3.3.8. Level two descriptor

If the level one fetch returns either a coarse page table descriptor or a fine page table descriptor, this provides the base address of the page table to be used. The page table is then accessed and a level two descriptor is returned. Figure 3.9 shows the format of level two descriptors.

Figure 3.9. Level two descriptor

A level two descriptor defines a tiny, a small, or a large page descriptor, or is invalid:

Coarse page tables provide base addresses for either small or large pages. Large page descriptors must be repeated in 16 consecutive entries. Small page descriptors must be repeated in each consecutive entry.

Fine page tables provide base addresses for large, small, or tiny pages. Large page descriptors must be repeated in 64 consecutive entries. Small page descriptors must be repeated in four consecutive entries and tiny page descriptors must be repeated in each consecutive entry.

Level two descriptor bit assignments are described in Table 3.7.

Table 3.7. Level two descriptor bits

Bits

Description

Large

Small

Tiny

31:16

31:12

31:10

These bits form the corresponding bits of the physical address

15:12

-

9:6

Should be zero

11:4

11:4

5:4

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

3:2

3:2

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

1:0

1:0

1:0

These bits indicate the page size and validity and are interpreted as shown in Table 3.8

The two least significant bits of the level two descriptor indicate the descriptor type as shown in Table 3.8.

Table 3.8. Interpreting page table entry bits [1:0]

Value

Meaning

Description

0 0

Invalid

Generates a page translation fault

0 1

Large page

Indicates that this is a 64KB page

1 0

Small page

Indicates that this is a 4KB page

1 1

Tiny page

Indicates that this is a 1KB page

Note

Tiny pages do not support subpage permissions and therefore only have one set of access permission bits.

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