.TA4 LDA INWK+7 ; If z_hi >= 3 then the ship is quite far away, so jump CMP #3 ; down to TA5 BCS TA5 LDA INWK+1 ; Otherwise set A = x_hi OR y_hi and extract bits 1-7 ORA INWK+4 AND #%11111110 BEQ TA15 ; If A = 0 then the ship is pretty close to us, so jump ; to TA15 so it heads away from us .TA5 ; If we get here then the ship is quite far away JSR DORND ; Set A and X to random numbers ORA #%10000000 ; Set bit 7 of A, so A is at least 128 CMP INWK+32 ; If A >= byte #32 (the ship's AI flag) then jump down BCS TA15 ; to TA15 so it heads away from us ; We get here if A < byte #32, and the chances of this ; being true are greater with high values of byte #32, ; as long as they are at least 128 ; ; In other words, higher byte #32 values increase the ; chances of a ship changing direction to head towards ; us - or, to put it another way, ships with higher ; byte #32 values of 128 or more are spoiling for a ; fight ; ; Thargoids have byte #32 set to 255, which explains ; an awful lot .TA20 ; If this is a missile we will have jumped straight ; here, but we also get here if the ship is either far ; away and aggressive, or not too close JSR TAS6 ; Call TAS6 to negate the vector in XX15 so it points in ; the opposite direction LDA CNT ; Change the sign of the dot product in CNT, so now it's EOR #%10000000 ; positive if the ships are facing each other, and ; negative if they are facing the same way .TA152 STA CNT ; Update CNT with the new value in A .TA15 ; If we get here, then one of the following is true: ; ; * This is a trader and XX15 is pointing towards the ; planet ; ; * The ship is pretty close to us, or it's just not ; very aggressive (though there is a random factor ; at play here too). XX15 is still pointing from our ; ship towards the enemy ship ; ; * The ship is aggressive (though again, there's an ; element of randomness here). XX15 is pointing from ; the enemy ship towards our ship ; ; * This is a missile heading for a target. XX15 is ; pointing from the missile towards the target ; ; We now want to move the ship in the direction of XX15, ; which will make aggressive ships head towards us, and ; ships that are too close turn away. Peaceful traders, ; meanwhile, head off towards the planet in search of a ; space station, and missiles home in on their targets LDY #16 ; Set (A X) = roofv . XX15 JSR TAS3 ; ; This will be positive if XX15 is pointing in the same ; direction as an arrow out of the top of the ship, in ; other words if the ship should pull up to head in the ; direction of XX15 TAX ; Copy A into X so we can retrieve it below EOR #%10000000 ; Give the ship's pitch counter the opposite sign to the AND #%10000000 ; dot product result, with a value of 0 STA INWK+30 TXA ; Retrieve the original value of A from X ASL A ; Shift A left to double it and drop the sign bit CMP RAT2 ; If A < RAT2, skip to TA11 (so if RAT2 = 0, we always BCC TA11 ; set the pitch counter to RAT) LDA RAT ; Set the magnitude of the ship's pitch counter to RAT ORA INWK+30 ; (we already set the sign above) STA INWK+30 .TA11 LDA INWK+29 ; Fetch the roll counter from byte #29 into A ASL A ; Shift A left to double it and drop the sign bit CMP #32 ; If A >= 32 then jump to TA6, as the ship is already BCS TA6 ; in the process of rolling LDY #22 ; Set (A X) = sidev . XX15 JSR TAS3 ; ; This will be positive if XX15 is pointing in the same ; direction as an arrow out of the right side of the ; ship, in other words if the ship should roll right to ; head in the direction of XX15 TAX ; Copy A into X so we can retrieve it below EOR INWK+30 ; Give the ship's roll counter a positive sign AND #%10000000 ; (clockwise roll) if the pitch counter and dot product EOR #%10000000 ; have different signs, negative (anti-clockwise roll) STA INWK+29 ; if they have the same sign, with a value of 0 TXA ; Retrieve the original value of A from X ASL A ; Shift A left to double it and drop the sign bit CMP RAT2 ; If A < RAT2, skip to TA12 (so if RAT2 = 0, we always BCC TA12 ; set the roll counter to RAT) LDA RAT ; Set the magnitude of the ship's roll counter to RAT ORA INWK+29 ; (we already set the sign above) STA INWK+29 .TA12 .TA6 LDA CNT ; Fetch the dot