Introduction to ARM
The world’s most popular 32-bit Embedded Processor
Dr. Bore Gowda S B
Associate Professor - Senior
Reference Books
1. Hohl W., ARM Assembly Language Fundamentals and
Techniques, CRC press, 2009
2. Sloss A.N., ARM System Developer’s Guide, Designing
and Optimizing System Software, Elsevier, 2004
3. Steve Furber., “ARM System-on-Chip Architecture”,
Second Edition, PEARSON EDUCATION, 2000
4. Gibson J. R., ARM Assembly Language-an Introduction,
Dept. of Electrical Engineering and Electronics, The
University of Liverpool, 2007.
5. Reay D., Digital Signal Processing and Applications with
the OMAP-L138 Experimenter, Wiley.
6.
ARM CORTEX MICROCONTROLLER DATA SHEET
History of ARM
In 1985, Acorn Computers Ltd. was in search of a new processor to
put up in the desktop. While the technocrats were contemplating
various design options, they came across a few papers published by a
set of students in the University of Berkely(USA) outlining a very
simple processor design based on RISC principles.
The computer architects of Acorn Computers found the design very
attractive and decided to build a new processor using some of these
principles.
This led to the development of ARM1, which had less than 25,000
transistors, and operated at 6MHz.
This was followed by ARM2(in 1987) with 30,000 transistors.
Comparing this to an Intel/Motorola’s processor of that time having
70,000 transistors, this was a beauty in terms of a smaller die size and
lower power dissipation.
History of ARM (contd…)
ARM2 was the first ARM processor which was produced in bulk
– It had a 32-bit bus, a 26-bit address space and sixteen 32-bit registers
– was clocked at 8 to 12 MHz.
– It dissipated much less power, and performed much better than Intel’s 80286
which came up around the same time (but focused on the desktop market).
ARM3, ARM4, ARM5 were also designed, but never produced,
because around this time, in 1990, Acorn Computers teamed up with
Apple Computers and VLSI Technology group to form a company
named Advanced RISC Machines Ltd.
This company continued with ARM6, ARM7, etc. The latter was the
processor which became very popular and led to ARM being used in
exotic products such as mobile phones, PDAs, Ipods, computer
harddisks, etc.
History of ARM (contd…)
As of 2011, ARM processors account for approximately 90 percent of
all embedded 32-bit RISC processors.
ARM Applications : PDAs, mobile phones, digital media and music
players, handheld game consoles, calculators and computer
peripherals such as hard drives and routers etc.
The subsequent and more advanced processors of the ARM family ARM9, ARM10, ARM11, Cortex
These have been built on the success of the ARM7 processor, which is
still the most popular and widely used member of the ARM family.
Over the years, many advanced features have been added to the
ARM processor, but the core has remained more or less the same.
The ARM Core
Core : It is the ‘processing unit’ or the ‘computing engine’ which has
all the computing power, and this aspect is decided by the
architecture, which represents the basic design of the processor.
Special and unique feature - ARM as a company it designs the core
and licenses this IP (Intellectual Property) to others. This simply
means that the company does not ‘fabricate’ the chip, but sells only
the design.
This design is taken by the licensee, who may or may not add more
features (usually peripherals) to the design.
Sometimes the buyer can also modify the basic design to a minor
extent
The buyer company fabricates the design and sells it/uses it for its
products.
The ARM Core (contd..)
Various ways ARM sells its IP in the form of :
1. Soft IP: In this case, the design is sold as RTL (VHDL/Verilog
code), and this allows the buyer to modify the design to a certain
extent.
2. Hard IP: It means the buyer gets only the layout or the netlist
(connection of nets or electronic wires). Thus the buyer can add
only peripherals to the ‘black box’ design he has purchased.
We can thus understand that ARM the company does not ‘fabricate’
ARM chips. (In contrast, Intel fabricates its processors and sells them
as chips.)
It is because of this, that we have ARM chips and boards of various
companies – Samsung, Philips, Atmel, Texas Instruments, ST
Microelectronics and so on – the list is very long.
