Oracle Database Discussions

Hi guys,

In my windows XP system, i got 9i and 10g both running.

9I ORACLE_SID = abc
10g ORACLE_SID = oic

I am testing my Logic of ORACLE_HOME+ORACLE_SID = memory of instance.
SO

i explicity set an environment variable ORACLE_SID = abc (9i).
i have no ORACLE_HOME environment variable setup. (The rest of the values are all in registry)

So, i to the exact folder of sqlplus (10g) c:\oracle\product\10.2.0\bin\sqlplusw.exe
By right, once i open the program, IF i am not wrong, it will search for a file call oracle.key in the BIN folder then from there, went to registry to get the ORACLE_HOME value.

and since it is going into the 10g BIN and getting the 10g HOME value.

10g ORACLE_HOME + 9i SID = no such memory exist.
However, i am still able to connect to the oracle 9i database locally.

May i know why ?

Any enlightenments gurus ?
Thanks