product, and if it's negative jump to BMI TA9 ; TA9, as the ships are facing away from each other and ; the ship might want to slow down to take another shot CMP CNT2 ; The dot product is positive, so the ships are facing BCC TA9 ; each other. If A < CNT2 then the ships are not heading ; directly towards each other, so jump to TA9 to slow ; down .PH10E LDA #3 ; Otherwise set the acceleration in byte #28 to 3 STA INWK+28 RTS ; Return from the subroutine .TA9 AND #%01111111 ; Clear the sign bit of the dot product in A CMP #18 ; If A < 18 then the ship is way off the XX15 vector, so BCC TA10 ; return from the subroutine (TA10 contains an RTS) ; without slowing down, as it still has quite a bit of ; turning to do to get on course LDA #$FF ; Otherwise set A = -1 LDX TYPE ; If this is not a missile then skip the ASL instruction CPX #MSL BNE P%+3 ASL A ; This is a missile, so set A = -2, as missiles are more ; nimble and can brake more quickly STA INWK+28 ; Set the ship's acceleration to A .TA10 RTS ; Return from the subroutine .TA151 ; This is called from part 3 with the vector to the ; planet in XX15, when we want the ship to turn towards ; the planet. It does the same dot product calculation ; as part 3, but it can also change the value of RAT2 ; so that roll and pitch is always applied LDY #10 ; Set (A X) = nosev . XX15 JSR TAS3 ; ; The bigger the value of the dot product, the more ; aligned the two vectors are, with a maximum magnitude ; in A of 36 (96 * 96 >> 8). If A is positive, the ; vectors are facing in a similar direction, if it's ; negative they are facing in opposite directions CMP #$98 ; If A is positive or A <= -24, jump to ttt BCC ttt LDX #0 ; A > -24, which means the vectors are facing in STX RAT2 ; opposite directions but are quite aligned, so set ; RAT2 = 0 instead of the default value of 4, so we ; always apply roll and pitch when we turn the ship ; towards the planet .ttt JMP TA152 ; Jump to TA152 to store A in CNT and move the ship in ; the direction of XX15Name: TACTICS (Part 7 of 7) [Show more] Type: Subroutine Category: Tactics Summary: Apply tactics: Set pitch, roll, and acceleration Deep dive: Program flow of the tactics routineContext: See this subroutine in context in the source code References: This subroutine is called as follows: * DOCKIT calls via TA151 * TACTICS (Part 3 of 7) calls via TA151 * TACTICS (Part 6 of 7) calls via TA9-1
This section looks at manoeuvring the ship. Specifically: * Work out which direction the ship should be moving, depending on the type of ship, where it is, which direction it is pointing, and how aggressive it is * Set the pitch and roll counters to head in that direction * Speed up or slow down, depending on where the ship is in relation to us
Other entry points: TA151 Make the ship head towards the planet TA9-1 Contains an RTS
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Subroutine DORND (category: Maths (Arithmetic))
Generate random numbers
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Configuration variable MSL = 1
Ship type for a missile
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Label TA10 is local to this routine
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Label TA11 is local to this routine
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Label TA12 is local to this routine
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Label TA15 is local to this routine
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Label TA152 is local to this routine
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Label TA5 is local to this routine
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Label TA6 is local to this routine
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Label TA9 is local to this routine
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Subroutine TAS3 (category: Maths (Geometry))
Calculate the dot product of XX15 and an orientation vector
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Subroutine TAS6 (category: Maths (Geometry))
Negate the vector in XX15 so it points in the opposite direction
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Label ttt is local to this routine