SP to INSERT or UPDATE

[spnovice]

I need to write an SP in SYBASE as follows : -
Passing in will be values X & Y.
If X exists in my 2-field table, use Y to update field 2.
If X does not exist, then insert a new record.
If either of the above 2 successful, then return a 0.
If neither succesful, then return a 1.
I have written straightforward Selects before with SP's but nothing like this.
The OS is INIX.
Help !