6. OCR A Level (H046-H446) SLR2 - 1.1 CISC vs RISC
Summary
TLDRThis video explores the differences between CISC and RISC processors. CISC processors use complex instructions to complete tasks in fewer lines of assembly, while RISC processors use simpler instructions executed in a single clock cycle. Despite requiring more lines of code, RISC's efficiency, lower power consumption, and use of pipelining make it dominant in modern devices. The video also discusses the performance equation and historical challenges RISC faced before becoming the industry standard.
Takeaways
- 💻 **Instruction Set Importance**: The script explains that at the core of all computers is the instruction set, which is the set of all machine code instructions recognized by the CPU.
- 🔍 **CISC vs RISC**: There are two main categories of instruction sets: Complex Instruction Set Computers (CISC) and Reduced Instruction Set Computers (RISC).
- 📚 **CISC Efficiency**: CISC processors aim to complete tasks with fewer lines of assembly code, which means the processor hardware and circuitry is more complex.
- 🔧 **CISC Example**: The script uses 'malt' as an example of a complex instruction in CISC that can multiply two numbers and store the result in one operation.
- 📉 **CISC Decline**: CISC architecture, once common in desktops and laptops, is now less common, with Intel's x86 processors being a notable exception.
- 🏃 **RISC Simplicity**: RISC processors use simple instructions that are executed within a single machine or clock cycle, which requires more lines of assembly code.
- 🚀 **RISC Performance**: RISC architecture can take advantage of pipelining to speed up the processor, as all instructions have uniform execution times.
- 🔋 **RISC Power Efficiency**: RISC processors are popular in low-power devices due to their lower energy requirements and simpler hardware.
- 📈 **RISC Dominance**: ARM and other RISC-based processors now make up over 90% of all processors in use, indicating their widespread adoption.
- 📊 **Performance Equation**: The script introduces a performance equation that contrasts CISC's focus on minimizing instructions per program with RISC's focus on reducing cycles per instruction.
- 📱 **RISC Adoption**: Despite early challenges, RISC has become dominant due to factors like the reduction in RAM prices and advancements in compiler technology.
Q & A
What is the core element of all computers?
-The core element of all computers is the instruction set, which is the set of all instructions written in machine code that can be recognized and executed by a given processing unit or CPU.
What are the two main categories of instruction sets?
-The two main categories of instruction sets are Complex Instruction Set Computer (CISC) and Reduced Instruction Set Computer (RISC).
What is the primary goal of CISC processors?
-The primary goal of CISC processors is to complete tasks in as few lines of assembly as possible, which means the processor hardware and circuitry is more complicated to understand and execute a series of operations.
What is an example of a complex instruction in CISC?
-An example of a complex instruction in CISC is 'malt', which would load two values into a register, multiply them together, and store the result back out.
Where is CISC architecture mainly found today?
-CISC architecture is mainly found in desktop computers and laptops, with Intel's x86 processors being an example, although they now operate much like RISC with the use of microcode.
What is the aim of RISC processors?
-The aim of RISC processors is to use simple instructions that will be executed within a single machine or clock cycle.
How does RISC handle complex commands that don't exist in its architecture?
-RISC handles complex commands by separating them into a number of simpler commands, requiring more lines of assembly but each executable in a single clock cycle.
What are the benefits of RISC architecture?
-RISC architecture benefits include lower energy requirements, simpler hardware, more room for general-purpose registers, and the ability to use pipelining to speed up the processor.
Why have RISC architectures become popular in low power and portable devices?
-RISC architectures have become popular in low power and portable devices due to their efficiency, lower energy requirements, and the fact that they can execute instructions in a single clock cycle.
What is the performance equation and how does it relate to CISC and RISC?
-The performance equation is used for expressing a computer's performance potential. CISC aims to minimize instructions per program but sacrifices cycles per instruction, while RISC aims to reduce cycles per instruction at the cost of increasing the number of instructions in the assembly program.
Why did it take RISC a long time to gain acceptance on a commercial scale?
-It took RISC a long time to gain acceptance on a commercial scale due to a lack of software support compared to CISC, and the influence of companies like Intel which were producing powerful CISC processors.
How has the reduction in price of RAM and increased sophistication of compiler technology affected RISC?
-The reduction in price of RAM and increased sophistication of compiler technology has made the RISC emphasis on software over hardware ideal, leading to the widespread adoption of RISC-based processors.
