Question

1. Describe the purpose of some components in Internal Memory of a computer.

2.  how Cache Memory functions

Answer 1

Internal memory is also known as main memory. It is typically smaller in size but has faster access times. External memory, also known as secondary memory, is separate from main memory.Internal memory is used to store data that is used by the system at start-up and to run various types of programs such as the operating system. Internal memory is mostly contained on small microchips that are either attached or connected to the computer's motherboard.

PURPOSE OF INTERNAL MEMORY

Internal memory is used to store data that is used by the system at start-up and to run various types of programs such as the operating system. Internal memory is mostly contained on small microchips that are either attached or connected to the computer's motherboard.

MORE DETAILED     

· There are three different main types of internal memory and one these internal memory

                   1)RAM

                  2)ROM

                  3)CACHE

1)RAM : it is also called main memory, primary memory, or system memory, RAM (random-access memory) is a hardware device that allows information to be stored and retrieved on a computer. It is a read/write memory which stores data until the machine is working. As soon as the machine is switched off, data is erased. RAM is volatile, i.e. data stored in it is lost when we switch off the computer or if there is a power failure.

Main Types of RAM

There are two main types of RAM:

1. DRAM (Dynamic Random Access Memory)

2. SRAM (Static Random Access Memory)

Static RAM (SRAM)

The word static indicates that the memory retains its contents as long as power is being supplied. 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 be refreshed on a regular basis.

There is extra space in the matrix, hence SRAM uses more chips than DRAM for the same amount of storage space, making the manufacturing costs higher. SRAM is thus used as cache memory and has very fast access.

Characteristic of Static RAM

• Long life
• No need to refresh
• Faster
• Used as cache memory
• Large size
• Expensive
• High power consumption

Dynamic RAM (DRAM)

DRAM, unlike SRAM, must be continually refreshed in order 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 as it is cheap and small. All DRAMs are made up of memory cells, which are composed of one capacitor and one transistor.

Characteristics of Dynamic RAM

• Short data lifetime
• Needs to be refreshed continuously
• Slower as compared to SRAM
• Used as RAM
• Smaller in size
• Less expensive
• Less power consumption

2) ROM, which stands for read only memory, is a memory device or storage medium that stores information permanently. It is also the primary memory unit of a computer along with the random access memory (RAM). It is called read only memory as we can only read the programs and data stored on it but cannot write on it. It is restricted to reading words that are permanently stored within the unit.

Types of ROM:

1) Masked Read Only Memory (MROM):

It is the oldest type of read only memory (ROM). It has become obsolete so it is not used anywhere in today's world. It is a hardware memory device in which programs and instructions are stored at the time of manufacturing by the manufacturer. So it is programmed during the manufacturing process and can't be modified, reprogrammed, or erased later.

The MROM chips are made of integrated circuits. Chips send a current through a particular input-output pathway determined by the location of fuses among the rows and columns on the chip. The current has to pass along a fuse-enabled path, so it can return only via the output the manufacturer chooses. This is the reason the rewriting and any other modification is not impossible in this memory.

2) Programmable Read Only Memory (PROM):

PROM is a blank version of ROM. It is manufactured as blank memory and programmed after manufacturing. We can say that it is kept blank at the time of manufacturing. You can purchase and then program it once using a special tool called a programmer.

In the chip, the current travels through all possible pathways. The programmer can choose one particular path for the current by burning unwanted fuses by sending a high voltage through them. The user has the opportunity to program it or to add data and instructions as per his requirement. Due to this reason, it is also known as the user-programmed ROM as a user can program it.

To write data onto a PROM chip; a device called PROM programmer or PROM burner is used. The process or programming a PROM is known as burning the PROM. Once it is programmed, the data cannot be modified later, so it is also called as one-time programmable device.

Uses: It is used in cell phones, video game consoles, medical devices, RFID tags, and more.

3) Erasable and Programmable Read Only Memory (EPROM):

EPROM is a type of ROM that can be reprogramed and erased many times. The method to erase the data is very different; it comes with a quartz window through which a specific frequency of ultraviolet light is passed for around 40 minutes to erase the data. So, it retains its content until it is exposed to the ultraviolet light. You need a special device called a PROM programmer or PROM burner to reprogram the EPROM.

Uses: It is used in some micro-controllers to store program, e.g., some versions of Intel 8048 and the Freescale 68HC11.

4) Electrically Erasable and Programmable Read Only Memory (EEPROM):

ROM is a type of read only memory that can be erased and reprogrammed repeatedly, up to 10000 times. It is also known as Flash EEPROM as it is similar to flash memory. It is erased and reprogrammed electrically without using ultraviolet light. Access time is between 45 and 200 nanoseconds.

