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Elite on the BBC Micro

Main loop: Main flight loop (Part 12 of 16) [Elite-A, Flight]

Name: Main flight loop (Part 12 of 16) [Show more] Type: Subroutine Category: Main loop Summary: For each nearby ship: Draw the ship, remove if killed, loop back Deep dive: Program flow of the main game loop Drawing ships
Context: See this subroutine in context in the source code References: No direct references to this subroutine in this source file

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: * Draw the ship * Process removal of killed ships * Loop back up to MAL1 to move onto the next ship in the local bubble
.MA8 JSR LL9 \ Call LL9 to draw the ship we're processing on-screen .MA15 LDY #35 \ Fetch the ship's energy from byte #35 and copy it to LDA INWK+35 \ byte #35 in INF (so the ship's data in K% gets STA (INF),Y \ updated) LDA NEWB \ If bit 7 of the ship's NEWB flags is set, which means BMI KS1S \ the ship has docked or been scooped, jump to KS1S to \ skip the following, as we can't get a bounty for a \ ship that's no longer around LDA INWK+31 \ If bit 7 of the ship's byte #31 is clear, then the BPL MAC1 \ ship hasn't been killed by collision or laser fire, \ so jump to MAC1 to skip the following AND #%00100000 \ If bit 5 of the ship's byte #31 is clear then the BEQ MAC1 \ ship is no longer exploding, so jump to MAC1 to skip \ the following \ --- Mod: Original Acornsoft code removed: -----------> \ LDA NEWB \ Extract bit 6 of the ship's NEWB flags, so A = 64 if \ AND #%01000000 \ bit 6 is set, or 0 if it is clear. Bit 6 is set if \ \ this ship is a cop, so A = 64 if we just killed a \ \ policeman, otherwise it is 0 \ ORA FIST \ Update our FIST flag ("fugitive/innocent status") to \ STA FIST \ at least the value in A, which will instantly make us \ \ a fugitive if we just shot the sheriff, but won't \ \ affect our status if the enemy wasn't a copper \ --- And replaced by: --------------------------------> \ We now update our FIST flag ("fugitive/innocent \ status") by 1 if we didn't kill a cop, or by a large \ amount if we did - specifically, if we killed a cop, \ then the most significant bit in FIST that is \ currently clear will be set, which means we increase \ FIST by the highest multiple of 2 that we can add and \ still fit the result in a byte \ \ Also, at this point, we know that A = %00100000 (as we \ didn't take the BEQ branch) BIT NEWB \ If bit 6 of the ship's NEWB flags is set, then this BVS n_badboy \ ship is a cop, so jump to n_badboy as we just killed a \ policeman BEQ n_goodboy \ The BIT NEWB instruction sets the Z flag according to \ the result of: \ \ A AND NEWB = %00100000 AND NEWB \ \ so this jumps to n_goodboy if bit 5 of NEWB is clear, \ so in other words, if the ship is no longer exploding, \ we don't update FIST LDA #%10000000 \ Set A so that the shift and rotate instructions we're \ about to do set A = %00000001, so we increase our FIST \ status by just 1 .n_badboy \ We get here with two possible values of A: \ \ * A = %00100000 if we just killed a cop \ * A = %10000000 otherwise ASL A \ Shift and rotate A so that we get: ROL A \ \ * A = %10000000 if we just killed a cop \ * A = %00000001 otherwise .n_bitlegal LSR A \ We now shift A to the right and AND it with FIST, BIT FIST \ repeating the process until the single set bit in A BNE n_bitlegal \ matches a clear bit in FIST, so this shifts A right \ so that the set bit matches the highest clear bit in \ FIST (if we just killed a cop), or it sets A to 0 and \ sets the C flag (if we didn't) ADC FIST \ Set A = A + C + FIST, so: \ \ * A = A + 0 + FIST if we just killed a cop \ * A = 0 + 1 + FIST otherwise \ \ so if we just killed a cop, this will effectively set \ the highest clear bit in FIST, otherwise we just add 1 \ to FIST BCS KS1S \ If the addition overflowed, jump to KS1S to skip \ showing an on-screen bounty for this kill, and without \ updating FIST first (as we are too bad to get any \ worse) STA FIST \ Otherwise update the value of FIST to the new value BCC KS1S \ Jump to KS1S to skip showing an on-screen bounty for \ this kill (the BCC is effectively a JMP as we just \ passed through a BCS) .n_goodboy \ --- End of replacement ------------------------------> LDA