Motorola MCF5282 ユーザーズマニュアル

8-14

MOTOROLA

 

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

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.

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.

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.