3.7.4. Permission fault

If the 2-bit domain field returns 01 (client) then access permissions are checked as follows:

Section

If the level one descriptor defines a section-mapped access, the AP bits of the descriptor define whether or not the access is allowed, according to Table 3.11. Their interpretation is dependent on the setting of the S and R bits (control register bits 8 and 9). If the access is not allowed, a section permission fault is generated.

Large page or small page

If the level one descriptor defines a page-mapped access and the level two descriptor is for a large or small page, four access permission fields (ap3-ap0) are specified, each corresponding to one quarter of the page. For small pages ap3 is selected by the top 1KB of the page and ap0 is selected by the bottom 1KB of the page. For large pages, ap3 is selected by the top 16KB of the page and 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 3.11). The only difference is that the fault generated is a page permission fault.

Tiny page

If the level one descriptor defines a page-mapped access and the level two descriptor is for a tiny page, the AP bits of the level one descriptor define whether or not the access is allowed in the same way as for a section. The fault generated is a page permission fault.

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