Outlines
💻 Introduction to CISC and RISC Architectures
This paragraph introduces the concept of instruction sets in computer processors, specifically focusing on Complex Instruction Set Computers (CISC) and Reduced Instruction Set Computers (RISC). It explains that the instruction set is the foundation of all computer operations, detailing how it is written in machine code and understood by the CPU. The paragraph uses a high-level abstraction of a computer's storage design to illustrate the process of executing a calculation using an execution unit and registers. It contrasts CISC processors, which have a wide range of instructions and can execute complex operations in a single instruction, with RISC processors, which use simpler instructions executed in a single machine cycle. The example of multiplying two numbers is used to demonstrate the difference in assembly code complexity between CISC and RISC, with CISC requiring less assembly code but more machine cycles, while RISC requires more assembly code but each instruction is simpler and faster to execute.
📈 Advantages and Popularity of RISC Architecture
The second paragraph delves into the advantages of RISC architecture, such as the use of simple instructions that can be executed in a single clock cycle, leading to faster processing. It mentions that RISC requires fewer transistors and less complex hardware, which allows for more general-purpose registers and benefits from techniques like pipelining to increase speed. The paragraph also discusses the historical context of RISC gaining popularity over CISC, especially in low-power and portable devices, due to its lower energy requirements and efficiency. It contrasts this with CISC's historical dominance in desktops and laptops, noting that even Intel's x86 processors, which are CISC-based, have adapted to operate more like RISC processors. The paragraph concludes with a summary of the key differences between CISC and RISC architectures, highlighting RISC's prevalence in modern devices.
📉 The Shift from CISC to RISC in the Market
The final paragraph of the script addresses the shift in the market from CISC to RISC processors. It discusses the performance equation used to express a computer's performance potential, where CISC aims to minimize instructions per program at the cost of more cycles per instruction, while RISC does the opposite. The paragraph notes the historical lack of software support for RISC compared to CISC, especially in the early days when operating systems like Windows 3.1 and 95 were designed with CISC in mind. It also touches on how Intel's dominance in CISC processors initially hindered the adoption of RISC. However, it concludes by stating that the reduction in RAM prices and advancements in compiler technology have made RISC's emphasis on software over hardware ideal, leading to a significant market share for RISC-based processors over CISC, as evidenced by sales data showing a peak for CISC processors in 2011.
Mindmap
Keywords
💡Instruction Set
💡CISC (Complex Instruction Set Computer)
💡RISC (Reduced Instruction Set Computer)
💡Assembly Language
💡Main Memory
💡Execution Unit
💡Registers
💡Pipelining
💡Microcode
💡Performance Equation
💡Compiler
Highlights
Instruction sets are the foundation of all computers, defining the operations CPUs can execute.
Two main categories of instruction sets: CISC (Complex Instruction Set Computer) and RISC (Reduced Instruction Set Computer).
CISC aims to complete tasks with fewer lines of assembly, resulting in more complex processor hardware.
RISC uses simple instructions executed within a single machine cycle, requiring less complex hardware.
CISC processors, like Intel's x86, are mainly found in desktops and laptops but operate more like RISC with microcode.
RISC architecture is prevalent in low-power devices such as smartphones, smart TVs, and tablets.
RISC processors, like ARM, now dominate the market, making up over 90% of all processors in use.
CISC processors may have more instructions but can be problematic for pipelining due to variable instruction length.
RISC's fixed-sized instructions facilitate pipelining, improving performance.
RISC's efficiency comes from completing each instruction in a single clock cycle.
The performance equation shows CISC aims to minimize instructions per program, sacrificing cycles per instruction.
RISC philosophy reduces cycles per instruction, increasing the number of instructions in the assembly program.
RISC's acceptance was slow due to a lack of software support and competition from established CISC processors.
The reduction in RAM prices and advancements in compiler technology have favored RISC's software-over-hardware approach.
Intel's influence and resources were a significant obstacle for RISC's early adoption.
Despite being CISC, Intel x86 processors implement features that allow RISC-like operation.
Sales of RISC-based processors have far surpassed those of CISC, with CISC peaking in 2011.
Transcripts
in this video we take a look at the
differences between
and the uses of sisk and risk processors
[Music]
so at the core of all computers is what
is known as the
instruction set this is effectively
the set of all instructions written in
machine code
that can be recognized and executed by
given processing unit or cpu
there are two main different categories
of instruction sets that you need to be
aware of
there's the complex instruction set
computer sisk
and the reduced instruction set computer
risk
so here on the screen is a high
level abstraction of a storage design
for a generic computer and it's going to
serve the purpose
of our illustration so important things
to note
we've got main memory at the top and
it's got ten locations
and they're addressed from zero zero
zero zero in binary
through to one zero zero one
we also have what we're labeling the
execution unit which is going to carry
out all calculations
now the execution unit can only operate
on data that's been loaded into one of
six registers
and we've labeled them r zero zero 0
to r 1 0 1.
