up-down-full counter using pic and dot matrix display

hi...i'm working on a project now..search in this forum...but not found any circuits....anyone could help me upload the circuit...???
the circuit have 3 inputs and all the inputs are come from the omron PLC and should working like this...

1st input=count up
2nd input=count down
3rd input=full

i would like to count the car park load of 100 cars..let say there are no cars in the car park..come 1 car and it count up...and so on...and if one car leaves....the total remaining car is less 1..let say it have already 20 cars..leaves 1..so it will be 19 cars..those process continuous untill it reach 100 and the display shows "full" at the dot matrix display...and if 1 car leave..the circuit continued counting again as 99....so the process is always running...

anyone that have this circuit and its pic program...plz reply...need to do some research..thank u very much

p/s..if dont have..any other circuit that using 7-seg and normal circuit is also allowed...but with the same working principle..tq

Nigel’s tutorials at http://www.winpicprog.co.uk have samples for most of that. A 4 digit 7 segment display would make FULL easily.

ok...thans..any other circuits from the other forumer please....