ARM Architecture Reference Manual ARMv7-A and ARMv7-R edition

B2.4.8 Access permission checking
Common Memory System Architecture Features
The function CheckPermission() is used by both the VMSA and PMSA architectures to perform access
permission checking based on attributes derived from the translation tables or region descriptors. The domain
and sectionnotpage arguments are only relevant for the VMSA architecture.
The interpretation of the access permissions is shown in:
Access permissions on page B3-28, for a VMSA implementation
Access permissions on page B4-9, for a PMSA implementation.
The following pseudocode describes the checking of the access permission:
// CheckPermission()
// =================
CheckPermission(Permissions perms, bits(32) mva,
boolean sectionnotpage, bits(4) domain, boolean iswrite, boolean ispriv)
if SCTLR.AFE == ‘0’ then
perms.ap = ‘1’;
case perms.ap of
when ‘000’ abort = TRUE;
when ‘001’ abort = !ispriv;
when ‘010’ abort = !ispriv && iswrite;
when ‘011’ abort = FALSE;
when ‘100’ UNPREDICTABLE;
when ‘101’ abort = !ispriv || iswrite;
when ‘110’ abort = iswrite;
when ‘111’
if MemorySystemArchitecture() == MemArch_VMSA then
abort = iswrite
else
UNPREDICTABLE;
if abort then
DataAbort(mva, domain, sectionnotpage, iswrite, DAbort_Permission);
return;
B2.4.9 Default memory access decode
The function DefaultTEXDecode() is used by both the VMSA and PMSA architectures to decode the texcb
and S attributes derived from the translation tables or region descriptors.
The interpretation of the arguments is shown in:
C, B, and TEX[2:0] encodings without TEX remap on page B3-33, for a VMSA implementation
C, B, and TEX[2:0] encodings on page B4-11, for a PMSA implementation.
The following pseudocode describes the default memory access decoding, when memory region remapping
is not implemented:
ARM DDI 0406B Copyright © 1996-1998, 2000, 2004-2008 ARM Limited. All rights reserved. B2-37