so our task is to find the product of
two numbers
the first number is going to be stored
in memory location zero
zero zero zero the second number in zero
zero zero one and then the result of the
calculation the product of those two
numbers
should be stored back in memory location
zero zero
one zero so in a high level language
this would be a line of code something
like product
equals num1 times num2
where each of those words are variables
or locations
so let's start with the complex
instruction set computer
so cis gains to complete the task in as
few lines
of assembly as possible now this means
the processor hardware
and the circuitry tends to be more
complicated
so it's able to understand and execute a
series of operations
the cisc processor could include a
specific instruction
for multiplying two numbers and we're
going to call that malt here we've
shown it on the screen when executed
this instruction would load the two
values into the register
multiply them together and store the
result back
out
so malt is an example in our abstraction
here
of a complex instruction it resembles
the original command from the high-level
language
the compiler therefore is having to do
very little work
to translate the original high-level
language statement
into low-level assembly code
however this complex instruction
might take more than one machine or
clock cycle in order to execute
now we're going to come back to that
point a bit later when we compare the
two
so although much less common today than
it used to be
the sisk architecture is mainly found in
desktop computers and laptops
intel's x86 processors still use the sys
architecture
although more recent changes beyond the
scope of this course
mean they operate very much like a risk
fashion
and they utilize micro code which allows
them to leverage many of the benefits
of the risk architecture which we're now
going to look at
so with risk processes the aim is to use
simple instructions which will be
executed within a single
machine or clock cycle now in our
generic computer example here that we're
using
at least we're going to say that the
complex command such as
malt doesn't exist we would need to
separate it into a number of simpler
commands
in this situation we're going to need
four lines of assembly
two for loading the different contents
of memory
one for providing the product and
another to store the result back out
whereas remember before with the sisk
architecture we had a single low-level
command that performed all of that
now there are more lines of code
required in the assembly version
than the high level version which only
had one line
this means the compiler is now being
required to do much of the conversion
work
more ram is needed to store the assembly
instructions
so on the face of it it would seem that
the risk architecture
is much less efficient
but we come back to the point that with
this
system each instruction can be completed
in a single clock cycle
these risk instructions require fewer
transistors
and less complex hardware this leaves
more room for general purpose registers
in cash
and we already know the benefits of that
at speeding up a processor from a
previous video
as all the instructions are uniform in
terms of their execution time
we can make use of pipelining another
way of speeding up the processor we
discussed earlier
the use of risk processing results also
in lower energy requirements
risk architectures have become
incredibly popular
in low power and portable devices such
as smart tvs
thermostats smart watches phones tablets
printers home assistants tb sticks and
many
many more as such arm processors
and other architectures that work off of
risk now make up
well over 90 percent of all processors
in use today
so here is a summary of sisk
architecture on the left
compared to risk architecture on the
right
i'm not going to read it all to you but
you might like to pause the video
and take some notes
so having watched this video you should
be able to answer the following key
question
what are the differences between the
risk and sisk architectures
so that's everything covered in the
specification so you can stop taking
notes
but if you're interested in learning a
little bit more and going a little bit
deeper
then watch the remainder of this video
so something not in the spec which is
quite interesting to learn about
is known as the performance equation now
this is an equation
commonly used for expressing a
computer's performance
potential
now the sisk approach attempts to
minimize
the number of instructions per program
but while sacrificing the number of
cycles per instruction
the risk philosophy does completely the
opposite
it attempts to reduce the cycles per
instruction
but at the cost of the number of
instructions that end up in the end
assembly program
you can clearly see here some of the
concepts that we've been talking about
in raw numbers
so look at the number of instructions
available in sisk versus
risk there are a lot more instructions
in the complex instruction sets
than the reduced instruction set
varieties
but you can see that there's a variable
length of sisk instructions
and of course this becomes problematic
when we
try to utilize pipelining to increase
performance
now risk gets around this by fixed sized
instructions
now even though risk-based processing
has many advantages
it did take well over a decade to gain
acceptance on a large commercial scale
risk lacks software support versus sisk
especially in the early days
windows 3.1 and 90 to 5 were designed
with cisc processors in mind
and most companies were unwilling to
take a chance on risk
which was seen as an emerging technology
in the beginning intel cisc processors
were a major setback for the emergence
of risk
intel had enormous influence and
resources
to throw at producing ever more powerful
cis processes
despite them becoming increasingly
unwieldy and difficult to develop
today though intel x86 is arguably the
only chip
that retains sisk architecture and even
then if you dig a bit deeper below the
surface
you'll realize it implements many
features such as micro codes
that allows it to operate in a risk
type fashion the reduction in price of
ram
and increased sophistication of compiler
technology
means the risk emphasis on software over
hardware has become ideal
you can easily see from this graph that
sales of risk-based processors
far outstrip intel sysversons which
peaked at
0.3 billion processors back in 2011.
you
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