To Next Page

To Previous Page

Memory Management Unit

ID012310 Non-Confidential, Unrestricted Access

Note

The use of the S and R bits is deprecated.

Table 6-1 lists the encoding of the access permission bits.

Restricted access permissions and the access bit

The Access bit is an ARMv6 enhancement, for full details see Access bit fault on page 6-32.

Some OSs only use a restricted set of the access permissions:

• APX and AP[1:0] = b111, Read-Only for both Privileged and Unprivileged code

• APX and AP[1:0] = b011, Read-Write for both Privileged and Unprivileged code

• APX and AP[1:0] = b101, Read-Only for Privileged code, No Access for Unprivileged

• APX and AP[1:0] = b001, Read-Write for Privileged code, No Access for Unprivileged.

For such OSs the encoding of the Read-Only or Read-Write and the User or Kernel access

permissions are orthogonal:

• APX selects the Read-Only or Read-Write permission

• AP[1] selects the User or Kernel access.

In this case, the AP[0] bit provides Access bit information so that software can optimize the

memory management algorithm.

The Access bit behaves in this way except in the deprecated case that uses the S and R bits, that

is when the S and R bits have opposite values, and when APX and AP[1:0] = b000.

6.5.3 Execute never bits in the TLB entry

Each memory region can be tagged as not containing executable code. If the Execute Never, XN,

bit of the TLB entry is set to 1, then any attempt to execute an instruction in that region results

in a permission fault. If the XN bit is cleared, then code can execute from that memory region.

When the MMU is in ARMv5 mode, see the XP bit in c1, Control Register on page 3-44, the

Table 6-1 Access permission bit encoding

APX AP[1:0] Privileged permissions User permissions

0 b00 No access, recommended use.

Read-only when S=1 and R=0 or when S=0 and R=1,

deprecated.

No access, recommended use.

Read-only when S=0 and R=1, deprecated.

0 b01 Read/write. No access.

0 b10 Read/write. Read-only.

0 b11 Read/write. Read/write.

1 b00 Reserved. Reserved.

1 b01 Read-only. No access.

1 b10 Read-only. Read-only.

1 b11 Read-only. Read-only.

ARM ARM1176JZF-S User Manual

Questions and Answers: