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6. OCR A Level (H046-H446) SLR2 - 1.1 CISC vs RISC
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- 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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