Hardware And Software
- 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:
- 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 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.
# 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.