MC68030 Datasheet, PDF (308/602 Pages) Motorola, Inc – ENHANCED 32-BIT MICROPROCESSOR

Memory Management Unit
DESCRIPTOR ADDRESS
This 30-bit field, which contains the physical address of a page descriptor, is only used in
short- and long-format indirect descriptors.
UNUSED
The bits in this field are not used by the MC68030 and may be used by the system soft-
ware.
RESERVED
Descriptor fields designated by a one or a zero are reserved by Motorola for future defini-
tion. These bits should be consistently written as either a one or a zero as appropriate. In
the root pointers, these bits are not alterable. In memory-resident descriptors, the values
in these fields are neither checked nor altered by the MC68030. Use of these bits by sys-
tem software for any purpose may not be supported in future products.
9.5.1.2 ROOT POINTER DESCRIPTOR. A root pointer descriptor contains the address of
the top-level pointer table of a translation tree. This type of descriptor is loaded into the CRP
and SRP registers with the PMOVE instruction. The field descriptions in the preceding
section apply to corresponding fields of the CRP and SRP with two minor exceptions. A
descriptor-type code of $00 (invalid) is not allowed; an attempt to load zero into the DT field
of the CRP or SRP register results in an MMU configuration exception. Also, when the FCL
field of the TC register is set, the L/U and LIMIT fields of the root pointer registers are
unused. Figure 9-9 shows the root pointer descriptor format.
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Figure 9-9. Root Pointer Descriptor Format
9.5.1.3 SHORT-FORMAT TABLE DESCRIPTOR. The field descriptions in 9.5.1.1
DESCRIPTOR FIELD DEFINITIONS apply to corresponding fields of this descriptor. Figure
9-10 shows the format of the short-format table descriptor.
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Figure 9-10. Short-Format Table Descriptor
9.5.1.4 LONG-FOMAT TABLE DESCRIPTOR. The field descriptions in 9.5.1.1
DESCRIPTOR FIELD DEFINITIONS apply to corresponding fields of this descriptor. During
address computations, the MC68030 internally replaces the UNUSED field with zeros.
Figure 9-11 shows the format of the long-format table descriptor.
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Figure 9-11. Long-Format Table Descriptor
9-20
MC68030 USER’S MANUAL
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