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Elite on the BBC Micro

Version analysis of Main game loop (Part 2 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 2 of 6) Type: Subroutine Category: Main loop

Code variation 1 of 15A variation in the comments only

Tap on a version to expand it, and tap it again to show to all variations.

Cassette, Flight, 6502SP, Master, Electron

Docked

Summary: Call the main flight loop, and potentially spawn a trader, an asteroid, or a cargo canister
Summary: Potentially spawn a trader, an asteroid, or a cargo canister (though this has no effect when docked)

Code variation 2 of 15A variation in the comments only

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

Docked

In the docked code, we start the main game loop at part 2 and then jump straight to part 5, as parts 1, 3 and 4 are not required when we are docked.
This section covers the following:

Code variation 3 of 15A variation in the comments only

This variation is blank in the Disc (docked) version.

Cassette, Flight, 6502SP, Master, Electron

* Call M% to do the main flight loop
* Potentially spawn a trader, asteroid or cargo canister Other entry points: TT100 The entry point for the start of the main game loop, which calls the main flight loop and the moves into the spawning routine me3 Used by me2 to jump back into the main game loop after printing an in-flight message
.TT100

Code variation 4 of 15Specific to an individual platform

This variation is blank in the Disc (docked) version.

Cassette, Flight, 6502SP, Master, Electron

JSR M% \ Call M% to iterate through the main flight loop
 DEC DLY                \ Decrement the delay counter in DLY, so any in-flight
                        \ messages get removed once the counter reaches zero

 BEQ me2                \ If DLY is now 0, jump to me2 to remove any in-flight
                        \ message from the space view, and once done, return to
                        \ me3 below, skipping the following two instructions

 BPL me3                \ If DLY is positive, jump to me3 to skip the next
                        \ instruction

 INC DLY                \ If we get here, DLY is negative, so we have gone too
                        \ and need to increment DLY back to 0

.me3

 DEC MCNT               \ Decrement the main loop counter in MCNT

 BEQ P%+5               \ If the counter has reached zero, which it will do
                        \ every 256 main loops, skip the next JMP instruction
                        \ (or to put it another way, if the counter hasn't
                        \ reached zero, jump down to MLOOP, skipping all the
                        \ following checks)

.ytq

 JMP MLOOP              \ Jump down to MLOOP to do some end-of-loop tidying and
                        \ restart the main loop

                        \ We only get here once every 256 iterations of the
                        \ main loop. If we aren't in witchspace and don't
                        \ already have 3 or more asteroids in our local bubble,
                        \ then this section has a 13% chance of spawning
                        \ something benign (the other 87% of the time we jump
                        \ down to consider spawning cops, pirates and bounty
                        \ hunters)
                        \
                        \ If we are in that 13%, then 50% of the time this will
                        \ be a Cobra Mk III trader, and the other 50% of the
                        \ time it will either be an asteroid (98.5% chance) or,
                        \ very rarely, a cargo canister (1.5% chance)

Code variation 5 of 15Related to the Electron version

As the Electron doesn't support witchspace, we always process ship spawning (the other versions skip the ship spawning logic when in witchspace, as the Thargoids are enough trouble without humans joining the fight).

This variation is blank in the Electron version.

Cassette, Flight, Docked, 6502SP, Master

LDA MJ \ If we are in witchspace following a mis-jump, skip the BNE ytq \ following by jumping down to MLOOP (via ytq above)
 JSR DORND              \ Set A and X to random numbers

Code variation 6 of 15Minor and very low-impact

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Cassette, Flight, 6502SP, Master, Electron

Docked

CMP #35 \ If A >= 35 (87% chance), jump down to MTT1 to skip BCS MTT1 \ the spawning of an asteroid or cargo canister and \ potentially spawn something else
CMP #35 \ If A >= 35 (87% chance), jump down to MLOOP to skip BCS MLOOP \ the following

Code variation 7 of 15Minor and very low-impact

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Cassette, Electron

Docked

Flight, 6502SP, Master

LDA MANY+AST \ If we already have 3 or more asteroids in the local CMP #3 \ bubble, jump down to MTT1 to skip the following and BCS MTT1 \ potentially spawn something else
LDA MANY+AST \ If we already have 3 or more asteroids in the local CMP #3 \ bubble, jump down to MLOOP to skip the following BCS MLOOP
LDA JUNK \ If we already have 3 or more bits of junk in the local CMP #3 \ bubble, jump down to MTT1 to skip the following and BCS MTT1 \ potentially spawn something else
 JSR ZINF               \ Call ZINF to reset the INWK ship workspace

 LDA #38                \ Set z_hi = 38 (far away)
 STA INWK+7

 JSR DORND              \ Set A, X and C flag to random numbers

 STA INWK               \ Set x_lo = random

 STX INWK+3             \ Set y_lo = random
                        \
                        \ Note that because we use the value of X returned by
                        \ DORND, and X contains the value of A returned by the
                        \ previous call to DORND, this does not set the new ship
                        \ to a totally random location. See the deep dive on
                        \ "Fixing ship positions" for details

 AND #%10000000         \ Set x_sign = bit 7 of x_lo
 STA INWK+2

 TXA                    \ Set y_sign = bit 7 of y_lo
 AND #%10000000
 STA INWK+5

 ROL INWK+1             \ Set bit 1 of x_hi to the C flag, which is random, so
 ROL INWK+1             \ this randomly moves us off-centre by 512 (as if x_hi
                        \ is %00000010, then (x_hi x_lo) is 512 + x_lo)

Code variation 8 of 15Specific to an individual platform

This variation is blank in the Disc (docked) version.

