.HITCH CLC ; Clear the C flag so we can return with it cleared if ; our checks fail LDA INWK+8 ; Set A = z_sign BNE HI1 ; If A is non-zero then the ship is behind us and can't ; be in our crosshairs, so return from the subroutine ; with the C flag clear (as HI1 contains an RTS) LDA TYPE ; If the ship type has bit 7 set then it is the planet BMI HI1 ; or sun, which we can't target or hit with lasers, so ; return from the subroutine with the C flag clear (as ; HI1 contains an RTS) LDA INWK+31 ; Fetch bit 5 of byte #31 (the exploding flag) and OR AND #%00100000 ; with x_hi and y_hi ORA INWK+1 ORA INWK+4 BNE HI1 ; If this value is non-zero then either the ship is ; exploding (so we can't target it), or the ship is too ; far away from our line of fire to be targeted, so ; return from the subroutine with the C flag clear (as ; HI1 contains an RTS) LDA INWK ; Set A = x_lo JSR SQUA2 ; Set (A P) = A * A = x_lo^2 STA S ; Set (S R) = (A P) = x_lo^2 LDA P STA R LDA INWK+3 ; Set A = y_lo JSR SQUA2 ; Set (A P) = A * A = y_lo^2 TAX ; Store the high byte in X LDA P ; Add the two low bytes, so: ADC R ; STA R ; R = P + R TXA ; Restore the high byte into A and add S to give the ADC S ; following: ; ; (A R) = (S R) + (A P) = x_lo^2 + y_lo^2 BCS TN10 ; If the addition just overflowed then there is no way ; our crosshairs are within the ship's targetable area, ; so return from the subroutine with the C flag clear ; (as TN10 contains a CLC then an RTS) STA S ; Set (S R) = (A P) = x_lo^2 + y_lo^2 LDY #2 ; Fetch the ship's blueprint and set A to the high byte JSR GetShipBlueprint ; of the targetable area of the ship CMP S ; We now compare the high bytes of the targetable area ; and the calculation in (S R): ; ; * If A >= S then then the C flag will be set ; ; * If A < S then the C flag will be C clear BNE HI1 ; If A <> S we have just set the C flag correctly, so ; return from the subroutine (as HI1 contains an RTS) DEY ; The high bytes were identical, so now we fetch the JSR GetShipBlueprint ; low byte of the targetable area into A CMP R ; We now compare the low bytes of the targetable area ; and the calculation in (S R): ; ; * If A >= R then the C flag will be set ; ; * If A < R then the C flag will be C clear .HI1 RTS ; Return from the subroutine .TN10 CLC ; Clear the C flag to indicate the ship is not in our ; crosshairs RTS ; Return from the subroutineName: HITCH [Show more] Type: Subroutine Category: Tactics Summary: Work out if the ship in INWK is in our crosshairs Deep dive: In the crosshairsContext: See this subroutine in context in the source code References: This subroutine is called as follows: * Main flight loop (Part 11 of 16) calls HITCH * ANGRY calls via HI1
This is called by the main flight loop to see if we have laser or missile lock on an enemy ship.
Returns: C flag Set if the ship is in our crosshairs, clear if it isn't
Other entry points: HI1 Contains an RTS
[X]
Subroutine GetShipBlueprint (category: Drawing ships)
Fetch a specified byte from the current ship blueprint
[X]
Subroutine SQUA2 (category: Maths (Arithmetic))
Calculate (A P) = A * A
[X]
Label TN10 is local to this routine