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BedRock Cache Coherence and Memory System Guide

BedRock encompasses both the cache coherence and memory systems used in BlackParrot. The principle component of BedRock is the specification of a cache coherence protocol and its required networks, which are collectively named BedRock. The BlackParrot implementation of BedRock, called BP-BedRock specifies the network message formats and implements the required coherence system components. BP-BedRock further defines a memory interface and system that is compatible with and complementary to the coherence system and network interfaces.

BedRock Cache Coherence Protocol

BedRock defines a family of directory-based invalidate cache coherence protocols based on the standard MOESIF coherence states. Protocol variants are defined for the MI, MSI, MESI, MOSI, MOESI, MESIF, and MOESIF subsets of states. The protocol relies on a duplicate tag, fully inclusive, standalone coherence directory to precisely track the coherence state of every block cached within the coherence system. A full description of the BedRock cache coherence protocol and system is available here. This description is system-agnostic, however its design has been influenced by its implementation within BlackParrot.

BlackParrot BedRock Cache Coherence and Memory Systems

BlackParrot implements BedRock to provide cache coherence between the processor cores and coherent accelerators in a multicore BlackParrot system. This system is called BlackParrot Bedrock (BP-BedRock). BP-BedRock also defines a BedRock compatible memory interface. The text below provides a brief overview of BP-Bedrock.

BP-BedRock Network Interface Specifications

The BlackParrot BedRock Interfaces are defined in the following files:

BP-BedRock defines a common message format with a unified header and parameterizable payload. The header includes message type, operation sub-type, address, and size fields, as well as the parameterizable payload. The payload is network-specific and carries metadata required to process messages on the selected network. The current implementation defines message formats for the four BedRock coherence protocol networks and a memory command/response network (discussed in the interface_specification).

The files above are the authoritative definitions for the BP-BedRock interface implementation. In the event that the code differs from any documentation on or referenced by this page, the code shall be considered as the current and authoritative specification.

BP-BedRock Coherence Interface

The BP-BedRock coherence interface (also called the LCE-CCE interface) carries messages between the BlackParrot LCEs (cache controllers) and CCEs (coherence directories). This interface implements the four BedRock coherence networks: Request, Command, Fill, and Response. Each network utilizes the BedRock message formats. For brevity, we outline the fields that differ for each network below. Fields not listed (e.g., message size, address) have common meanings across all message types.

The Request network has the following message types and payload fields:

  • Message type
    • Read miss
    • Write miss
    • Uncached read/load
    • Uncached write/store
    • Uncached Atomic
  • Payload
    • Destination CCE
    • Requesting LCE
    • Requesting Way ID
    • Non-exclusive request hint (request block in read-only state without write permissions)

The Command network has the following message types and payload fields:

  • Message type
    • Sync
    • Invalidate
    • Set State
    • Data (cache block data, tag, and state)
    • Set State and Wakeup (cache block permission upgrade, no data)
    • Writeback
    • Set State and Writeback
    • Transfer
    • Set State and Transfer
    • Set State, Transfer, and Writeback
    • Uncached Data (uncached load request data from memory)
    • Uncached Store Done (uncached store request has been completed to memory)
  • Payload
    • Destination LCE
    • CCE sending command
    • Cache Way ID
    • Coherence State
    • Target cache, state, and way ID for cache to cache transfer

The Fill network is a special network which comprises a subset of the Command network messages. Fills can be casted safely to Cmds.

  • Message type
    • Data (cache to cache block transfer)
  • Payload
    • Destination LCE
    • CCE sending command
    • Cache Way ID
    • Coherence State
    • Target cache, state, and way ID for cache to cache transfer

The Response network has the following message types and payload fields:

  • Message type
    • Sync Ack
    • Invalidation Ack
    • Coherence Transaction Ack
    • Writeback
    • Null Writeback
  • Payload
    • Destination CCE
    • Responding LCE

Address and Data Alignment

All four LCE-CCE networks have the same address and data alignment properties. Uncached accesses are naturally aligned to the size of the request, and behavior of a misaligned request is undefined.

Cacheable accesses are block-based and support critical word first behavior. Data is returned to the cache beginning with the byte at the LCE Request address, then wrapping around at the natural cache block boundary. In other words, data is returned as found in the cache block, from LSB to MSB, but left rotated to place the requested byte at the LSB of the message data field. This behavior naturally supports networks that serialize the cache block data and send the block in multiple data beats, as well as conversion between different serialization widths without requiring re-alignment of message data.

The BlackParrot LCEs and CCEs expect that cacheable requests are issued aligned to the BedRock network data channel width (which is currently the same as the cache fill width) at the LCE.

BedRock Network Protocol

BP-BedRock defines the BedRock network protocol to exchange BedRock messages between modules. The BedRock protocol comprises the following signals:

  • header
  • data
  • valid
  • ready_and

The last signal is raised with data_valid when the last beat of the message is being sent. The width of the data channel must be a power-of-two number of bits, in the inclusive range of 64- to 1024-bits. The data channel should not be wider than the size of a cache block.

BP-BedRock Local Cache Engine (LCE) Microarchitecture

Coming Soon!

BP-BedRock Cache Coherence Engine (CCE) Microarchitecture

Refer to the BedRock Microarchitecture Guide for an overview of the cache coherence directory designs employed in BlackParrot.