Motorola MCF5282, MCF5281 user manual System Access Control Unit Sacu, Features

Models: MCF5282 MCF5281

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System Access Control Unit (SACU)

NOTE

The M1_PRTY field should not be set for a priority higher than third (default).

8.6System Access Control Unit (SACU)

This section details the functionality of the System Access Control Unit (SACU) which provides the mechanism needed to implement secure bus transactions to the address space mapped to the internal modules.

8.6.1Overview

The SACU supports the traditional model of two privilege levels: supervisor and user. Typically, memory references with the supervisor attribute have total accessibility to all the resources in the system, while user mode references cannot access system control and configuration registers. In many systems, the operating system executes in supervisor mode, while application software executes in user mode.

The SACU further partitions the access control functions into two parts: one control register defines the privilege level associated with each bus master, and another set of control registers define the access levels associated with the peripheral modules and the memory space.

The SACU’s programming model is physically implemented as part of the System Control Module (SCM) with the actual access control logic included as part of the arbitration controller. Each bus transaction targeted for the IPS space is first checked to see if its privilege rights allow access to the given memory space. If the privilege rights are correct, the access proceeds on the bus. If the privilege rights are insufficient for the targeted memory space, the transfer is immediately aborted and terminated with an exception, and the targeted module not accessed.

8.6.2Features

Each bus transfer can be classified by its privilege level and the reference type. The complete set of access types includes:

Supervisor instruction fetch

Supervisor operand read

Supervisor operand write

User instruction fetch

User operand read

User operand write

Instruction fetch accesses are associated with the execute attribute.

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MCF5282 User’s Manual

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Motorola MCF5282, MCF5281 user manual System Access Control Unit Sacu, Features