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Revs on the BBC Micro

Extra tracks: HookSlopeJump

Name: HookSlopeJump [Show more] Type: Subroutine Category: Extra tracks Summary: Jump the car when driving fast over sloping segments Deep dive: Secrets of the extra tracks Code hooks in the extra tracks
Context: See this subroutine in context in the source code References: This subroutine is called as follows: * ModifyGameCode (Part 3 of 3) calls HookSlopeJump

This routine is called from part 5 of ApplyElevation to jump the car off the ground when driving fast over sloping segments. If the car is on the ground, replace the heightAboveTrack * 4 part of the car's y-coordinate calculation with playerSpeedHi * yTrackSegmentI * 4, to give: (yPlayerCoordTop yPlayerCoordHi) = (ySegmentCoordIHi ySegmentCoordILo) + carProgress * yTrackSegmentI + playerSpeedHi * yTrackSegmentI * 4 + 172 So driving fast over sloping segments can make the car jump. Arguments: A Current value of heightAboveTrack
.HookSlopeJump BNE slop1 \ If A is non-zero, skip the following (so the hook has \ no effect when the car is off the ground) \ If we get here then heightAboveTrack = 0, so the car \ is on the ground LDA playerSpeedHi \ Set A = the high byte of the current speed JSR MultiplyHeight \ Set: \ \ A = A * yTrackSegmentI \ = playerSpeedHi * yTrackSegmentI \ \ The value given in yTrackSegmentI is the y-coordinate \ of the segment vector, i.e. the vector from this \ segment to the next, which is the same as the change \ in height as we move through the segment \ \ So this value is higher with greater speed and on \ segments that have higher slopes BPL slop1 \ If A is positive, skip the following instruction DEC W \ Decrement W to &FF, so (W A) has the correct sign .slop1 ASL A \ Implement the shifts that we overwrote with the call ROL W \ to the hook routine, so we have effectively inserted \ the above code into the main game RTS \ Return from the subroutine