bit 7 is clear), and a 1 in 127
                        \ chance of it having no damping

 ROL INWK+31            \ Set bit 0 of the ship's missile count ramdomly (as the
                        \ C flag was set), giving the ship either no missiles or
                        \ one missile

Code variation 4 of 8Minor and very low-impact

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Cassette, 6502SP, Master, Electron

Flight

AND #31 \ Set the ship speed to our random number, set to a ORA #16 \ minimum of 16 and a maximum of 31 STA INWK+27
AND #15 \ Set the ship speed to our random number, set to a ORA #16 \ minimum of 16 and a maximum of 31 STA INWK+27

Code variation 5 of 8Related to an enhanced feature

Traders in the enhanced versions can spawn in docking mode, in which case they mind their own business.

This variation is blank in the Cassette and Electron versions.

Flight, 6502SP, Master

JSR DORND \ Set A and X to random numbers, plus the C flag BMI nodo \ If A is negative (50% chance), jump to nodo to skip \ the following \ If we get here then we are going to spawn a ship that \ is minding its own business and trying to dock LDA INWK+32 \ Set bits 6 and 7 of the ship's AI flag, to make it ORA #%11000000 \ aggressive if attacked, and enable its AI STA INWK+32 LDX #%00010000 \ Set bit 4 of the ship's NEWB flags, to indicate that STX NEWB \ this ship is docking .nodo

Code variation 6 of 8Related to an enhanced feature

Traders in the enhanced versions can be one of the following: Cobra Mk III, Python, Boa or Anaconda (in the cassette version, they are always Cobras).

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Cassette, Electron

Flight, 6502SP, Master

LDA #CYL \ Add a new Cobra Mk III to the local bubble and fall JSR NWSHP \ through into the main game loop again
AND #2 \ If we jumped here with a random value of A from the \ BMI above, then this reduces A to a random value of \ either 0 or 2; if we didn't take the BMI and made the \ ship hostile, then A will be 0 ADC #CYL \ Set A = A + C + #CYL \ \ where A is 0 or 2 and C is 0 or 1, so this gives us a \ ship type from the following: Cobra Mk III, Python, \ Boa or Anaconda

Code variation 7 of 8Related to an advanced feature

The advanced versions have rock hermits, which are classed as junk but can release ships if attacked.

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

6502SP, Master

CMP #HER \ If A is now the ship type of a rock hermit, jump to BEQ TT100 \ TT100 to skip the following instruction

Code variation 8 of 8Specific to an individual platform

This variation is blank in the Cassette and Electron versions.

Flight, 6502SP, Master

JSR NWSHP \ Add a new ship of type A to the local bubble and fall \ through into the main game loop again
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