Year 12 (1.1.2)

OCR AS Level

(H046)

OCR A Level

(H446)

Teacher site: craigndave.org | Student site: student.craigndave.org | Smart revise: smartrevise.co.uk

SLR2 Types of processor

• The differences between and uses of CISC and

RISC processors

OCR H046/H466

SLR2 Types of processor| CISC vs RISC

•At the core of all computers is the
instruction set: “Set of all
instructions written in machine code
that can be recognised and executed
by a given processing unit/CPU.”

oCISC: Complex Instruction Set
Computer

oRISC: Reduced Instruction Set
Computer

OCR H046 & H466

SLR2 Types of processor| CISC vs RISC

What is an instruction set?

CISC

RISC

OCR H046/H466

SLR2 Types of processor| CISC vs RISC

Our task is to find the product of two
numbers.

•The first number is stored in memory
location 0000.

•The second number is stored in
memory location 0001.

•The result of the calculation should
be stored back in 0010.

•In a high-level language, this would
be a line of code such as:
product = num1 * num2

Multiplying two numbers in memory

Main memory (RAM)

0000

0001

0010

0011

0100

0101

0110

0111

1000

1001

Registers

r000

r001

r010

r011

r100

r101

Execution
unit

OCR H046/H466

SLR2 Types of processor| CISC vs RISC

•CISC aims to complete the task in as
few lines of assembly as possible –
this means the processor hardware
and circuitry has to be more
complicated so it can understand
and execute a series of operations.

Complex Instruction Set Computer (CISC)

Main memory (RAM)

0000

0001

0010

0011

0100

0101

0110

0111

1000

1001

Registers

r000

r001

r010

r011

r100

r101

Execution
unit

OCR H046/H466

SLR2 Types of processor| CISC vs RISC

•CISC aims to complete the task in as
few lines of assembly as possible –
this means the processor hardware
and circuitry has to be more
complicated so it can understand
and execute a series of operations.

•A CISC processor could include a
specific instruction for multiplying
two numbers – e.g., MULT.

•When executed, this instruction
would load the two values into the
register, multiple them together and
store the result back out.

Complex Instruction Set Computer (CISC)

Main memory (RAM)

0000

0001

0010

0011

0100

0101

0110

0111

1000

1001

Registers

r000

r001

r010

r011

r100

r101

Execution
unit

MULT 0000, 0001

OCR H046/H466

SLR2 Types of processor| CISC vs RISC

•MULT is an example of a complex
instruction.

•It resembles the original command
from a high-level language:
product = num1 * num2

•The compiler has to do very little
work to translate the high-level
language statement into assembly.

Complex Instruction Set Computer (CISC)

Main memory (RAM)

0000

0001

0010

0011

0100

0101

0110

0111

1000

1001

Registers

r000

r001

r010

r011

r100

r101

Execution
unit

MULT 0000, 0001

OCR H046/H466

SLR2 Types of processor| CISC vs RISC

•MULT is an example of a complex
instruction.

•It resembles the original command
from a high-level language:
product = num1 * num2

•The compiler has to do very little
work to translate the high-level
language statement into assembly.

•However, this complex instruction
might take more than one
machine/clock cycle to execute.

Complex Instruction Set Computer (CISC)

Main memory (RAM)

0000

0001

0010

0011

0100

0101

0110

0111

1000

1001

Registers

r000

r001

r010

r011

r100

r101

Execution
unit

MULT 0000, 0001

OCR H046/H466

SLR2 Types of processor| CISC vs RISC

•Although less common today than it
used to be, CISC architecture is
mainly found in desktop computers
and laptops.

•Intel's x86 processors still use CISC
architecture, although more recent
changes beyond the scope of this
course mean they operate in a RISC
fashion, as they utilise microcodes to
leverage many RISC benefits.

Complex Instruction Set Computer (CISC)

OCR H046/H466

SLR2 Types of processor| CISC vs RISC

•RISC processors aim to use simple
instructions that will be executed
within a single machine/clock cycle.

•In our generic computer example, at
least, a complex command such as
MULT won’t exist.

•We would need to separate it into a
number of simpler commands.

•In this situation, we would need four
lines of assembly, whereas before,
we only needed one.

Reduced Instruction Set Computer (RISC)

Main memory (RAM)

0000

0001

0010

0011

0100

0101

0110

0111

1000

1001

Registers

r000

r001

r010

r011

r100

r101

Execution
unit

LDA r000, 0000

LDA r001, 0001

PROD r000, r001

STA 0010, r000

OCR H046/H466

SLR2 Types of processor| CISC vs RISC

•There are more lines of code
required in the assembly version of
high-level code line:
product = num1 * num2

•The compiler has to do more
conversion work.

•More RAM is needed to store the
assembly instructions.

•The process seems less efficient.

Reduced Instruction Set Computer (RISC)

Main memory (RAM)

0000

0001

0010

0011

0100

0101

0110

0111

1000

1001

Registers

r000

r001

r010

r011

r100

r101

Execution
unit

LDA r000, 0000

LDA r001, 0001

PROD r000, r001

STA 0010, r000

OCR H046/H466

SLR2 Types of processor| CISC vs RISC

•However, each instruction can be
completed in a single clock cycle.

