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Elite on the BBC Micro

Version analysis of Main flight loop (Part 7 of 16)

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Name: Main flight loop (Part 7 of 16) Type: Subroutine Category: Main loop Summary: For each nearby ship: Check whether we are docking, scooping or colliding with it Deep dive: Program flow of the main game loop
The main flight loop covers most of the flight-specific aspects of Elite. This section covers the following: * Continue looping through all the ships in the local bubble, and for each one: * Check how close we are to this ship and work out if we are docking, scooping or colliding with it
LDA INWK+31 \ Fetch the status of this ship from bits 5 (is ship AND #%10100000 \ exploding?) and bit 7 (has ship been killed?) from \ ship byte #31 into A JSR MAS4 \ Or this value with x_hi, y_hi and z_hi BNE MA65 \ If this value is non-zero, then either the ship is \ far away (i.e. has a non-zero high byte in at least \ one of the three axes), or it is already exploding, \ or has been flagged as being killed - in which case \ jump to MA65 to skip the following, as we can't dock \ scoop or collide with it LDA INWK \ Set A = (x_lo OR y_lo OR z_lo), and if bit 7 of the ORA INWK+3 \ result is set, the ship is still a fair distance ORA INWK+6 \ away (further than 127 in at least one axis), so jump BMI MA65 \ to MA65 to skip the following, as it's too far away to \ dock, scoop or collide with

Code variation 1 of 2A variation in the comments only

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Cassette, Flight, 6502SP, Master

Electron

LDX TYPE \ If the current ship type is negative then it's either BMI MA65 \ a planet or a sun, so jump down to MA65 to skip the \ following, as we can't dock with it or scoop it
LDX TYPE \ If the current ship type is negative then it's the BMI MA65 \ planet, so jump down to MA65 to skip the following, \ as we can't dock with it or scoop it
 CPX #SST               \ If this ship is the space station, jump to ISDK to
 BEQ ISDK               \ check whether we are docking with it

 AND #%11000000         \ If bit 6 of (x_lo OR y_lo OR z_lo) is set, then the
 BNE MA65               \ ship is still a reasonable distance away (further than
                        \ 63 in at least one axis), so jump to MA65 to skip the
                        \ following, as it's too far away to dock, scoop or
                        \ collide with

 CPX #MSL               \ If this ship is a missile, jump down to MA65 to skip
 BEQ MA65               \ the following, as we can't scoop or dock with a
                        \ missile, and it has its own dedicated collision
                        \ checks in the TACTICS routine

Code variation 2 of 2Specific to an individual platform

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

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Cassette

Electron

CPX #OIL \ If ship type >= OIL (i.e. it's a cargo canister, BCS P%+5 \ Thargon or escape pod), skip the JMP instruction and JMP MA58 \ continue on, otherwise jump to MA58 to process a \ potential collision
CPX #OIL \ If ship type >= OIL (i.e. it's a cargo canister or BCS P%+5 \ escape pod), skip the JMP instruction and continue JMP MA58 \ on, otherwise jump to MA58 to process a potential \ collision
 LDA BST                \ If we have fuel scoops fitted then BST will be &FF,
                        \ otherwise it will be 0

 AND INWK+5             \ Ship byte #5 contains the y_sign of this ship, so a
                        \ negative value here means the canister is below us,
                        \ which means the result of the AND will be negative if
                        \ the canister is below us and we have a fuel scoop
                        \ fitted

 BPL MA58               \ If the result is positive, then we either have no
                        \ scoop or the canister is above us, and in both cases
                        \ this means we can't scoop the item, so jump to MA58
                        \ to process a collision

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