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By: Abhishek Pande
Submitted to: Prof. V
Processor modes refer to the various ways that
the processor creates an operating environment for itself.
Specifically, the processor mode controls how
the processor sees and manages the system memory
and the tasks that use it.
In the old days, you had a processor and it executed
instructions. When an interrupt occured, the processor
would save its current state and then branch to a specific
place in order to service the interrupt. Thus, essentially,
the processor had just two 'modes' - dealing with an
interrupt, and not.
But now the processors are much more capable and
ARM processors support different processor modes,
depending on the architecture version:-
Processor mode Architectures Mode number
User All 0b10000
FIQ - Fast
IRQ - Interrupt
Supervisor All 0b10011
Abort All 0b10111
Undefined All 0b11011
• User mode: It is the usual ARM program execution
state, and is used for executing most application
• Fast Interrupt (FIQ): This mode supports a data
transfer or channel process.
• Interrupt (IRQ) mode is used for general-purpose
• Supervisor mode is a protected mode for the operating
• Abort mode is entered after a data or instruction
• System mode is a privileged user mode for the
• Undefined mode is entered when an undefined
instruction is executed.
Mode Mode identifier
Fast interrupt fiq
Table : Register mode identifiers
• It is the unprivileged mode under which most tasks run.
Unprivileged mode means that it doesn’t have access to
the full system resources and cannot change the mode
• It has access to the base register set i.e. at any time all
the 16 registers from R0 to R15(pc) can be accessed.
• CPSR (Current Program Status Register) is a 32-bit
wide register used in the ARM architecture to record
various pieces of information and flags regarding the
state of the program being executed by the processor
and the state of the processor. This register’s values can
be read from the User mode.
• Every processor mode except user mode can change
mode by writing directly to the mode bits of the CPSR.
• A banked register maps one-to-one onto a user mode
• SPSR (Saved Program Status Register) is not available
in this mode.
• It is a privileged mode unlike the User mode but uses
the same set of registers as the User mode.
• All regular application tasks can be performed in this
mode. Also all the files except the kernel files can be
accessed through this mode. Both R/W is possible.
• We can only enter System mode from another
privileged mode by modifying the mode bit of
the Current Program Status Register (CPSR) and it
cannot be entered by an exception.
• System mode doesn’t have a set of associated banked
registers which all the other privileged modes have.
• A problem with the original design of the ARM is that as
processor vectors modify R14 with the return address, an
exception handler (for example, IRQ) that calls
subroutines cannot act in a re-entrant way; for if the IRQ
handler is taken while in the IRQ handler and having
called a subroutine to handle the (first) IRQ, the return
address in R14 will be trashed by the second IRQ
exception. Ways around this involve clever coding to
avoid an exception of the same type occurring, or to
avoid taking subroutine calls, or switching to USR mode.
Or in the case of interrupts, disabling them completely
during the interrupt process.
• But as of ARMv4, an alternative is proposed viz.
the System mode. It is like a cross between SVC and
USR. System mode offers the privileges of SVC mode,
however it uses the USR registers. An exception handler
can safely deal with R14 and CPSR and so re-entrancy is
possible. This deals with the corruption of the link
• This is a privileged mode which can be entered by
pressing RESET when a software interrupt instruction is
• Being in this mode we can breach the kernel files.
Updating these kernel files and even modifying them is
also possible but the warranty of the OS is rendered void
if it’s done.
• For e.g: Modifying the kernel files to install a new OS in
devices such as cell phones is done by entering into this
mode. This process is more popularly called “rooting” for
Android OS wherein we can access various privileged
• It’s basically a protected mode for the Operating
System which is uniquely reserved for it.
• In this mode R13(sp), R14(lr) and CPSR registers are
• OS calls the SWI to enter into SVC mode and then
processor jumps to &FFFF0008 location. After
subsystem reset, the ARM begins processing at address
&FFFF0000(for high vector config.) viz. the reset vector
address with interrupts disabled.
• To handle the problem of link register corruption, Linux
kernel does it this way: whenever any interrupt occurs in
• When power is supplied to the core, it starts in the SVC
• A Software Interrupt (SWI) exception occurs when the
SWI instruction is executed and none of the other higher-
priority exceptions have been ﬂagged. On entry to the
handler, the CPSR will be automatically set to the
• FIQ or Fast Interrupt mode is a privileged mode which
can be entered when a high priority interrupt is raised.
• This mode is useful for digital data processors that have
the ability to handle multiple interrupts.
• When a fast interrupt request is received a flag is set
and the program counter(pc or R15) and condition code
registers are stored on a stack.
• FIQ is just a higher priority interrupt request, that is
prioritized by disabling IRQ and other FIQ handlers during
request servicing. Therefore, no other interrupts can
occur during the processing of the active FIQ interrupt.
• At the end of the ISR the return from interrupt
instructions retrieves the condition code register which
contains the status of the digital data processor and
checks to see whether the flag has been set or not. If the
flag is set it indicates that a fast interrupt was serviced
and therefore only the program counter(R15) is
• It is based on the same concept of a two-level interrupt
system where a more important interrupt can interrupt an
• FIQ mode provides a large number of banked registers
(R8 to R14, CPSR) and is useful for things that must
• The original (8MHz) ARM used FIQ for networking and
floppy disc which had to be serviced as soon as data
was available. Modern ARMs would probably use FIQ
for high speed DMA-style transfers.
