System Control Module (SCM)
MCF52235 ColdFire® Integrated Microcontroller Reference Manual, Rev. 6
Freescale Semiconductor
13-13
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 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 is not accessed.
13.7.2
Features
Each bus transfer can be classified by its privilege level and the reference type. The complete set of access
types includes the following:
•
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.
It should be noted that while the bus does not implement the concept of reference type (code versus data)
and only supports the user/supervisor privilege level, the reference type attribute is supported by the
system bus. Accordingly, the access checking associated with privilege level and reference type is
performed in the IPS controller using the attributes associated with the reference from the system bus.
The SACU partitions the access control mechanisms into three distinct functions:
•
Master privilege register (MPR)
— Allows each bus master to be assigned a privilege level:
– Disable the master’s user/supervisor attribute and force to user mode access
– Enable the master’s user/supervisor attribute
— The reset state provides supervisor privilege to the processor core (bus master 0).
— Input signals allow the non-core bus masters to have their user/supervisor attribute enabled at
reset. This is intended to support the concept of a trusted bus master, and also controls the
ability of a bus master to modify the register state of any of the SACU control registers; that is,
only trusted masters can modify the control registers.
•
Peripheral access control registers (PACRs)
— Provide read/write access rights, supervisor/user privilege levels.
— Reset state provides supervisor-only read/write access to these modules.
— Nine 8-bit registers control access to 17 of the on-chip peripheral modules
•
Grouped peripheral access control registers (GPACR0, GPACR1)
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