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

Notes on the ARM processor modes
The System Level Programmers’ Model
User mode User mode enables the operating system to restrict the use of system resources. Application
programs normally execute in User mode. In User mode, the program being executed:
cannot access protected system resources
cannot change mode except by causing an exception, see Exceptions on page B1-30.
Privileged modes
Exception modes
The modes other than User mode are known as privileged modes. In their security state they
have full access to system resources and can change mode freely.
The exception modes are:
FIQ mode
IRQ mode
Supervisor mode
Abort mode
Undefined mode
Monitor mode.
Each of these modes normally handles the corresponding exceptions, as shown in
Table B1-1 on page B1-6.
Each exception mode has some banked registers to avoid corrupting the registers of the
mode in use when the exception is taken, see ARM core registers on page B1-9.
System mode System mode has the same registers available as User mode, and is not entered by any
exception.
System mode is intended for use by operating system tasks that must access system
resources, but do not want to use the exception entry mechanism and the associated
additional registers. Also, it is used when the operating system has to access the User mode
registers.
Monitor mode
Monitor mode is only implemented as part of the Security Extensions, and is always in the
Secure state, regardless of the value of the SCR.NS bit. For more information, see The
Security Extensions on page B1-25.
Code running in Monitor mode has access to both the Secure and Non-secure copies of
system registers. This means Monitor mode provides the normal method of changing
between the Secure and Non-secure security states.
ARM DDI 0406B Copyright © 1996-1998, 2000, 2004-2008 ARM Limited. All rights reserved. B1-7