pass values to-and-fro from foreign function - oracle

Can you pass values to-and-fro from foreign function ? how ? 
Yes . You obtain a return value from a foreign function by assigning the return value to an Oracle Forms variable or item. Make sure that the Oracle Forms variable or item is the same data type as the return value from the foreign function.
After assigning an Oracle Forms variable or item value to a PL/SQL variable, pass the PL/SQL variable as a parameter value in the PL/SQL interface of the foreign function. The PL/SQL variable that is passed as a parameter must be a valid PL/SQL data type; it must also be the appropriate parameter type as defined in the PL/SQL interface.

What is IAPXTB structure ? 
The entries of Pro * C and user exits and the form which simulate the proc or user_exit are stored in IAPXTB table in d/b.

Can you call WIN-SDK thru user exits? 
YES.

What command would you use to create a backup control file?
Alter database backup control file to trace.

Give the stages of instance startup to a usable state where normal users may access it. 
STARTUP NOMOUNT - Instance startup
STARTUP MOUNT - The database is mounted
STARTUP OPEN - The database is opened

What column differentiates the V$ views to the GV$ views and how? 
The INST_ID column which indicates the instance in a RAC environment the information came from.

How would you go about generating an EXPLAIN plan? 
Create a plan table with utlxplan.sql.
Use the explain plan set statement_id = 'tst1' into plan_table for a SQL statement
Look at the explain plan with utlxplp.sql or utlxpls.sql

How would you go about increasing the buffer cache hit ratio? 
Use the buffer cache advisory over a given workload and then query the v$db_cache_advice table. If a change was necessary then I would use the alter system set db_cache_size command.