7.2.2. RAM protection summary

Table 7.1 shows how the different types of RAM are protected.

Table 7.1. RAM protection summary

RAM typeProtectionParity/ECC chunkCorrectable errorFatal errorHard error support
Data tag RAMSEC-DED ECC

34 bits:

  • 25 bits for tag and status.

  • 9 bits for index.

Error seen as a single bit errorError seen as a multiple bit errorUp to three hard errors, including the SCU hard errors
Data data RAMSEC-DED ECC32-bit data wordError seen as a single bit errorError seen as a multiple bit error on dirty lines
Instruction tag RAMSEC-DED ECC

28 bits:

  • 23 bits for tag.

  • 5 bits for index.

Any error, single or double, on the tag or valid stored in the RAMNoneUp to three hard errors
Instruction data RAMSEC-DED ECC[a] 64-bit data wordAny error on the data stored in the RAMNone
BTACParity[b]8-bit--None
PREDParity[b] 1-bit (copy)--None
SCU tag RAMSEC-DED ECC

28 bits:

  • 23 bits for tag.

  • 5 bits for index.

Any errorNoneUp to two hard errors
Data TCMSEC-DED ECC32-bit data wordError seen as a single bit errorError seen as a multiple bit errorUp to one hard error
Instruction TCMSEC-DED ECC64-bit wordError seen as a single bit errorError seen as a multiple bit errorUp to one hard error

[a] The SEC-DED ECC is used as a Double Error Correction because the lines are clean.

[b] BTAC and PRED do not prevent the system from operating correctly and only impact the performance, even if hard errors occur. Parity provides a compromise between the area overhead in the RAMs and the ability to detect errors.


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