Tuesday, June 26, 2007

Sized Read And Sized Write Requests

The eight-byte sized read and sized write packets (abbreviated RdSized and WrSized in the Specification) are the mainstream commands used to perform most of the data transfers to both memory or I/O in HyperTransport. Some of the options available with sized read and write requests are:

  • Byte or dword read/write data transfers; valid data transferred ranges from 0 bytes to 64 bytes (16 dwords).

  • Posted or non-posted virtual channel for writes. Reads are always split transactions traveling in the non-posted virtual channel.

  • Isochronous posted or non-posted virtual channels for the request and any subsequent response. Isochronous flow control buffers are required to support this traffic.

  • Coherency option bit which indicates whether the transaction requires enforcement of host cache coherency. If the transaction does not target host memory, this feature does not apply.

  • Assignment of a non-zero Sequence ID attribute to requests forces other devices to maintain strict ordering for all requests from same source. A Sequence ID of 0 indicates that there is no strict ordering required.

  • Use of reserved ranges in RdSized and WrSized request packet address fields to support special-case transactions, including configuration cycles, interrupt requests, and End-Of-Interrupt (EOI) messages, etc.

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