
10-2
ColdFire CF4E Core User’s Manual
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Virtual Memory Management Architecture
read, write, and execute permission checking on a per-memory request basis. The optional
MMU is placed with a software-controlled TLB and associated logic in the core at the
K-Bus level of the ColdFire memory/bus hierarchy. Other changes better isolate supervisor
and user modes and make core access error exceptions precise.
The architecture defines the MMU TLB, associated control logic, TLB hit/miss logic,
address translation based on the TLB contents, and access faults due to TLB misses and
access violations. It intentionally leaves some virtual environment details undefined to
maximize the software-defined flexibility. These include the exact structure of the
memory-resident pointer descriptor/page descriptor tables, the base registers for these
tables, the exact information stored in the tables, the methodology (if any) for maintenance
of access, and written information on a per-page basis.
10.2.1 MMU Architecture Features
To add optional virtual addressing support, demand-page support, permission checking,
and hardware address translation acceleration to the ColdFire architecture, the MMU
architecture features the following:
Addresses from the core to the K-Bus are treated as physical or virtual addresses.
The address access control logic, address attribute logic, K-Bus memories, and
K-Bus to M-Bus controller function as in previous ColdFire versions with the
addition of the MMU. The MMU, its TLB, and associated control reside in the
K-Bus logic.
The MMU appears as a memory-mapped device in the K-Bus space. Information for
access error fault processing is stored in the MMU.
A precise K-Bus fault (transfer error acknowledge) signals the core on translation
(TLB miss) and access faults. The core supports an instruction restart model for this
fault class. Note that this structure uses the existing ColdFire access error fault
vector and needs no new ColdFire exception stack frames.
The following additions are made to the K-Bus memory access control to better
support the fault processing and memory maintenance necessary for this virtual
addressing environment. These additions improve K-Bus memory performance and
functionality for physical and virtual address environments:
— New supervisor-protect bits to the access control registers (ACRs) and the cache
control register (CACR)
— Improved addressing of the ACRs
10.2.2 MMU Architectural Location
Figure 10-1 shows the placement of the MMU/TLB hardware. It follows a traditional
model in which it is closely coupled to the processor local-memory controllers.
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