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

Virtual Memory System Architecture (VMSA)
Second-level descriptors
Table B3-2 shows the possible formats of a second-level descriptor, where bits [1:0] of the descriptor
identify the descriptor type:
0b00 Invalid or fault entry. The associated MVA is unmapped, and attempting to access it
generates a Translation fault. Software can use bits [31:2] of an invalid descriptor for its own
purposes, because these bits are ignored by the hardware.
0b01 Large page descriptor. Bits [31:16] of the descriptor are the base address of the Large page.
0b1X Small page descriptor. Bits [31:12] of the descriptor are the base address of the Small page.
In this descriptor format, bit [0] of the descriptor is the XN bit.
Table B3-2 VMSAv7 second-level descriptor formats
31 16 15 14 12 11 10 9 8 7 6 5 4 3 2 1 0
Fault IGNORE 0 0
Large page Large page base address, PA[31:16]
Small page Small page base address, PA[31:12]
The address information in the second-level descriptors is:
Large page Bits [31:16] of the descriptor are bits [31:16] of the physical address of the Large page.
Small page Bits [31:12] of the descriptor are bits [31:12] of the physical address of the Small page.
The other fields in the descriptors are:
XN bit The execute-never bit. Determines whether the region is executable, see The Execute Never
(XN) attribute and instruction prefetching on page B3-30.
TEX[2:0], C, B
Memory region attribute bits, see Memory region attributes on page B3-32.
AP[2], AP[1:0]
Access Permissions bits, see Memory access control on page B3-28.
AP[0] can be configured as the access flag, see The access flag on page B3-21.
S bit The Shareable bit. Determines whether the translation is for Shareable memory, see Memory
region attributes on page B3-32.
nG bit The not global bit. Used in the TLB matching process, see Global and non-global regions
in the virtual memory map on page B3-54.
B3-10 Copyright © 1996-1998, 2000, 2004-2008 ARM Limited. All rights reserved. ARM DDI 0406B
X
N
TEX
[2:0]
n
G S
n
G S
A
P
[2]
A
P
[2]
SBZ
TEX
[2:0]
AP
[1:0]
AP
[1:0]
C B 0 1
C B 1 X
N