• CPSR bit 6/F controls the masking of FIQ.
• FIQ vectors are similar to IRQ vectors but they are
reserved for hardware requiring faster response times.
• IRQ or Interrupt mode is a privileged mode which can
be entered when a low priority interrupt is raised.
• This is the other, regular, interrupt mode. Only R13,
R14, and CPSR registers are banked.
• Since IRQ has a lower priority than FIQ. This means
that when the core takes an FIQ exception, it
automatically masks out IRQs. An IRQ cannot interrupt
the FIQ handler.
• All interrupts that don't require extreme speed (clock
ticks, screen VSync, keyboard, etc...) will use IRQ
• When the processor is in the IRQ mode, the instructions
you execute still access registers R13 and R14. However,
these registers are the banked registers r13_irq and
r14_irq. The user mode registers r13_usr and r14_usr are
not affected by the instructions referencing these registers.
The program still has normal access to other registers R0
• The following figure shows mode change from User to
IRQ when an interrupt request occurs due to an external
device raising interrupt to the processor core. This change
causes user registers R13 and R14 to be banked. The
user registers are replaced with registers r13_irq and
r14_irq, respectively. Note r14_irq contains the return
• The fig also shows a new register appearing in interrupt
request mode: the Saved Program Status Register
(SPSR), which stores the previous mode’s CPSR. You
can see in the diagram the cpsr being copied into
• CPSR bit 7/I controls the IRQ masking.
• So why do many systems use IRQ and not FIQ?
It’s because all of the interrupt controller hardware is
typically on the IRQ pin and using FIQ only makes sense
if you have a single highest priority interrupt source
connected to the nFIQ input and many systems do not
• This privileged mode is used to handle memory
• An abort is signalled by the memory system as a result
of a failure to load either an instruction (Prefetch Abort)
or data (Data abort).
• A Prefetch Abort occurs if the processor attempts
to execute a failed instruction load (note - no abort
happens if the processor fails to load an instruction, but
said instruction is not executed due to a branch or
In ARMv5 a Prefetch Abort can be generated
• A Data Abort occurs if the processor attempts to fetch
data but the memory system says it is unable to due to
incorrect access permissions. The abort occurs before
the failed instruction alters the processor state.
• In both cases, interrupts are disabled and the branch
• When we acess data through protected memory, we
can go into Abort mode.
• We can reserve any amount of data in protected
memory but it’s in ‘read only’ form so if a user tries to
write data into the protected memory, the system goes
• It’s not a working mode as such but a warning mode
which if triggered, the processor cannot perform any other
• In general, a processor enters abort mode when there is
a failed attempt to access memory.
• This mode has R13, R14 and SPSR as banked registers.
• It’s underlying mechanism is that whenever an abort
signal is routed to the processor core, it responds to this
signal by taking an exception and vectoring to the abort
handler. The abort handler then determines the abort type
as either a prefetch or a data abort, and based on the
• It’s a privileged mode and not a working mode as such.
It’s also an error mode.
• When the processor tries to execute an invalid or
undefined instruction, it automatically goes to the
• An Undefined instruction vector is used when the
processor cannot decode an instruction. Since this is an
ARM exception so instead of loading regular
instructions, some special instructions are loaded from
the exception vector table. Each vector table entry
contains a form of branch instruction pointing to the start
• When an undefined instruction is encountered, the
ARM will wait for a coprocessor to acknowledge that it
can deal with the instruction (if in co-processor
instruction space). If no coprocessor responds, or the
instruction is one that is not defined, then the undefined
instruction vector is taken. This will branch to
&FFFF0004 address to allow such things as software
emulation of coprocessors, or other extensions to the
• An Undeﬁned Instruction exception occurs when an
instruction not in the ARM or Thumb instruction set
reaches the execute stage of the pipeline and none of
• Since coprocessors follow the pipeline, instruction
identiﬁcation can take place in the execute stage of the
core. If none of the coprocessors claims the
instruction, an Undeﬁned Instruction exception is
• Both the SWI instruction and Undeﬁned instruction
have the same level of priority.
• If this mode is triggered, then the processing is
stalled like the Abort mode.
• Unlike the yesteryears processors, today’s competetive
market requires higher level of functionality and much
greater freedom to the developer or even the end user to
use a system in his/her on personalised manner.
• Multitasking or the ability to run various programs at
the same time is of great importance today.
• Moreover these multiple modes provide some
banked registers. These extra registers allow us
to write much less complicated exception routines.
Without them the function return address in LR
register would blow up everytime an interrupt
• With these extra registers at our disposal, we have the
option of not having to save and restore more processor
context in software which in turn speeds up the interrupt
• Furthermore, with such high level of complexity in the
instuction set used, it should be made sure that none of
the mere programs should be able to mess around with
the OS or the machine's hardware. This is managed, in
by the use of an MMU or other memory
management system, and in part by the use
of privilege. It is the various processor modes