Memory Management Unit
3-28 Copyright © 2001-2003 ARM Limited. All rights reserved. ARM DDI0198D
3.5.4 Permission faults
If the two-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 3-12 on page 3-24. Their interpretation is dependent on the setting
of the S and R bits (CP15 c1 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
to 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 3-12 on page 3-24), 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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