OPERATING MODES IN ARM

 OPERATING MODES in ARM

ARM supports 7 modes of operation.

• ARM core modes of operation:

• User (usr): Normal program execution state

• FIQ (fiq): Data transfer state (fast irq, DMA-type transfer)

• IRQ (iqr): Used for general interrupt services

• Supervisor (svc): Protected mode for operating system support

• Abort mode (abt): Selected when data or instruction fetch is aborted

• System (sys): Operating system ‘privilege’-mode for user

• Undefined (und): Selected when undefined instruction is fetched

The ARM processor has several Operating Modes which are described in the table below. The User Mode has limited access to the hardware (non-privileged) whereas all other modes have full access (privileged) to the CPU resources. In an Embedded System, the Operating Modes are used to limit access of the user application to the interrupt system. On some systems there is an additional memory protection unit that limits access in the User Mode. The Mode Bits of the Program Status Register (PSR M[4:0]) contains the current Operating Mode.