The entire main memory that is available for I/O operations is managed in the I/O buffer cache.
When the database instance is started, the I/O buffer is generated in the main memory in accordance with the size given in the general database parameter CACHE_SIZE.
The converter and the data cache are the most important main memory consumers that are managed in the I/O buffer cache. If the converter grows while the database is running, and requires more pages, then extra pages are assigned to it from the I/O buffer cache. If no more pages are available there, data is displaced from the data cache and made available to the converter.
See also:
Concepts of the Database System, General Database Parameters