MC68030 Datasheet, PDF (312/602 Pages) Motorola, Inc – ENHANCED 32-BIT MICROPROCESSOR

Memory Management Unit
9.5.1.12 LONG-FORMAT INDIRECT DESCRIPTOR. The long-format indirect descriptor
has all the attributes of the short-format indirect descriptor described in the preceding
section. The only differences are that it is used in a page table that contains long-format
descriptors and that it has two unused fields. The field descriptions in 9.5.1.1 Descriptor
Field Definitions apply to corresponding fields of this descriptor. Figure 9-18 shows the
format of a long-format indirect descriptor.
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Figure 9-18. Long-Format Indirect Descriptor
9.5.2 General Table Search
When the ATC does not contain a descriptor for the logical address of a processor access
and a translation is required, the MC68030 searches the translation tables in memory and
obtains the physical address and status information for the page corresponding to the logical
address. When a table search is required, the CPU suspends instruction execution activity
and, at the end of a successful table search, stores the address mapping in the ATC and
retries the access. The access then results in a match (it hits) and the translated address is
transferred to the bus controller (provided no exceptions were encountered).
The table search begins by selecting the translation tree, using function code bit FC2 and
the SRE bit of the TC register, as shown in Table 9-2. SRE is set to enable the supervisor
root pointer, and FC2 is set for supervisor-level accesses. The translation tree with its root
defined by the SRP register is selected only when SRE and FC2 are both set. Otherwise,
the translation table with its root defined by the CRP register is selected. A simplified
flowchart of the table search procedure is shown in Figure 9-19.
9-24
MC68030 USER’S MANUAL
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