Multimedia input/output (MMIO) services provides both I/O and CODEC procedures. I/O procedures are message-based handlers, which direct the input and output associated with reading and writing to different types of storage systems or file formats. Applications and the MMIO subsystem communicate to I/O procedures (DLL files) through the use of MMIO messages. When MMIO receives a request from an application through a function call, the MMIO Manager sends a predefined message for that operation to the I/O procedure responsible for that particular file format or storage system. In turn, the I/O procedure performs operations based on the messages it receives from the MMIO Manager or an application.

These messages are designed for efficient communications to all I/O procedures. The I/O procedures, however, must be able to process the messages or pass them on to a child I/O procedure. For example, if an I/O procedure receives a message requesting the compression of data object, the I/O procedure must be able to process the message, or pass the message to a CODEC procedure. The following figure illustrates the interaction of I/O and CODEC procedures in the MMIO subsystem.

The MMIO Manager calls the following types of procedures:

File Format

A file format procedure is an I/O procedure that manipulates data at the element level, with each procedure handling a different element type such as audio, image, or MIDI. It processes the data "within" the object and does not rely on any other file format I/O procedures to process the data. However, a file format procedure might need to call a storage system I/O procedure to obtain data within a file containing multiple elements.
Storage System
A storage system procedure is an I/O procedure that "unwraps" a data object for a file format procedure to access. Storage system procedures are unaware of the format of the data object contained within the wrapper.
CODEC
A CODEC procedure operates on data within a file or buffer. Based on the data content, an I/O procedure can load a CODEC procedure to either compress or decompress data.

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