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AXI3 & AXI4 wrap burst length

Utkarsh Jain

Hi,

Was going through AXI spec.

As per AXI spec:

"AXI3 supports burst lengths of 1 to 16 transfers, for all burst types."

"AXI4 extends burst length support for the INCR burst type to 1 to 256 transfers. Support for all other burst types in

AXI4 remains at 1 to 16 transfers."

Also, as per AXI spec:

"for wrapping bursts, the burst length must be 2, 4, 8, or 16"

These are two contradicting lines for WRAP burst type. Can anyone confirm what's allowed specifically for AXI3 & AXI4?

Thanks

Utkarsh

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  • 0 Arm Employee Badge in reply to Utkarsh Jain

    Hi Utkarsh,

    In both AXI3 and AXI4, WRAPx bursts can only have lengths 2, 4, 8 or 16 transfers, so exactly as the later comment describing wrapping bursts states. The concept of a WRAP burst allows you to read in a cache line with the critical word first, and then fetching the remainder of the cache line after the first access. Cache lines are (usually) powers of 2 in length, hence these specific lengths.

    The earlier comment about burst lengths of 1 to 16 transfers for all burst types was trying to be generic about the burst length limitations of AXI3 before then stating what AXI4 has added.

    So FIXED bursts can have any length from 1-16 transfers in both AXI3 and AXI4.

    And WRAP bursts can have lengths 2, 4, 8 or 16 in both AXI3 and AXI4.

    But INCR bursts can have lengths 1-16 in AXI3 and 1-256 in AXI4.

    JD

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  • 0 Arm Employee Badge in reply to Utkarsh Jain

    Hi Utkarsh,

    In both AXI3 and AXI4, WRAPx bursts can only have lengths 2, 4, 8 or 16 transfers, so exactly as the later comment describing wrapping bursts states. The concept of a WRAP burst allows you to read in a cache line with the critical word first, and then fetching the remainder of the cache line after the first access. Cache lines are (usually) powers of 2 in length, hence these specific lengths.

    The earlier comment about burst lengths of 1 to 16 transfers for all burst types was trying to be generic about the burst length limitations of AXI3 before then stating what AXI4 has added.

    So FIXED bursts can have any length from 1-16 transfers in both AXI3 and AXI4.

    And WRAP bursts can have lengths 2, 4, 8 or 16 in both AXI3 and AXI4.

    But INCR bursts can have lengths 1-16 in AXI3 and 1-256 in AXI4.

    JD

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