3.6. Domain access control

MMU accesses are primarily controlled through the use of domains. There are 16 domains and each has a 2-bit field to define access to it. Two types of user are supported, clients and managers. The domains are defined in the domain access control register. Figure 3.13 shows how the 32 bits of the register are allocated to define the 16 2-bit domains.

Figure 3.13. Domain access control register format

Table 3.10 defines how the bits within each domain are interpreted to specify the access permissions.

Table 3.10. Interpreting access control bits in domain access control register

Value

Meaning

Description

00

No access

Any access generates a domain fault

01

Client

Accesses are checked against the access permission bits in the section or page descriptor

10

Reserved

Reserved. Currently behaves like the no access mode

11

Manager

Accesses are not checked against the access permission bits so a permission fault cannot be generated

Table 3.11 shows how to interpret the Access Permission (AP) bits and how their interpretation is dependent on the S and R bits (control register bits 8 and 9).

Table 3.11. Interpreting access permission (AP) bits

AP

S

R

Supervisor

permissions

User permissions

Description

00

0

0

No access

No access

Any access generates a permission fault

00

1

0

Read-only

No access

Only Supervisor read permitted

00

0

1

Read-only

Read-only

Any write generates a permission fault

00

1

1

Reserved

-

-

01

x

x

Read/write

No access

Access allowed only in Supervisor mode

10

x

x

Read/write

Read-only

Writes in User mode cause permission fault

11

x

x

Read/write

Read/write

All access types permitted in both modes

xx

1

1

Reserved

-

-

Copyright © 2000, 2001 ARM Limited. All rights reserved.ARM DDI 0184B
Non-Confidential