The multimedia system supports sharing of physical devices among multiple applications. If a device is capable of being shared; that is, if it can maintain state information, the system can establish a unique device state, much like a Presentation Manager device context, for each application that uses the device.
The scope of a device state is defined by each device. The state of a simple device like the digital video player contains information about the current frame position, whether the device is playing or stopped, what its current playback speed is set to, and so on. The state of a compound device can include the name of the currently selected file, RIFF object, and playback position.
Media devices vary in their ability to support multiple device contexts concurrently. The different types of device use that are supported by media devices are:
The following table contains descriptions and examples of these device use types.
旼컴컴컴컴컴컴컴컴컴컴컴컴쩡컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴엿Context Use Type 쿏escription � 쳐컴컴컴컴컴컴컴컴컴컴컴컴탠컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� 쿑ixed single-context 쿌 fixed single-context device can establish � � 쿽nly one device context. The state of a � � 쿯ixed single-context device cannot be queried� � 쿽r set by software. � � 쿌n example of a fixed single-context device � � 쿶s a video cassette recorder that does not � � 퀁eport the tape position to the driver. � 쳐컴컴컴컴컴컴컴컴컴컴컴컴탠컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� 쿏ynamic single-context 쿌 dynamic single-context device is serially � � 퀂hareable. That is, the device can be used by� � 쿽nly one application at a time but can be � � 쿾assed from one application to another. A � � 쿭evice state for each application is saved � � 쿪nd restored appropriately. � � 쿟his is the most common concurrent use type � � 쿯or a media device. An example of a dynamic � � 퀂ingle-context device is a CD-ROM player. � 쳐컴컴컴컴컴컴컴컴컴컴컴컴탠컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� 쿗imited multiple-context 쿌 limited multiple-context device can � � 쿮stablish multiple device contexts, but the � � 쿻umber of device contexts is limited by the � � 쿾hysical device. � � 쿌n example of a limited multiple-context � � 쿭evice is a 4-channel amp-mixer audio device,� � 퀇hich can concurrently support any of the � � 쿯ollowing multiple-contexts: � � 쿑our monaural contexts, two stereo contexts, � � 쿪nd one stereo and two monaural contexts. � 쳐컴컴컴컴컴컴컴컴컴컴컴컴탠컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� 쿢nlimited context 쿌n unlimited context device can support an � � 쿪rbitrary number of concurrent device � � 쿬ontexts. The number of concurrent contexts � � 쿶s limited only by the resource limits of the� � 퀂ystem. � 읕컴컴컴컴컴컴컴컴컴컴컴컴좔컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴�