7.14.4. Permission fault

There are two types of permission fault:

Permission fault is checked at the same time as Domain fault. If the 2-bit domain field returns client (01), the permission access check is invoked as follows:

section

If the Level 1 Descriptor defines a section-mapped access, the AP bits of the descriptor define whether or not the access is allowed according to Table 7.3. Their interpretation depends on the setting of the S bit (Control Register bit 8). If the access is not allowed, a Section Permission fault is generated.

sub-page

If the Level 1 Descriptor defines a page-mapped access, the Level 2 Descriptor specifies four access permission fields (ap3...ap0) each corresponding to one quarter of the page.

For small pages:

  • ap3 is selected by the top 1KB of the page

  • ap0 is selected by the bottom 1KB of the page

For large pages:

  • ap3 is selected by the top 16KB of the page

  • ap0 is selected by the bottom 16KB of the page.

The selected AP bits are then interpreted in exactly the same way as for a section (see Table 7.3), the only difference is that the fault generated is a sub-page permission fault.

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