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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.
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:
coarse page tables have 256 entries, splitting the 1MB that the table describes into 4KB blocks
fine page tables have 1024 entries, splitting the 1MB that the table describes into 1KB blocks.
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 |
| Copyright © 2000, 2001 ARM Limited. All rights reserved. | ARM DDI 0184B |
| Non-Confidential | |
| PDF version | |
| Home > Memory Management Unit > Address translation > Level one descriptor |