The two types of memory
There are two fundamentally different types of memory:
-main memory and secondary storage.
Characteristics of Main Memory
Main memory has the following characteristics:
- Very fast access times
- Relatively small capacity
What is ROM ?
ROM stands for Read Only Memory: it cannot be changed by a program or user.
ROM is non-volatile which means data is retained when power is switched off.
ROM is used for software needed when the computer switches on (such as the instructions for booting the computer) or software that will ‘never’ change, such as the BIOS.
Firmware is stored on the ROM. Firmware is programming added by the manufacturer of a device and used to run user programs on the device.
What is RAM ?
RAM stands for Random Access Memory
RAM is volatile which means all data is lost when power is switched off.
RAM is used for holding data temporarily for use by the CPU.
What is cache memory ?
Cache memory is a special type of RAM and is part of the primary store.
It is almost always located on the same microchip as the CPU.
Cache memory is much more expensive than ordinary RAM so computers don’t have much of it.
The cache only stores a copy of the data also stored in RAM but the CPU looks in the cache first.
Level 1 Cache
This is extremely fast, but relatively small, and is usually embedded in the processor chip. It is called Level 1 because it is closest to the processor.
Level 2 Cache
This is often larger than L1 but one stage further from the processor, though still on the same chip.
Level 3 Cache
This is typically specialised memory that works to improve the performance of L1 and L2. It can be significantly slower than L1 or L2, but still quicker than RAM.
What is a HDD ?
A magnetic hard disk drive (HDD) uses moving read/write heads that contain electromagnets.
The disk’s surface contains iron particles that can be given a polarity. Each polarity represents 0 or 1.
The data is stored in tracks. The sections within each track are called sectors.
The head can move to any given sector to read or write data from or to that sector.
Advantages of HDD
Advantages
- Very large storage capacity
- Much faster than an optical disk
- Data is not lost when power is switched off
- Cheaper per MB than other storage types
Disadvantages of HDD
Disadvantages
- Moving parts that can fail
- Easily damaged if dropped
- Uses a large amount of power compared to other storage types
- Can be noisy
How does an Optical Drive work ?
An optical drive uses reflective light to read data. These include CDs, DVDs and Blu-ray.
Once written, the optical disk’s surface is covered with microscopic dents (pits) and flat spots (lands).
As the laser light reflection is detected by a sensor, it assigns a 1 for pits and 0 for lands.
What are the advantages of Optical Drives ?
Advantages
- Easy to store and carry
- Read in many devices, such as audio or TV systems
- Long lasting if looked after
What are the disadvantages of Optical Drives ?
Disadvantages
- Data on write-once disks (CD-R, DVD-R) is permanent
- Require special (large) drives to read/write
- Expensive per MB
- Can easily be broken, scratched or damaged
Do SSD contain moving parts ?
Solid-state disks contain no moving parts. They do not rely on magnets, so are unaffected by magnetic fields.
Name three common formates SSD are found in
They are found in three common formats:
Hard disks
Memory cards (SD, etc.)
USB flash drives
How do SSD record data ?
They record data using special transistors that retain their state even when there is no power to them.
Because there are no moving parts, data access times are quicker.
What is Cloud Storage ?
Cloud storage is where data is stored on servers (magnetic or SSD) at a remote location, often in another country. Data is usually stored across multiple servers, in different locations.
Advantages of Cloud Storage
Advantages
- Unlimited storage capacity (i.e. more than any person or company would require)
- Automatic backup in case of personal computer or local storage failure
- Universal access wherever you are
- Universal access across all your connected devices: you can begin work on a PC and finish it on your phone
Disadvantages of Cloud Storage
Disadvantages
- Cloud storage requires a reliable (and fast) internet connection: you can’t access data without it
- Even on a very fast connection, it is slower than accessing data stored locally on a machine
- The user loses control over what happens to the data as this is managed by the cloud service provider. This can raise concerns over security of sensitive data and privacy
How many types of RAM are there ?
There are two types of RAM: Dynamic RAM (DRAM) and Static RAM (SRAM). These store data in slightly different ways, meaning that SRAM is quicker, but also more expensive.
