3.3.3. Level one descriptor

The level one descriptor returned is either a section descriptor, a coarse page table descriptor, or a fine page table descriptor, or is invalid. Figure 3.4 shows the format of a level one descriptor.

Figure 3.4. Level one descriptor

A section descriptor provides the base address of a 1MB block of memory.

The page table descriptors provide the base address of a page table that contains level two descriptors. There are two sizes of page table:

Level one descriptor bit assignments are shown in Table 3.2.

Table 3.2. Level one descriptor bits

Bits

Description

Section

Coarse

Fine

31:20

31:10

31:12

These bits form the corresponding bits of the physical address

19:12

-

-

Should be zero

11:10

-

-

Access permission bits. Domain access control and Fault checking sequence show how to interpret the access permission bits

9

9

11:9

Should be zero

8:5

8:5

8:5

Domain control bits

4

4

4

Must be 1

3:2

-

-

These bits, C and B, indicate whether the area of memory mapped by this page is treated as write-back cachable, write-through cachable, noncached buffered, or noncached nonbuffered

-

3:2

3:2

Should be zero

1:0

1:0

1:0

These bits indicate the page size and validity and are interpreted as shown in Table 3.3

The two least significant bits of the level one descriptor indicate the descriptor type as shown in Table 3.3.

Table 3.3. Interpreting level one descriptor bits [1:0]

Value

Meaning

Description

0 0

Invalid

Generates a section translation fault

0 1

Coarse page table

Indicates that this is a coarse page table descriptor

1 0

Section

Indicates that this is a section descriptor

1 1

Fine page table

Indicates that this is a fine page table descriptor

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