Unit -3 :Memories

Memory Devices

A memory is just like a human brain. It is used to store data and instruction. Computer memory is the storage space in computer where data is to be processed and instructions required for processing are stored.

The memory is divided into large number of small parts. Each part is called a cell. Each location or cell has a unique address which varies from zero to memory size minus one.

For example if computer has 64k words, then this memory unit has 64 * 1024 = 65536 memory location. The address of these locations varies from 0 to 65535.

Memory is primarily of two types

  • Internal Memory − cache memory and primary/main memory
  • External Memory − magnetic disk / optical disk etc.
Memory Hiearchy

Characteristics of Memory Hierarchy are following when we go from top to bottom.

  • Capacity in terms of storage increases.
  • Cost per bit of storage decreases.
  • Frequency of access of the memory by the CPU decreases.
  • Access time by the CPU increases.

RAM

A RAM constitutes the internal memory of the CPU for storing data, program and program result. It is read/write memory. It is called random access memory (RAM).

Since access time in RAM is independent of the address to the word that is, each storage location inside the memory is as easy to reach as other location & takes the same amount of time. We can reach into the memory at random & extremely fast but can also be quite expensive.

RAM is volatile, i.e. data stored in it is lost when we switch off the computer or if there is a power failure. Hence, a backup uninterruptible power system (UPS) is often used with computers. RAM is small, both in terms of its physical size and in the amount of data it can hold.

RAM is of two types

  • Static RAM (SRAM)
  • Dynamic RAM (DRAM)

Static RAM (SRAM)

The word static indicates that the memory retains its contents as long as power remains applied. However, data is lost when the power gets down due to volatile nature. SRAM chips use a matrix of 6-transistors and no capacitors. Transistors do not require power to prevent leakage, so SRAM need not have to be refreshed on a regular basis.

Because of the extra space in the matrix, SRAM uses more chips than DRAM for the same amount of storage space, thus making the manufacturing costs higher.

Static RAM is used as cache memory needs to be very fast and small.

Dynamic RAM (DRAM)

DRAM, unlike SRAM, must be continually refreshed in order for it to maintain the data. This is done by placing the memory on a refresh circuit that rewrites the data several hundred times per second. DRAM is used for most system memory because it is cheap and small. All DRAMs are made up of memory cells. These cells are composed of one capacitor and one transistor.

ROM

ROM stands for Read Only Memory. The memory from which we can only read but cannot write on it. This type of memory is non-volatile. The information is stored permanently in such memories during manufacture.

A ROM, stores such instruction as are required to start computer when electricity is first turned on, this operation is referred to as bootstrap. ROM chip are not only used in the computer but also in other electronic items like washing machine and microwave oven.

Following are the various types of ROM −

MROM (Masked ROM)

The very first ROMs were hard-wired devices that contained a pre-programmed set of data or instructions. These kind of ROMs are known as masked ROMs. It is inexpensive ROM.

PROM (Programmable Read Only Memory)

PROM is read-only memory that can be modified only once by a user. The user buys a blank PROM and enters the desired contents using a PROM programmer. Inside the PROM chip there are small fuses which are burnt open during programming. It can be programmed only once and is not erasable.

EPROM (Erasable and Programmable Read Only Memory)

The EPROM can be erased by exposing it to ultra-violet light for a duration of upto 40 minutes. Usually, an EPROM eraser achieves this function. During programming an electrical charge is trapped in an insulated gate region. The charge is retained for more than ten years because the charge has no leakage path. For erasing this charge, ultra-violet light is passed through a quartz crystal window (lid). This exposure to ultra-violet light dissipates the charge. During normal use the quartz lid is sealed with a sticker.

EEPROM (Electrically Erasable and Programmable Read Only Memory)

The EEPROM is programmed and erased electrically. It can be erased and reprogrammed about ten thousand times. Both erasing and programming take about 4 to 10 ms (millisecond). In EEPROM, any location can be selectively erased and programmed. EEPROMs can be erased one byte at a time, rather than erasing the entire chip. Hence, the process of re-programming is flexible but slow.

Serial Access Memory

Sequential access means the system must search the storage device from the beginning of the memory address until it finds the required piece of data. Memory device which supports such access is called a Sequential Access Memory or Serial Access Memory. Magnetic tape is an example of serial access memory.

Direct Access Memory

Direct access memory or Random Access Memory, refers to conditions in which a system can go directly to the information that the user wants. Memory device which supports such access is called a Direct Access Memory. Magnetic disks, optical disks are examples of direct access memory.

Cache Memory

Cache memory is a very high speed semiconductor memory which can speed up CPU. It acts as a buffer between the CPU and main memory. It is used to hold those parts of data and program which are most frequently used by CPU. The parts of data and programs, are transferred from disk to cache memory by operating system, from where CPU can access them.

Advantages

  • Cache memory is faster than main memory.
  • It consumes less access time as compared to main memory.
  • It stores the program that can be executed within a short period of time.
  • It stores data for temporary use.

Disadvantages

  • Cache memory has limited capacity.
  • It is very expensive.

How Does Cache Memory Work?

Cache memory works by taking data or instructions at certain memory addresses in RAM and copying them into the cache memory, along with a record of the original address of those instructions or data.

This results in a table containing a small number of RAM memory addresses, and copies of the instructions or data that those RAM memory address contain.

Memory Cache “Hit”

When the processor requires instructions or data from a given RAM memory address, then before retrieving them from RAM it checks to see if the cache memory contains a reference to that RAM memory address. If it does, then it reads the corresponding data or instructions from the cache memory instead of from RAM. This is known as a “cache hit”. Since the cache memory is faster than RAM, and because it is located closer to the CPU, it can get and start processing the instructions and data much more quickly.

The same procedure is carried out when data or instructions need to be written back to memory. However, in this case there is an additional step because if anything is written to cache memory than ultimately it must also be written RAM.

Memory Cache “Miss”

If data or instructions at a given RAM memory address are not found in cache memory, then this is known as a “cache miss.” In this case, the CPU is forced to wait while the information is retrieved from RAM.

In fact, the data or instructions are retrieved from RAM and written to cache memory, and then sent on to the CPU. The reason for this is that data or instructions that have been recently used are very likely to be required again in the near future. So anything that the CPU requests from RAM is always copied to cache memory.

This begs the question of what happens if the cache memory is already full. The answer is that some of the contents of the cache memory has to be “evicted” to make room for the new information that needs to be written there.

If a decision needs to be made then the memory cache will apply a “replacement policy” to decide which information is evicted.

There are a number of possible replacement policies. One of the most common ones is a least recently used (LRU) policy. This policy uses the principal that if data or instructions have not been used recently, then they are less likely to be required in the immediate future than data or instructions that have been required more recently.

Types of Cache Memory

  • Primary Cache  Most cache memory is physically located on the same die as the CPU itself, and the part closest to the CPU cores is sometimes called primary cache, although the term is not commonly used any more.
  • Secondary Cache  This often refers to a further piece of cache memory, which is located on a separate chip on the motherboard close to the CPU. This term is also not commonly used anymore, because most cache memory is now located on the CPU die itself.

Levels  of Cache Memory

Modern computer systems have more than one piece of cache memory, and these caches vary in size and proximity to the processor cores, and therefore also in speed. These are known as cache levels.

The smallest and fastest cache memory is known as Level 1 cache, or L1 cache, and the next is L2 cache. Most  systems now have L3 cache, and since the introduction of its Skylake chips, Intel has added L4 cache to some of its processors as well.

Level 1

L1 cache is cache memory that is built into the CPU itself. It runs at the same clock speed as the CPU. It is the most expensive type of cache memory so its size is extremely limited. But because it is very fast it is the first place that a processor will look for data or instructions that may have been buffered there from RAM.

In fact, in most modern CPUs, the L1 cache is divided into two parts: a data section (L1d) and an instruction section (L1i). These hold data and instructions, respectively. 

A modern CPU may have a cache size on the order of 32 KB of L1i and L1d per core.

Level 2

L2 cache may also be located in the CPU chip, although not as close to the core as L1 cache. Or more rarely, it may be located on a separate chip close to the CPU. L2 caches are less expensive and larger than L1 caches, so L2 cache sizes tend to be larger, and may be of the order of 256 KB per core.

Level 3

Level 3 cache tends to be much larger than either L1 or L2 cache, but it also different in another important way. Whereas L1 and L2 caches are private to each core of a processor, L3 tends to be a shared cache that is common to all the cores. This allows it to play an important role in data sharing and inter-core communication. L3 cache may be of the order of 2 MB per core.

Auxiliary Memory

Auxiliary memory is much larger in size than main memory but is slower. It normally stores system programs, instruction and data files. It is also known as secondary memory. It can also be used as an overflow/virtual memory in case the main memory capacity has been exceeded. Secondary memories cannot be accessed directly by a processor. First the data/information of auxiliary memory is transferred to the main memory and then that information can be accessed by the CPU. Characteristics of Auxiliary Memory are following −

  • Non-volatile memory − Data is not lost when power is cut off.
  • Reusable − The data stays in the secondary storage on permanent basis until it is not overwritten or deleted by the user.
  • Reliable − Data in secondary storage is safe because of high physical stability of secondary storage device.
  • Convenience − With the help of a computer software, authorised people can locate and access the data quickly.
  • Capacity − Secondary storage can store large volumes of data in sets of multiple disks.
  • Cost − It is much lesser expensive to store data on a tape or disk than primary memory.
  • Floppy Disk: A floppy disk is a flexible disk with a magnetic coating on it. It is packaged inside a protective plastic envelope. These are one of the oldest type of portable storage devices that could store up to 1.44 MB of data but now they are not used due to very less memory storage.
  • Hard disk: A hard disk consists of one or more circular disks called platters which are mounted on a common spindle. Each surface of a platter is coated with a magnetic material. Both surfaces of each disk are capable of storing data except the top and bottom disk where only the inner surface is used. The information is recorded on the surface of the rotating disk by magnetic read/write heads. These heads are joined to a common arm known as access arm.Hard disk drive components:
    Most of the basic types of hard drives contains a number of disk platters that are placed around a spindle which is placed inside a sealed chamber. The chamber also includes read/write head and motors. Data is stored on each of these disks in the arrangement of concentric circles called tracks which are divided further into sectors. Though internal Hard drives are not very portable and used internally in a computer system, external hard disks can be used as a substitute for portable storage. Hard disks can store data upto several terabytes.

CD- ROM -It stands for Compact Disk – Read Only Memory.Data is written on these disks at the time of manufacture. This data cannot be changed, once is it written by the manufacturer, but can only be read. CD- ROMs are used for text, audio and video distribution like games, encyclopedias and application software.