sql> select object_name,object_type from dba_objects where object_name like 'idl%';

object_name            object_type
------------------- ------------------
idl_char$                 table
idl_sb4$                  table
idl_ub1$                  table
idl_ub2$                  table

ora-00600: internal error code, arguments: [17069],

[0xc0000000dddfa690], [], [], [], [], [], []

idl_ub1$ is one of four tables that hold compiled pl/sql code:

idl_ub1$
idl_char$
idl_ub2$
idl_sb4$

"pl/sql is based on the programming language ada. as a result, pl/sql uses a
variant of descriptive intermediate attributed notation for ada (diana), which
is a tree-structured intermediate language. it is defined using a meta-notation
called interface definition language (idl). diana provides for communication
internal to compilers and other tools.

"at compile time, pl/sql source code is translated into machine-readable
m-code.both the diana and m-code for a procedure or package are stored in the
database.at run time, they are loaded into the shared (memory) pool. the diana is
used to compile dependent procedures; the m-code is simply executed."

these four tables hold the diana and the so-code m-code. i think "m-code" is
short for machine-dependent byte code but there is a sizable machine-indenpendent part

as well. if you have a look at sql.bsq, you can see
that oracle documents the "type" column of these tables as follows:
part number not null,
/* part: 0 = diana, 1 = portable pcode, 
2 = machine-dependentpcode */