DLY \ If we already have an in-flight message on-screen (in ORA MJ \ which case DLY > 0), or we are in witchspace (in BNE KS1S \ which case MJ > 0), jump to KS1S to skip showing an \ on-screen bounty for this kill \ --- Mod: Original Acornsoft code removed: -----------> \ LDY #10 \ Fetch byte #10 of the ship's blueprint, which is the \ LDA (XX0),Y \ low byte of the bounty awarded when this ship is \ BEQ KS1S \ killed (in Cr * 10), and if it's zero jump to KS1S as \ \ there is no on-screen bounty to display \ --- And replaced by: --------------------------------> LDY #10 \ Fetch byte #10 of the ship's blueprint, which is the LDA (XX0),Y \ low byte of the bounty awarded when this ship is \ killed (in Cr * 10) \ --- End of replacement ------------------------------> TAX \ Put the low byte of the bounty into X INY \ Fetch byte #11 of the ship's blueprint, which is the LDA (XX0),Y \ high byte of the bounty awarded (in Cr * 10), and put TAY \ it into Y JSR MCASH \ Call MCASH to add (Y X) to the cash pot LDA #0 \ Print control code 0 (current cash, right-aligned to JSR MESS \ width 9, then " CR", newline) as an in-flight message .KS1S JMP KS1 \ Process the killing of this ship (which removes this \ ship from its slot and shuffles all the other ships \ down to close up the gap) \ --- Mod: Code added for Elite-A: --------------------> .n_hit \ If we get here then we need to apply a hit of strength \ A to the enemy ship STA T \ Store the strength of the hit in T SEC \ Set the C flag so we can do some subtraction LDY #14 \ Fetch byte #14 of the enemy ship's blueprint into A, LDA (XX0),Y \ which gives the ship's maximum energy/shields AND #7 \ Reduce the maximum energy/shields figure to the range \ 0-7 SBC T \ Subtract the hit strength from the maximum shields, so \ A = ship energy - hit strength BCS n_kill \ If the subtraction didn't underflow, then the hit was \ weaker than the ship's shields, so jump to n_kill \ with the C flag set to indicate that the ship has \ survived the attack \BCC n_defense \ These instructions are commented out in the original \LDA #&FF \ source \.n_defense CLC \ Otherwise the hit was stronger than the enemy shields, ADC INWK+35 \ so the ship's energy level needs to register some STA INWK+35 \ damage. A contains a negative number whose magnitude \ is the amount by which the attack is greater than the \ shield defence, so we can simply add this figure to \ the ship's energy levels in the ship's byte #35 to \ reduce the energy by the amount that the attack was \ stronger than the defence (i.e. the shields absorb the \ amount of energy that is defined in the blueprint, and \ the rest of the hit makes it through to damage the \ energy levels) BCS n_kill \ Adding this negative number is the same as subtracting \ a positive number, so having the C flag set indicates \ that the subtraction didn't underflow - in other words \ the damage isn't greater than the energy levels, and \ the ship has survuved the hit. In this case we jump to \ n_kill with the C flag set to indicate that the ship \ has survived the attack JSR TA87+3 \ If we get here then the ship has not survived the \ attack, so call TA87+3 to set bit 7 of the ship's byte \ #31, which marks the ship as being killed .n_kill RTS \ Return from the subroutine with the C flag set if the \ ship has survived the onslaught, or clear if it has \ been destroyed \ --- End of added code -------------------------------> .MAC1 LDA TYPE \ If the ship we are processing is a planet or sun, BMI MA27 \ jump to MA27 to skip the following two instructions JSR FAROF \ If the ship we are processing is a long way away (its BCC KS1S \ distance in any one direction is > 224, jump to KS1S \ to remove the ship from our local bubble, as it's just \ left the building .MA27 LDY #31 \ Fetch the ship's explosion/killed state from byte #31 LDA INWK+31 \ and copy it to byte #31 in INF (so the ship's data in STA (INF),Y \ K% gets updated) LDX XSAV \ We're done processing this ship, so fetch the ship's \ slot number, which we saved in XSAV back at the start \ of the loop INX \ Increment the slot number to move on to the next slot JMP MAL1 \ And jump back up to the beginning of the loop to get \ the next ship in the local bubble for processing