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Elite on the BBC Micro

Main loop: Main game loop (Part 2 of 6) [Disc version, Docked]

Name: Main game loop (Part 2 of 6) [Show more] Type: Subroutine Category: Main loop Summary: Potentially spawn a trader, an asteroid, or a cargo canister (though this has no effect when docked) Deep dive: Program flow of the main game loop Ship data blocks Fixing ship positions
Context: See this subroutine in context in the source code Variations: See code variations for this subroutine in the different versions References: This subroutine is called as follows: * Main game loop (Part 2 of 6) calls entry point me3 * me2 calls entry point me3 * Main game loop (Part 6 of 6) calls entry point TT100

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: * 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 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) 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 CMP #35 \ If A >= 35 (87% chance), jump down to MLOOP to skip BCS MLOOP \ the following 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 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) \ Fall through into part 5 (parts 3 and 4 are not \ required when we are docked)