ARM processors are widely used in consumer electronics, especially smartphones, forming the backbone of over 95% of devices. Originally designed in the 1980s, ARM has evolved to provide architectures that prioritize low power consumption while supporting high performance, especially for mobile and embedded systems. The architecture features novel elements like a load-store design and conditional execution, establishing it as a leading choice for various applications.

1. ARM PROCESSORS-I

2. Why ARM?
• ARM processors have extensive applications in
consumer electronic devices such as smart phones,
tablets, multimedia players and other mobile
devices, such as wearables .
• Arm remains at the center of the smartphone
evolution, with more than 95 percent of the world’s
smartphones built on ARM’s IP.
• Arm’s IP designs for smartphones enable the most
popular ML-based real-world applications.
• ARMv7 and ARMv8 are the most recent and
popular today.

3. contd
• ARM allowed smartphone manufacturers like Qualcomm ,
Nvidia and Apple etc. to design their own CPU cores .
• For ex: The very recent mobile phone Redmi 9 is powered
by Octa core (2 GHz, Dual core, ARM Cortex A75 + 2
GHz, Hexa Core, ARM Cortex A55) processor.
• Similarly the Oppo Reno 2F is powered by Octa
core(4x2.1G.Hz ARM Cortex-A73 &4X2.0 G.Hz ARM
Cortex A53.
• Recently Apple announced its first homegrown ARM
CPU intended for desktop and laptop computing
applications.

4. PROLOGUE
• The ARM Processor was originally designed at
Acorn Computers Limited of Cambridge, England,
during the years 1983-1985.
• It was the first RISC microprocessor developed for
commercial use and has some significant
differences from subsequent RISC architectures.
• 1990 ARM Limited was established as a separate
company specifically to widen the exploitation of
ARM technology and it is established as a market-
leader for low-power and cost-sensitive embedded
applications.
• Later it was named as ARM Holdings.

5. contd
• In fact, ARM does not fab the microprocessors. It is
an IP(intellectual property) company that design
systems and give licenses to other companies like,
Intel, Texas Instruments, Samsung etc. purchase
licenses and fabricate the chips.
• The basic reason behind the origin of ARM
processor was, the 16-bit CISC microprocessors
(mostly Intel) that were available during 1983 were
slower than standard memory parts.
• They also had instructions that took many clock
cycles to complete (in some cases, many hundreds
of clock cycles), resulting very long interrupt
latencies.

6. contd
• As a result of these limitations with the commercial
microprocessor offerings, the design of a proprietary
microprocessor was considered & hence ARM chip
was emerged.
• The ARM processor is supported by a toolkit which
includes an instruction-set emulator for hardware
modeling and software testing and benchmarking, an
assembler, C and C++ compilers, a linker and a
symbolic debugger.

7. contd
• So, ARM is not a Fab company, it only gives
licenses to companies which manufacture ARM
based CPUs or System On Chip products.
• The architecture license enables the licensee to
develop their own processors compliant with ARM
ISA.
• To understand the architectural features of ARM
processors, ARM7TDMI which is popularly known
as ARM7 is considered as an example.

8. ARM FAMILY
• Here an interesting thing s to be observed.ARM 7
processor is the family name not the version.AS
shown below ARM7TDMI belongs to ARM v4.
• The version and family
don't sync.
Another interesting thing
is after ARM 11 a new
series of processors by
name Cortex was started.

9. contd
• These Cortex processors are of three types.Cortex-A,
Cortex-M and Cortex-R.

10. contd
• Cortex-A means Cortex Application .This finds
applications in mobile phones as SOC.
• Cortex M means Cortex Micro-controller. For Ex:
The low power Cortex M0 is used in Arduino Nano
33 IoT board.
• Also Raspberry Pi board is powered by ARM 11
processor
• Cortex –R denotes Cortex Real time which are
optimized for hard real-time and safety-
critical applications.

11. ARM 7 TDMI-S
• The ARM7TDMI was introduced in 1995 by ARM
• The ARM7TDMI-S processor is a member of the
ARM family of general-purpose 32-bit
microprocessors.
• T: THUMBMODE(16 bit instruction support).
• D for on-chip Debug support, enabling the
processor to halt in response to a debug request.
• M: enhanced(fast) Multiplier, yield a full 64-bit
result, high performance.
• I: Embedded ICE hardware (In Circuit
emulator).The Embedded ICE macro cell consists of
on-chip logic to support debug operations.

12. contd
• S: Synthesizable. What is its meaning?
• In early days ARMs were designed as a hard macro,
i.e the physical design at the transistor layout level,
and the fab companies were taking this fixed
physical block and used to place it into their chip
designs.
• But due to the complexities, a demand increased for
a more flexible and configurable solution, hence
ARM decided to deliver processor designs as a
behavioral description at the "register transfer level"
(RTL) written in a hardware description language
(HDL), typically Verilog HDL.

13. contd
• The process of converting this behavioral
description into a physical network of logic gates is
called "synthesis", and several major EDA
companies sell automated synthesis tools for this
purpose.
• A processor design distributed to licensees as an
RTL description (such as ARM7TDMI-S) is
therefore described as "synthesizable“.

14. Designer of ARM PROCESSOR
• Sophie Wilson in 1980s, with his colleague Steve Furber
(Author of a very popular the book on ARM” ARM System-
on-Chip Architecture ”) designed the ARM architecture, a
new CPU design that made mobile computing possible.
• The low-power architecture has become the dominant
architecture in mobile devices as smartphones and tablets,
and has become a significant player in the embedded system
space
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15. Novel Features
• ARM has many special features that made which
make them most suitable in Embedded Design.
• What is the reason for its High Performance?
• The performance of a processor is mainly decided
by two factors.
• One is clock frequency(operational frequency ) and
the other is the architecture.
• By increasing the clock frequency the performance
can be improved , but it has a limitation also.
• Because, with increase of clock frequency the
power dissipation also increases.(Power dissipation
being proportional to operating frequency)

16. ARM Architecture
• So, the designers opted a novel design architectures
which reduce the power dissipation and enhance the
performance.
• ARM is basically a RISC architecture processor
which incorporated a number of features from the
Berkeley RISC design.
• A load-store architecture: Only Load /Store
instructions access memory.
• Fixed-length 32-bit instructions: All instructions
have only a fixed length of 32 bits.
• An orthogonal instruction set.

17. contd
(i).Conditional execution of Instructions:
All instructions are conditionally executed. i.e an
instruction is executed only if the current values of
the condition code flags .
Ex: ADDNE r1, r2, r3 i.e Add the registers r2 and r3,
if they are not equal and keep the result in the register
r1.
If the condition is not satisfied ,the instruction acts as
a NOP.

18. contd
• As RISC architecture is known for rich register
set, the ARM processor contains nearly 32
registers of width 32 bits.
• Most of the instructions are register based .So, the
time of execution is minimum.
• As you know the accessing time for memory is
large when compared to registers it will reduce the
time of execution and also the power dissipation .
• The order of execution times is
Hard disc ----milli secs
Primary memory (like SRAM)…. micro secs
Register file …..nano secs.

19. contd
• ARM was one of the first architectures to implement
load-store multiple instructions.
• These can transfer multiple registers between memory
and processor in a single instruction.
• ARM processor include an inline barrel shifter to pre-
process one of the input registers.
• This barrel shifter helps in executing arithmetic
instructions like multiplication and multiply
accumulate (MAC)etc .
• The simplicity in architecture reduces the overhead on
each instruction allowing the clock cycles to be
shortened.

20. 19-07-2020 yayavaram@yahoo.com 20