Hardware And Software

Memory

• The space where programs and data are stored. Data in memory is accessed through instructions from the CPU.
• Classified into primary and secondary.

Primary Memory

• Directly accessed by the CPU, ie RAM, ROM, Cache

Primary memory types listed in order of closeness to the CPU:

• Registers
• Cache
• RAM
• Virtual Memory

The closer to the chip, the faster the memory, however the more expensive the memory is.

Secondary memory

• External devices such as hard drives, SSDs, flash drives

Latency

• Latency is the time taken by components to respond to the request.
• To avoid short delays between the CPU’s request for data and finding the data in memory some instructions and data are copied to the cache.

Random Access Memory (RAM)

• Temporary memory which stores data, files and parts of the operating system
• All data is loaded from secondary storage into RAM.
• Is volatile.

Dynamic RAM (DRAM)

• DRAM consists of transistors that act as switches and capacitors that hold binary data.
• DRAM has to be constantly refreshed (every 15microseconds) to maintain capacitor charge.
• DRAM is used for main RAM memory.
• Is volatile

Static RAM (SRAM)

• SRAM uses “flip-flops” which hold each bit of memory.
• SRAM need not to be refreshed constantly.
• SRAM is faster than DRAM.
• SRAM is used in cache memory.
• SRAM is more complex to build and therefore more expensive than DRAM.
• Comes in smaller capacities than SRAM.
• Non-volatile

Read-Only Memory (ROM)

• Read only memory is a permanent memory that is used to store the instructions that are executed once a computer is switched ON. This set of instructions is called a boot process.
• This is responsible for initializing the hardware and operating system soon after the power is switched ON.
• The contents of ROM are not erased, even when the power is switched OFF.
• The contents of ROM can only be read and cannot be changed.
• ROM is made by interconnecting several transistors. It is an example of non-volatile memory.

Flash memory

• A type of ROM
• Programmed by applying a slightly larger electric current that forces an electron through a barrier.
• Can be rewritten

Hard disk drives (HDD)

• Also called magnetic disk drives, used in computers and laptops.

• It provides high storage capacity and is cost-effective.

• Large storage facilities also use this technology.

• In a computer, it can store operating systems, user data and programs.

• The disk is made of a magnetic surface, known as a platter

• Digital data is stored in these magnetic platters

• The disk can spin at around 7000 revolutions per second

• Data can be accessed by a number of read-write heads on the surface

• Moves from centre of disk to edge and back

Latency of HDD

• Relatively high
• Latency is defined as the time taken for a specific block of data to rotate around to the read-write head.
• High latency leads to “not responding” and “please wait” messages increasing in frequency.

Solid State Drives (SSD)

• The latency is reduced in SSD compared to HDD as there is no read/write head that needs to be moved.
• Data is stored and retrieved using the electronic properties of NAND chips.
• This type of memory is used in USB devices to transfer information from one device to another.
• Data stored in millions of transistors within the chip.

HDD vs SSD

• SSD have faster speeds but cost more
• A combination of both can be used, with frequently needed data store on an SSD and less frequently loaded data stored on an HDD.

Optical Storage Systems

• CD, DVD, Blu-ray discs use optics to store data.

• The surface of CDs and DVDs are made of light-sensitive organic dyes or metal alloys.

• Data is read and written using a laser light.

• DVDs (4.7 GB) can store large amounts of data compared to CDs (800 MB).

• Blu-ray discs use blue laser light and can hold up to 50 GB. Used to store music, movies, and games.

• The time taken to transfer information to these devices is comparatively higher than HDD.

• After data is stored, it cannot be overwritten. Some do support being overwritten, however it is very limited.

• The clear plastic layer on the surface allows the laser light to pass through.

• The colour changes in the dye layer when light falls on it.

Types of memory access

• Memory can be accessed sequentially or directly. Different devices work in different ways.

Sequential Memory Access

• Allows user to access data one by one in a sequence.
• Sequential memory access is slower than direct memory access.
• Eg, film reel

Direct (Random) Memory Access

• Any storage location can be accessed at any moment—such as a DVD.

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