read/write allocate

 

Certainly! Here's an explanation of write allocate and read allocate in the context of AXI4's AWCACHE and ARCACHE signals:

Write Allocate and Read Allocate in AXI4

Context

  • Both write allocate and read allocate are bits within the 4-bit AWCACHE (write address channel) and ARCACHE (read address channel) signals.
  • They control how the cache behaves when a write or read transaction occurs, specifically whether the cache allocates a cache line on that access.

1. Read Allocate

  • Definition:
    When the read allocate bit is set, it indicates that the cache should allocate a cache line on a read miss.

  • Behavior:

    • If the requested data is not present in the cache (a cache miss), the cache will fetch the data from memory and allocate a new cache line to store it.
    • This improves performance for subsequent accesses to the same data.

  • If Read Allocate is cleared:

    • The cache does not allocate a new cache line on a read miss.
    • The data is fetched and returned to the master but not cached.
    • Useful for streaming or non-reusable data.

2. Write Allocate

  • Definition:
    When the write allocate bit is set, it indicates that the cache should allocate a cache line on a write miss.

  • Behavior:

    • If the cache line corresponding to the write address is not present in the cache (write miss), the cache will allocate a new cache line.
    • This usually involves fetching the existing data from memory (read-for-ownership) before updating it.
    • This allows the cache to hold the updated data and maintain coherency.

  • If Write Allocate is cleared:

    • The cache does not allocate a cache line on a write miss.
    • The write is typically passed directly to memory (write-no-allocate or write-through).
    • Useful for write-only or streaming data where caching the data is not beneficial.

Summary Table

Allocate TypeBit in AWCACHE/ARCACHEMeaning when SetMeaning when Cleared
Read AllocateBit 1Allocate cache line on read missDo not allocate cache line on read miss
Write AllocateBit 0Allocate cache line on write missDo not allocate cache line on write miss

Example

  • AWCACHE = 0b0011 (Bufferable=0, Cacheable=0, Read Allocate=1, Write Allocate=1)
    Cache allocates lines on both read and write misses.

  • AWCACHE = 0b0010 (Read Allocate=1, Write Allocate=0)
    Cache allocates lines on read misses only, not on write misses.

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