The data in this memory is written or erased one byte at a time; byte per byte, whereas, in flash memory data is written and erased in blocks. So, it is faster than EEPROM. It is used for storing a small amount of data in computer and electronic systems and devices such as circuit boards.

Uses: The BIOS of a computer is stored in this memory.

5) FLASH ROM:

It is an advanced version of EEPROM. It stores information in an arrangement or array of memory cells made from floating-gate transistors. The advantage of using this memory is that you can delete or write blocks of data around 512 bytes at a particular time. Whereas, in EEPROM, you can delete or write only 1 byte of data at a time. So, this memory is faster than EEPROM.

It can be reprogrammed without removing it from the computer. Its access time is very high, around 45 to 90 nanoseconds. It is also highly durable as it can bear high temperature and intense pressure.

Uses: It is used for storage and transferring data between a personal computer and digital devices. It is used in USB flash drives, MP3 players, digital cameras, modems and solid-state drives (SSDs). The BIOS of many modern computers are stored on a flash memory chip, called flash BIOS.

3)Cache memory: It is also called CPU memory, is high-speed static random access memory (SRAM) that a computer microprocessor can access more quickly than it can access regular random access memory (RAM). This memory is typically integrated directly into the CPU chip or placed on a separate chip that has a separate bus interconnect with the CPU. The purpose of cache memory is to store program instructions and data that are used repeatedly in the operation of programs or information that the CPU is likely to need next. The computer processor can access this information quickly from the cache rather than having to get it from computer’s main memory. Fast access to these instructions increases the overall speed of the program.

Levels of memory:

· Level 1 or Register –
It is a type of memory in which data is stored and accepted that are immediately stored in CPU. Most commonly used register is accumulator, Program counter, address register etc.

· Level 2 or Cache memory –
It is the fastest memory which has faster access time where data is temporarily stored for faster access.

· Level 3 or Main Memory –
It is memory on which computer works currently. It is small in size and once power is off data no longer stays in this memory.

· Level 4 or Secondary Memory –
It is external memory which is not as fast as main memory but data stays permanently in this memory.

How does cache memory work?

Cache memory temporarily stores information, data and programs that are commonly used by the CPU. When data is required, the CPU will automatically turn to cache memory in search of faster data access. This is because RAM is slower and is further away from the CPU. When data is found in cache memory, this is called a cache hit. A cache hit enables the processor to retrieve data quickly, making your overall system more efficient.

Since cache memory is much smaller than RAM, the data it stores is only temporary, and so it may not hold the information that the processor needs. When the cache does not have the processor’s required data, this is called a cache miss, and in this instance the CPU will move onto the hard drive and use RAM.

MORE DETAILED:

Memory caching works by first setting aside a portion of RAM to be used as the cache. As an application tries to read data, typically from a data storage system like a database, it checks to see if the desired record already exists in the cache. If it does, then the application will read the data from the cache, thus eliminating the slower access to the database. If the desired record is not in the cache, then the application reads the record from the source. When it retrieves that data, it also writes the data to the cache so that when the application needs that same data in the future, it can quickly get it from the cache.

Since the cache is limited in size, eventually some data already in the cache will have to be removed to make room for new data that the application most recently accessed. This means the caching system needs a strategy on which records to remove to make room. The strategy will depend on the nature of the application’s data accesses, and will generally try to remove records that are not expected to be accessed again soon. For example, a least-recently-used (LRU) strategy will remove the record whose last access was before any other record in the cache. The assumption here is that if it has been a long time since the record was accessed, it will likely not be accessed again soon. Or to put it another way, the records that were most used recently will likely to be used again soon. A least-frequently-used (LFU) strategy entails tracking the number of accesses of each record in the cache and removing the record with the least amount of accesses. The assumption here is that an infrequently used record will not likely be used again soon.

The challenge with caches is how to minimize “cache misses,” i.e., attempted reads by the application for records that are not in the cache. If you have too many misses, the efficiency of your cache decreases. An application that only reads new data would not benefit from a cache, and in fact, would exhibit lower performance because of the extra work of checking the cache yet not finding the desired record in it. One way this challenge can be mitigated is by leveraging larger caches. This is often not practical on a single computer, which is why distributed caches are popular choices for speeding up applications that need to access larger data sets. A distributed cache pools together the RAM of multiple computers connected in a cluster so that you can create a bigger cache that can continue to grow by adding more computers to the cluster. Technologies like Hazelcast IMDG can be used as a distributed cluster to accelerate large-scale applications.

Another challenge of caches is the risk of reading “stale” data, in which the data in the cache does not reflect the latest data in the underlying source. Oftentimes this risk is an acceptable trade-off for the sake of application performance. In cases where it is not, it is up to the application that updates the underlying data source to update the record in question in the cache.