Cassette, Flight, 6502SP, Master, Electron

JSR DORND \ Set A, X and V flag to random numbers BVS MTT4 \ If V flag is set (50% chance), jump up to MTT4 to \ spawn a trader

Code variation 9 of 15Other (e.g. bug fix, optimisation)

A bug fix for the first version of disc Elite, in which asteroids never appeared.

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

Flight

IF _STH_DISC NOP \ In the first version of disc Elite, asteroids never NOP \ appeared. It turned out that the authors had put in a NOP \ jump to force traders to spawn, so they could test \ that part of the code, but had forgotten to remove it, \ so this was fixed in later versions by replacing the \ JMP instruction with NOPs... and this is where that \ was done ELIF _IB_DISC JMP MTT4 \ In the first version of disc Elite, asteroids never \ appeared. It turned out that the authors had put in a \ jump to force traders to spawn, so they could test \ that part of the code, but had forgotten to remove it, \ so this was fixed in later versions by replacing the \ JMP instruction with NOPs. The version on Ian Bell's \ site still contains the test jump, so asteroids never \ appear in this version ENDIF

Code variation 10 of 15Specific to an individual platform

This variation is blank in the Disc (docked) version.

Cassette, Flight, 6502SP, Master, Electron

ORA #%01101111 \ Take the random number in A and set bits 0-3 and 5-6, STA INWK+29 \ so the result has a 50% chance of being positive or \ negative, and a 50% chance of bits 0-6 being 127. \ Storing this number in the roll counter therefore \ gives our new ship a fast roll speed with a 50% \ chance of having no damping, plus a 50% chance of \ rolling clockwise or anti-clockwise LDA SSPR \ If we are inside the space station safe zone, jump BNE MTT1 \ down to MTT1 to skip the following and potentially \ spawn something else TXA \ Set A to the random X we set above, which we haven't BCS MTT2 \ used yet, and if the C flag is set (50% chance) jump \ down to MTT2 to skip the following 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 BCC MTT3 \ Jump down to MTT3, skipping the following (this BCC \ is effectively a JMP as we know the C flag is clear, \ having passed through the BCS above) .MTT2 ORA #%01111111 \ Set bits 0-6 of A to 127, leaving bit 7 as random, so STA INWK+30 \ storing this number in the pitch counter means we have \ full pitch with no damping, with a 50% chance of \ pitching up or down .MTT3 JSR DORND \ Set A and X to random numbers

Code variation 11 of 15Related to an advanced feature

In the advanced versions, 1.2% of asteroids spawned are rock hermits.

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

6502SP, Master

CMP #252 \ If random A < 252 (98.8% of the time), jump to thongs BCC thongs \ to skip the following LDA #HER \ Set A to #HER so we spawn a rock hermit 1.2% of the \ time STA INWK+32 \ Set byte #32 to %00001111 to give the rock hermit an \ E.C.M. BNE whips \ Jump to whips (this BNE is effectively a JMP as A will \ never be zero) .thongs

Code variation 12 of 15Related to a standard feature

In the cassette version, 1.5% of asteroids are actually spawned as cargo canisters, while in the enhanced versions 4% of asteroids are spawned as either cargo canisters or alloy plates, with an even chance of each.

This variation is blank in the Disc (docked) version.

Tap on a version to expand it, and tap it again to show to all variations.

Cassette, Electron

Flight, 6502SP, Master

CMP #5 \ Set A to the ship number of an asteroid, and keep LDA #AST \ this value for 98.5% of the time (i.e. if random BCS P%+4 \ A >= 5 then skip the following instruction) LDA #OIL \ Set A to the ship number of a cargo canister
CMP #10 \ If random A >= 10 (96% of the time), set the C flag AND #1 \ Reduce A to a random number that's 0 or 1 ADC #OIL \ Set A = #OIL + A + C, so there's a tiny chance of us \ spawning a cargo canister (#OIL) and an even chance of \ us spawning either a boulder (#OIL + 1) or an asteroid \ (#OIL + 2)

Code variation 13 of 15A variation in the labels only

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

6502SP, Master

.whips

Code variation 14 of 15Specific to an individual platform

This variation is blank in the Disc (docked) version.

Cassette, Flight, 6502SP, Master, Electron

JSR NWSHP \ Add our new asteroid or canister to the universe

Code variation 15 of 15Specific to an individual platform

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

Docked

\ Fall through into part 5 (parts 3 and 4 are not \ required when we are docked)
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