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Hello fellas.

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Welcome back.

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So in this lesson we shall look at another essential components of the embedded system the memory an

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embedded system has to have some form of memory to store and execute code.

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And when we look up memory we can compare them in terms of price performance and power consumption.

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For instance if a memory has to run twice as fast to maintain desired bond with the memory power consumption

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requirements may be higher.

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This curve depicts the the memory hierarchy.

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First you must know that all computer systems have memory arranged in some form of hierarchy.

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This graph shows the arm.

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The memory trade of the fastest memory is the cache memory and is physically located nearer to the process

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of call and the slowest memory system is the secondary storage which is set further away from the processor.

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Generally speaking the closer the memory is to the process all the more its costs and the smaller its

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capacity the cache is placed between the main memory and the call and its use to speed up data transfer

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between the process all and the main memory which all week some into cash on its own later but for now

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you only need to remember that the cash provides an overall increase in performance and it comes with

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a loss of predictable execution time though.

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And this only becomes a problem when when you do it in a real time application the main memory is large.

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It's in it's an megabytes mostly and sometimes even an gigabyte depending on the application and and

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it may memory is generally a separate chip from the from the process so as we shall see later load and

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store instructions access the main memory unless the value have been stored into cash for fast access.

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You can think of the young secondary memory as your everyday flash disc your hard disk or your seedy

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rom.

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Now let's take a look at some parameters we pay attention to when looking up memory.

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So let's start with the memory with the memory with is basically the number of bits the memory returns

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on each on each access and it comes in the sizes of 16 bit 32 bits and 64 bit memory with us direct

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effect on overall performance.

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For instance if you have a system without cash memory using 32 bit instructions and 16 bits wide memory

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chips to process or have to make to memory fetches pet instruction because each fetch can only carry

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sixteen bits load.

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This of course has the effect of reducing the system performance but the upside is sixteen bits memory

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is less expensive.

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In contrast if the call executed 16 bit thumb instructions it will achieve better up to four months

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with a sixteen bits memory because the call will make a single fetch to memory to load an instruction

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as we shall see in this course.

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The ARM processor is capable of executing three instruction sets the instruction sets which has 32 bit

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instructions the thumb instruction sets which are sixteen bits instructions and a thumb to instruction

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sets which uses both sixteen bits and sets two bits instructions.

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Let's see how the arm and thumb instructions compare when using different memory with the ARM instruction

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the ARM instruction set is that two bits in size like we mentioned.

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So if they're with off the memory is 8 bits the processor would need four cycles to fetch a single instruction

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if the width of the memory is 16 bits the process will require two cycles better instruction.

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And if the width of the memory is 32 bits then the process will require a single cycle pair instruction.

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On the other hand the instruction set has instructions with 16 bits of size.

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Therefore we turn 8 bits memory with the process eight uses two cycles to fetch a single instruction

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and we have sixteen bits memory with the process or will require a single cycle to fetch a single instruction

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with a 32 bit memory with the process will still require a single cycle to fetch a single instruction.

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Now let's take a look at some of the commonly used memory types we would find in embedded devices.

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The room cannot be reprogrammed and the flush room is a type of from that can return to as well as read

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but it is slow to write too so it's not used for hold in dynamic data.

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Its main uses for holding the device firm were storing long term data that needs to be preserved.

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After the power is off the dynamic random access memory is the most commonly used type of Rob for devices.

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It has the lowest cost per megabyte compared with other types of firearm the s ROM which stands for

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static random access memory is faster than D D ROM its access time is considerably shorter than the

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equivalent do run because s run does not require pauses between data access the SD Iran is a subcategory

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of the RAM and it is capable of running at fast faster clock frequencies.

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The EP from which stands for the electrically reasonable programmable read only memory is used for program

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cold storage most often for saving critical data.

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Not all ARM chips have an on chip approach.

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Now let's look at a simplified memory space allocation diagram for an armed device typically has four

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gigabytes of directly accessible memory space.

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Now why is that.

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Remember cause it two bits course and you are fit two bits cause because the internal registers of that

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two bits the data paths out there's two bits and a boss interfaces off it to bits.

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The bits size allows the CPSU to address a memory for an individual a process so an expert can handle

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two to the power X byte of memory the higher the bid size the higher the performance.

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Therefore a 32 bit can handle approximately 40 get byte of memory.

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The memory space can be divided into five sections.

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The ownership pair for an eye or register section which can occupy any of the empty areas here.

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Depending on the particular silicon manufacturer the s room section which is used for the data variables

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and stack the EP section and the Flash section the SFR section indicated up here is for special function

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registers.

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We should talk about the arm the arm registers in sections.

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So this is all there is to it.

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To summarize the s Rom is used by the CPI for data variables and stock whereas the EP roms are considered

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to be the memory.

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You can add externally to the chip the flash from is useful program code while the apron is used most

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often for critical system data that must not be lost if power is cut to the system.

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This all there is to it and I'll see you in the next lesson.