Flight: CheckAltitude

       Name: CheckAltitude                                           [Show more]
       Type: Subroutine
   Category: Flight
    Summary: Perform an altitude check with the planet, ending the game if we
             hit the ground
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * Main flight loop (Part 15 of 16) calls CheckAltitude
             * WARP calls CheckAltitude
             * Main flight loop (Part 16 of 16) calls via MA232


 Other entry points:

   MA232                Contains an RTS


.CheckAltitude

 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 MA232              ; If A > 0 then we are a fair distance away from the
                        ; planet in at least one axis, so jump to MA232 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 MA232              ; 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 MA232 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 MA282              ; If A < 0 then jump to MA282 as we have crashed into
                        ; the planet

 STA R                  ; Set (R Q) = (A Q)

 SETUP_PPU_FOR_ICON_BAR ; If the PPU has started drawing the icon bar, configure
                        ; the PPU to use nametable 0 and pattern table 0

 JSR LL5                ; We are getting close to the planet, so we need to
                        ; 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 MA232              ; If our altitude is non-zero then we haven't crashed,
                        ; so jump to MA232 to skip to the next section

.MA282

 JMP DEATH              ; If we get here then we just crashed into the planet
                        ; or got too close to the sun, so jump to DEATH to start
                        ; the funeral preparations and return from the main
                        ; flight loop using a tail call

.MA232

 RTS                    ; Return from the subroutine