Two types of I/O procedures are as follows:
Storage System IOProcs unwrap data objects such as RIFF files, compound RIFF files, or AVC files. These IOProcs are ignorant to the content of the data they contain. A storage system IOProc goes directly to the OS/2 (or native) file system (memory in the case of a MEM file) and does not pass information to any other file format or storage system IOProc. The internal I/O procedures provided for DOS files, memory files, and RIFF compound files are examples of storage system IOProcs because they operate on the storage mechanism rather than the data content itself. See the table in the Internal Storage System Procedures section.
File Format IOProcs manipulate multimedia data at the element level (not to be confused with an element of a RIFF compound file). Each IOProc handles a different element type such as audio, image, or MIDI. A file format IOProc handles the element type it was written for and does not rely on any other file format IOProcs to do any processing. However, a file format IOProc might need to call a storage system IOProc to obtain data within a file containing multiple file elements. For example, the MIDI IOProc calls MMIO functions to access data from other storage system IOProcs supported by MMIO. See the table in the File Format I/O Procedures Provided With OS/2 Multimedia section.