•These RISC instructions require
fewer transistors and less complex
hardware, leaving more room for
general-purpose registers and cache.

•As all instructions are uniform in
terms of their execution time,
pipelining is possible.

•The use of RISC processing results in
lower energy requirements.

Reduced Instruction Set Computer (RISC)

Main memory (RAM)

0000

0001

0010

0011

0100

0101

0110

0111

1000

1001

Registers

r000

r001

r010

r011

r100

r101

Execution
unit

LDA r000, 0000

LDA r001, 0001

PROD r000, r001

STA 0010, r000

OCR H046/H466

SLR2 Types of processor| CISC vs RISC

•RISC architectures have become
incredibly popular in low-power and
portable devices such as smart TVs,
thermostats, smart watches,
smartphones, tablets, printers, home
assistants and TV sticks.

•As such, ARM processors and other
architectures that work off RISC now
make up well over 90% of all
processors in use today.

Reduced Instruction Set Computer (RISC)

OCR H046/H466

SLR2 Types of processor| CISC vs RISC

CISC vs RISC

Complex Instruction Set Computing

Reduced Instruction Set Computing

Used in laptops/desktops computers.
x86 processors made mainly by Intel and AMD.

Used in smartphones, tablets, embedded systems.
Made popular by ARM processors.

Has more complex hardware.

Has simpler hardware.

Larger number of complex instructions of variable length.

Smaller number of simple, standardised instructions that are
all of a fixed length.

Multiple machine/clock cycles per instruction.

Single machine cycle per instruction.

Physically larger, more complex circuitry, requires more
silicon to make, typically more expensive.

Smaller in size, less complex circuitry required, less silicon,
typically cheaper to produce.

Greater energy consumption.

Lower energy requirements.

Can’t make use of pipelining.

Can support pipelining.

As emphasis is on the hardware, the compiler (software) has
to do less work.

As the emphasis is on the software, the compiler has to do
more work.

Small code sizes, higher cycles per second.

Large code sizes, lower cycles per second.

Makes more efficient use of RAM than RISC.

Heavy use of RAM (can cause bottlenecks if RAM is limited).

Multiple addressing modes available.

Fewer addressing modes available.

OCR H046/H466

SLR2 Types of processor| CISC vs RISC

•You can clearly see some of the
concepts we have been talking about
in the raw numbers here.

Architecture implementation in numbers

Performance equation:

CISC examples

RISC examples

IBM 370/168

VAX 11/780

88000

R4000

Year developed

1973

1978

1988

1991

Number of instructions

208

303

51

94

Instruction sizes (bytes)

2 - 6

2 - 57

4

4

Addressing modes

4

22

3

1

Cache size (KB)

64

64

16

128

Multiple Choice

Used in smartphones and tablets, based around ARM processor

• CISC

• RISC

Multiple Choice

Lower energy requirements and can go into “sleep mode” when not actively processing

• CISC

• RISC

Multiple Choice

Multiple machine cycles per instruction 

• CISC

• RISC

Multiple Choice

Greater energy consumption

• CISC

• RISC

Multiple Choice

Smaller in size as less complex circuitry required, less silicon needed to make thus cheaper

• CISC

• RISC

Multiple Choice

Several CISC instructions may be equivalent to a single RISC instruction

• TRUE

• FALSE

Multiple Choice

CISC aims to improve processing time by increasing the number of instructions understood by the CPU in the instruction set.

• TRUE

• FALSE

Multiple Choice

RISC aims to improve processing speed by simplifying instructions so that one line takes one clock cycle.

• TRUE

• FALSE

Multiple Choice

CISC code may have fewer lines of code, but will take more processor time.

• TRUE

• FALSE

Multiple Choice

RISC code can better utilise pipe-lining as the time needed for each operation can be predicted.

• TRUE

• FALSE

Multiple Select

CISC processors...

• Are physically larger

• Are better at complex tasks

• Use more electricity than RISC

• Expect more work from the compiler to generate more code

• Not as good at pipelining

Multiple Select

An example of where you would find CISC processors:

• Tablets

• Smartphones

• Laptops

• Desktop Computers

• Smartwatches

Multiple Select

RISC Processors...

• Minimise the number of clock cycles per instruction

• Have a larger instruction set

• Tend to run at a lower clock speed

• Make use of pipelining

• Use less power and generate less heat

Multiple Select

Pipelining means...

• Running the fetch-decode-execute cycle more efficiently

• Waiting for a full fetch-decode-execute cycle to complete before starting the next

• Fetches the next instruction while the previous instruction is still decoding

• CISC processors can run more efficiently

• RISC processors can run more efficiently

OCR H466

SLR2 Types of processor| GPUs and their uses

•The CPU is often referred to as the
brain of the computer.

•It drives and carries out the overall
operations of the system.

•In the early days of computing,
systems had a single CPU responsible
for carrying out and controlling all
instructions.

