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7: Memory Management Unit
7-8 EPSON ARM720T CORE CPU MANUAL
7.3.4 Section descriptor
A section descriptor provides the base address of a 1MB block of memory. Figure 7-5 shows the
format of a section descriptor.
Figure 7-5 Section descriptor
Section descriptor bit assignments are described in Table 7-4.
7.3.5 Coarse page table descriptor
A coarse page table descriptor provides the base address of a page table that contains level two
descriptors for either large page or small page accesses. Coarse page tables have 256 entries,
splitting the 1MB that the table describes into 4KB blocks. Figure 7-6 shows the format of a
coarse page table descriptor.
Figure 7-6 Coarse page table descriptor
Note: If a coarse page table descriptor is returned from the level one fetch, a level two
fetch is initiated.
Table 7-4 Section descriptor bits
Bits Description
31:20 Form the corresponding bits of the physical address for a section
19:12 Always written as 0
11:10 (AP) Specify the access permissions for this section
9 Always written as 0
8:5 Specify one of the 16 possible domains (held in the Domain
Access Control Register) that contain the primary access
controls
4 Should be written as 1, for backward compatibility
3:2 These bits, C and B, indicate whether the area of memory
mapped by this page is treated as cachable or noncachable, and
bufferable or nonbufferable. (The system is always
write-through.)
1:0 These bits must be b10 to indicate a section descriptor
SBZ
31 2019 12111098 543210
Section base address AP Domain 1 C B 1 0SBZ
31 1098 543210
Coarse page table base address Domain 1 0 1
SBZ
SBZ