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3.2.1. The AMBA3 AXI system bus

The most significant feature of the extended bus design is the addition of an extra control signal for each of the read and write channels on the main system bus. These bits are known as the Non-Secure, or NS bits, and are defined in the public AMBA3 Advanced eXtensble Interface (AXI) bus protocol specification.

  • AWPROT[1]: Write transaction - low is Secure and high is Non-secure.

  • ARPROT[1]: Read transaction - low is Secure and high is Non-secure.

All bus masters set these signals when they make a new transaction, and the bus or slave decode logic must interpret them to ensure that the required security separation is not violated. All Non-secure masters must have their NS bits set high in the hardware, which makes it impossible for them to access Secure slaves. The address decode for the access will not match any Secure slave and the transaction will fail.

If a Non-secure master attempts to access a Secure slave it is implementation defined whether the operation fails silently or generates an error. An error may be raised by the slave or the bus, depending on the hardware peripheral design and bus configuration, consequently a SLVERR (slave error) or a DECERR (decode error) may occur.

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