What is a co-processor?

OCR H466

SLR2 Types of processor| GPUs and their uses

•As technology advanced,
ever-increasing demand was placed
on this single central processing unit.

What is a co-processor?

OCR H466

SLR2 Types of processor| GPUs and their uses

•As technology advanced,
ever-increasing demand was placed
on this single central processing unit.

•At this point special, co-processors
began to emerge.

•Co-processor: “Any additional
processor used for a specialised
task”.

•A co-processor’s purpose is to
improve the overall speed of the
computer by executing concurrently
with the main CPU, aided by a highly
bespoke set of electronics.

What is a co-processor?

OCR H466

SLR2 Types of processor| GPUs and their uses

•The most well known type of
co-processor is the Graphical
Processing Unit (GPU).

•As the name suggests, GPUs were
initially used for rendering graphics.

•As technology advanced, the large
number of cores in GPUs have been
exploited to process many parallel
streams of data at the same time, no
matter what that data might be.

•The use of GPUs is no longer limited
to graphics processing.

What is a co-processor?

The term “GPU” was

coined by Sony in

1994, referring to the

32-bit Sony GPU in

the PlayStation video

game console.

OCR H466

SLR2 Types of processor| GPUs and their uses

•While GPUs have thousands of
stream processors, they typically run
slower than a standard CPU core.

•However, be careful of simply saying
CPUs are faster than GPUs – CPUs
are generic processing units and
good all-rounders, whereas GPUs are
highly specialised.

Differences between CPUs and GPUs

OCR H466

SLR2 Types of processor| GPUs and their uses

•While GPUs have thousands of
stream processors, they typically run
slower than a standard CPU core.

•However, be careful of simply saying
CPUs are faster than GPUs – CPUs
are generic processing units and
good all-rounders, whereas GPUs are
highly specialised.

•GPUs are superior in speed and
efficiency, but only for certain tasks.

•Remember, CPUs and GPUs are
designed with two different goals in
mind, so they have different
performance characteristics.

Differences between CPUs and GPUs

OCR H466

SLR2 Types of processor| GPUs and their uses

•CPUs excel at performing complex
operations on small data sets.

•GPUs tend to be better for simple
operations on larger data sets.

•Think of GPUs as a special-purpose
processor, designed so a single
instruction can work quickly over a
large block of similar data, applying
the same operation again and again.

•This is not to say the GPUs can’t
perform complex numerical
calculations, indeed they can and do.

•They simply excel and performing
calculations on matrices / vectors
and multiple data at the same time
such rendering graphics and other
modelling tasks.

Differences between CPUs and GPUs

OCR H466

SLR2 Types of processor| GPUs and their uses

•The mass calculations typically
associated with graphics processing
(known as Single Instruction Multiple
Data) can take full advantage of
parallel processing.

•Although a typical CPU might
perform one floating-point operation
quicker than a typical GPU, a GPU
will be able to perform a thousand
floating-point operations much
quicker due to the benefits of its
bespoke design for parallel
processing.

Why are GPUs so good at rendering graphics?

Time

OCR H466

SLR2 Types of processor| GPUs and their uses

•From around 2006 onwards, we
realised that this highly specialised
SIMD processor technique could be
utilised for operations other than
simply handling graphics – this lead
to the birth of General Purpose
Computing on GPU or GPGPU.

•Any situation that required
processing thousands or even
millions of simple calculations over
and over on multiple points of data
became much quicker to perform on
a GPU than a generic CPU.

Beyond handling graphics

OCR H466

SLR2 Types of processor| GPUs and their uses

•Some examples of computing tasks
performed on GPUs:

oOil exploration

oWeather modelling

oMachine learning

oLinear algebra

oStatistical analysis

oStock option predictions

oGravitational field theory

Beyond handling graphics

Uses for

GPUs
beyond
graphics

Multiple Select

What are GPUs used for now?

• image processing
• machine learning
• controlling peripherals
• managing user access levels
• system security

Multiple Choice

What doesn’t affect the performance of the computer

• Clock Speed

• Number of Cores

• The number of instructions in the program

• Cache Size

Multiple Choice

If the processing speed goes from 1GHZ to 4GHZ and the processor is changed from an 8 core to a dual core processor – what is the exact effect on the performance of the computer?

• The number of instructions executed per second quadruples

• The number of instructions executed per second doubles

• The number of instructions executed per second stays the same

• The number of instructions executed per second is halved

Multiple Choice

Which of the following best defines clock speed?

• Used to count the number of instructions.
• The number of instructions stored in memory.
• The number of cycles performed in a second.
• Used perform mathematical calculations.

Multiple Choice

A multi-core processor is?

• Two or more cache units inside a CPU.
• Multiple clock speed CPU
• Two or more processors inside a CPU.
• Multiple counter systems inside the CPU.

Multiple Choice

Cache for a CPU is?

• Temporary data used to store data for websites.
• Super fast memory located on main memory.
• Super fast memory located in or nearby the processor.
• Temporary data used to store data used in apps.