well i'm also workin on a map with a save feature although mine is much more advanced and much harder to crack. I'll try to keep things simple when i explain this.
First of all when your making your code you have to predetermine how much information your going to allow the user to save. Say you want your code to save exp and the characters inventory. You might only need 1 number slot for each space in the inventory your going to use up and say 4 or 5 slots for the exp. also lets just say your code saves 4 inventory slots making it a 9 number code.
Next you need to decide how your going to rearrange your code. You should try something like:
[exp2][item4][exp1][item3][exp5][exp3][item2][item1][exp4]
and not like:
[exp1][exp2][exp3][exp4][exp5][item1][item2][item3][item4]
next you need to decide what number each item will represent
for example:
0:empty
1:potion
2:key
3:ether.... and so on
(you will need to use death counts for each inventory slot)
exp will be a little more complicated and use death counters.
when a character goes to save you will need to first subtract 10,000 and add 1 death counter to whatever you want (obviously a unit you dont use). then when that player has at most 9,999 you subtract 1,000 and add 1 death counter to something diffrent. at most 999 you subtract 100, at most 99 you subtract 10, and at most 9 you subtract 1. Lets say the player had 2,513 exp. He will now have
0 death counts for 1 unit in the 1st exp slot
2 death counts for 1 unit in the 2nd exp slot
5 death counts for 1 unit in the 3rd exp slot
1 death counts for 1 unit in the 4th exp slot
3 death counts for 1 unit in the 5th exp slot
if you put this information into the code i suggested above and lets say the person had 2 potions and an ether in their inventory their code would now be:
200335111
For uploading it will just be the same but in reverse. I believe that xuru used some sort of scv to a beacon method which you could use as well. just use triggers like this
C:
current player brings at least 1 scv to "slot 1"
A:
kill 1 scv for current player at "slot 1"
add 1 death count for psi emulator for current player
preserve trigger
you will also need a "finished" beacon to let the game know that the player is done typing it the code. also a subtract 1 unit like a drone or something would be nice incase a player messes up.
well... thats it

i tried to explain it all as simply as possible so i hope you understand it
