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 flight loop (Part 15 of 16) Type: Subroutine Category: Main loop
Deep dive: Program flow of the main game loop Scheduling tasks with the main loop counter
The main flight loop covers most of the flight-specific aspects of Elite. This section covers the following: * Perform an altitude check with the planet (every 32 iterations of the main loop, on iteration 10 of each 32)
* Perform an altitude check with the sun and process fuel scooping (every 32 iterations of the main loop, on iteration 20 of each 32)
.MA22
This variation is blank in the Electron version.
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LDA MCNT \ Fetch the main loop counter and calculate MCNT mod 32, AND #31 \ which tells us the position of this loop in each block \ of 32 iterations .MA93
If speech is enabled on the Executive version, it will say "Energy low" every time the "ENERGY LOW,SIR" message flashes on-screen.
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LDY #&FF \ Set our altitude in ALTIT to &FF, the maximum STY ALTIT INY \ Set Y = 0 JSR m \ Call m to calculate the maximum distance to the \ planet in any of the three axes, returned in A BNE MA23 \ If A > 0 then we are a fair distance away from the \ planet in at least one axis, so jump to MA23 to skip \ the rest of the altitude check JSR MAS3 \ Set A = x_hi^2 + y_hi^2 + z_hi^2, so using Pythagoras \ we now know that A now contains the square of the \ distance between our ship (at the origin) and the \ centre of the planet at (x_hi, y_hi, z_hi) BCS MA23 \ If the C flag was set by MAS3, then the result \ overflowed (was greater than &FF) and we are still a \ fair distance from the planet, so jump to MA23 as we \ haven't crashed into the planet SBC #36 \ Subtract 36 from x_hi^2 + y_hi^2 + z_hi^2 \ \ When we do the 3D Pythagoras calculation, we only use \ the high bytes of the coordinates, so that's x_hi, \ y_hi and z_hi and \ \ The planet radius is (0 96 0), as defined in the \ PLANET routine, so the high byte is 96 \ \ When we square the coordinates above and add them, \ the result gets divided by 256 (otherwise the result \ wouldn't fit into one byte), so if we do the same for \ the planet's radius, we get: \ \ 96 * 96 / 256 = 36 \ \ So for the planet, the equivalent figure to test the \ sum of the _hi bytes against is 36, so A now contains \ the high byte of our altitude above the planet \ surface, squared BCC MA28 \ If A < 0 then jump to MA28 as we have crashed into \ the planet STA R \ We are getting close to the planet, so we need to JSR LL5 \ work out how close. We know from the above that A \ contains our altitude squared, so we store A in R \ and call LL5 to calculate: \ \ Q = SQRT(R Q) = SQRT(A Q) \ \ Interestingly, Q doesn't appear to be set to 0 for \ this calculation, so presumably this doesn't make a \ difference LDA Q \ Store the result in ALTIT, our altitude STA ALTIT BNE MA23 \ If our altitude is non-zero then we haven't crashed, \ so jump to MA23 to skip to the next section .MA28
.MA29
The docking computer in the enhanced versions updates its position every 32 iterations round the main loop (on the 15th iteration), at which point it displays "DOCKING COMPUTERS ON" as an in-flight message.
This variation is blank in the Cassette and Electron versions.
CMP #15 \ If this is the 15th iteration in this block of 32, BNE MA33 \ do the following, otherwise jump to MA33 to skip the \ docking computer manoeuvring LDA auto \ If auto is zero, then the docking computer is not BEQ MA23 \ activated, so jump to MA23 to skip to the next \ section LDA #123 \ Set A = 123 and jump down to MA34 to print token 123 BNE MA34 \ ("DOCKING COMPUTERS ON") as an in-flight message .MA33
As there are no suns in the Electron version, we don't need to set the cabin temperature based on the altitude from the sun.
This variation is blank in the Electron version.
CMP #20 \ If this is the 20th iteration in this block of 32, BNE MA23 \ do the following, otherwise jump to MA23 to skip the \ sun altitude check LDA #30 \ Set CABTMP to 30, the cabin temperature in deep space STA CABTMP \ (i.e. one notch on the dashboard bar) LDA SSPR \ If we are inside the space station safe zone, jump to BNE MA23 \ MA23 to skip the following, as we can't have both the \ sun and space station at the same time, so we clearly \ can't be flying near the sun LDY #NI% \ Set Y to NI%, which is the offset in K% for the sun's \ data block, as the second block at K% is reserved for \ the sun (or space station) JSR MAS2 \ Call MAS2 to calculate the largest distance to the BNE MA23 \ sun in any of the three axes, and if it's non-zero, \ jump to MA23 to skip the following, as we are too far \ from the sun for scooping or temperature changes JSR MAS3 \ Set A = x_hi^2 + y_hi^2 + z_hi^2, so using Pythagoras \ we now know that A now contains the square of the \ distance between our ship (at the origin) and the \ heart of the sun at (x_hi, y_hi, z_hi) EOR #%11111111 \ Invert A, so A is now small if we are far from the \ sun and large if we are close to the sun, in the \ range 0 = far away to &FF = extremely close, ouch, \ hot, hot, hot! ADC #30 \ Add the minimum cabin temperature of 30, so we get \ one of the following: \ \ * If the C flag is clear, A contains the cabin \ temperature, ranging from 30 to 255, that's hotter \ the closer we are to the sun \ \ * If the C flag is set, the addition has rolled over \ and the cabin temperature is over 255 STA CABTMP \ Store the updated cabin temperature BCS MA28 \ If the C flag is set then jump to MA28 to die, as \ our temperature is off the scale CMP #224 \ If the cabin temperature < 224 then jump to MA23 to BCC MA23 \ skip fuel scooping, as we aren't close enough
This variation is blank in the Cassette, Disc (flight), 6502 Second Processor and Electron versions.
\CMP #&F0 \ These instructions are commented out in the original \BCC nokilltr \ source \LDA #5 \JSR SETL1 \LDA VIC+&15 \AND #&3 \STA VIC+&15 \LDA #4 \JSR SETL1 \LSR TRIBBLE+1 \ROR TRIBBLE \.nokilltr
As there are no suns in the Electron version, we don't need to implement fuel scooping.
This variation is blank in the Electron version.
LDA BST \ If we don't have fuel scoops fitted, jump to BA23 to BEQ MA23 \ skip fuel scooping, as we can't scoop without fuel \ scoops LDA DELT4+1 \ We are now successfully fuel scooping, so it's time LSR A \ to work out how much fuel we're scooping. Fetch the \ high byte of DELT4, which contains our current speed \ divided by 4, and halve it to get our current speed \ divided by 8 (so it's now a value between 1 and 5, as \ our speed is normally between 1 and 40). This gives \ us the amount of fuel that's being scooped in A, so \ the faster we go, the more fuel we scoop, and because \ the fuel levels are stored as 10 * the fuel in light \ years, that means we just scooped between 0.1 and 0.5 \ light years of free fuel ADC QQ14 \ Set A = A + the current fuel level * 10 (from QQ14) CMP #70 \ If A > 70 then set A = 70 (as 70 is the maximum fuel BCC P%+4 \ level, or 7.0 light years) LDA #70 STA QQ14 \ Store the updated fuel level in QQ14