Micro Channel Participants

There are two basic types of devices, which may exist on and communicate via the Micro Channel:

  • A master is an intelligent device, which may contend for control of the channel.

  • A slave is a unintelligent device, which merely acts as the source or target of a data transfer, in conjunction with a master.

    Figure "Micro Channel Participants and Data Transfer Paths" shows the interaction between masters and slaves over the Micro Channel. The master devices are of three kinds:

  • The system master is the processor provided with the system hardware, and is thus also known as the system processor. The system master assumes ownership of the channel when no other master has arbitrated for and won control of it. The system master is, therefore, also known as the default master.

  • The DMA controller is typically provided on the system board. It supports multiple independent DMA channels, each allowing the attachment of a DMA slave. Each DMA slave is allocated its own dedicated channel. The DMA controller manages the transfer of data between a DMA slave and a memory slave, and supports burst mode data transfer if the DMA slave requests it. The DMA controller does not arbitrate for the channel, but requires the DMA slave to do the arbitration. Once this is done, the data transfer is completed independently of the system processor.

  • The bus master is an intelligent device or adapter on the Micro Channel and is typically an advanced adapter, which functions as a subsystem within the system. It arbitrates for the channel and manages the data transfer to or from an I/O slave or a memory slave. The bus master is described further in Bus Master Adapters.

    Slave devices on the Micro Channel may be of three kinds:

  • The I/O slave is selected via its address in the I/O address space. The system processor or a bus master is required to actually perform the data transfer.

  • The memory slave is selected via its address in the memory address space. Any master may perform the data transfer. The memory may reside on the system board or on an adapter on the Micro Channel. It can also be non-system memory (such as memory-mapped I/O) on an adapter, used for communication of information between the system and the adapter.

  • The DMA I/O slave contends for control of the Micro Channel on behalf of the DMA controller, and is thus an exception to the normal rule that slaves do not contend for control of the Micro Channel. It relies on the DMA controller to be the controlling master and manage the data transfer between the DMA slave and memory slave.

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