The RISC Design philosophy
ARM core uses a RISC architecture
RISC is a design philosophy aimed at delivering simple but powerful
instructions that execute within a single cycle at a high clock speed.
RISC philosophy concentrates on reducing the complexity of
instructions performed by the hardware because it is easier to provide
greater flexibility and intelligence in software than hardware.
In contrast, the traditional complex instruction set computer (CISC)
relies more on the hardware for instruction functionality, and
consequently the CISC instructions are more complicated.
The RISC Design philosophy (contd..)
The RISC Philosophy is implemented with four major design rules:
1. Instructions
– RISC processors have a reduced number of instruction classes.
These classes provide simple operations that can each execute in a
single cycle.
– The compiler or programmer synthesizes complicated operations
by combining several simple instructions.
– Each instruction is a fixed length to allow the pipeline to fetch
future instructions before decoding the current instruction.
– In contrast, in CISC processors the instructions are often of
variable size and take many cycles to execute.
2. Pipelines
– The processing of instructions is broken down into smaller units
that can be executed parallel by pipelines.
The RISC Design philosophy (contd..)
3. Registers
– RISC machines have a large general-purpose register set. Any
register can contain either data or an address.
4. Load – store architecture
– The processor operates on data held in registers.
– Separate load and store instructions transfer data between
register bank and external memory.
– Memory accesses are costly, so separating memory access
from data processing provides an advantage because you can
use data items held in the register bank multiple times
without needing multiple memory access.
– In contrast, with a CISC design the data processing operations
can act on memory directly.
The ARM Design philosophy
There are number of physical features that have driven the ARM
processor design:
1. Reduced power consumption
o Portable embedded systems require some form of battery power
o ARM processor has been specifically designed to be small to
reduce power consumption and extend battery operation
o Essential for applications such as mobile phones and personal
digital assistants (PDAs)
2. High code density
o Major requirement since embedded systems have limited
memory due to cost and/or physical size restrictions
o High code density is useful for applications that have limited onboard memory, such as mobile phones and mass storage devices
The ARM Design philosophy (contd..)
3. Price sensitive
o Embedded systems use slow and low-cost memory devices
o Ability to use low-cost memory devices produces substantial savings
4. Size
o Another important requirement is to reduce the area of the die
taken up by the embedded processor
o For a single-chip solution, the smaller the area used by the
embedded processor, the more available space for specialized
peripherals
o This in turn reduces the cost of the design and manufacturing since
fewer discrete chips are required for the end product
ARM has incorporated hardware debug technology within the processor
so that software engineers can view what is happening while the
processor is executing code.
With greater visibility, software engineers can resolve issues faster,
which has a direct effect on the time to market and reduces overall
development costs.
ARM Instruction set Vs RISC instruction set
The ARM core is not a pure RISC architecture because of the
constraints of its primary application – the embedded system.
In today’s systems the key is not raw processor speed but total
effective system performance and power consumption.
The ARM instruction set differs from the pure RISC definition in
several ways that make the ARM instruction set suitable for
embedded applications:
– Variable cycle execution for certain instructions:
• Not every ARM instruction executes in a single cycle.
• For example: load-store-multiple instructions vary in the
number of execution cycles depending upon the number of
registers being transferred.
– Instruction barrel shifter leading to more complex instructions:
• The inline barrel shifter is a hardware component that
preprocesses one of the input registers before it is used by an
instruction. This expands the capability of many instructions to
improve core performance and code density.
The ARM Design philosophy (contd..)
– Thumb 16-bit instruction set:
• ARM enhanced the processor core by adding a second 16-bit
instruction set called Thumb that permits the ARM core to
execute either 16- or 32-bit instructions. The 16-bit
instructions improve code density by about 30% over 32-bit
fixed length instructions
– Conditional execution:
• An instruction is only executed when a specific condition has
been satisfied. This feature improves performance and code
density by reducing branch instructions
– Enhanced instructions:
• The enhanced digital signal processor (DSP) instructions were
added to the standard ARM instruction set to support fast
16x16-bit multiplier operations and saturation. These
instructions allow a faster-performing ARM processor in some
cases to replace the traditional combinations of a processor
plus a DSP