Memory

Types of Memory

Random Access Memory (RAM) is one of the two types of main memory found in computers. General purpose computers usually contain a large amount of RAM. The features of RAM are:

• Expensive - RAM is much more expensive per unit capacity than secondary storage media.
• Volatile - RAM is volatile. This means that it will lose its contents when power is lost (i.e. if the computer is switched off).
• Fast - RAM is much faster to access than secondary storage such as Hard Disk Drives (HDDs).
• Random access - Random Access means that any part of the memory can be accessed as quickly as any other part of the memory.

Read Only Memory (ROM) is another type of main memory found in computers. It comes on a small, factory made chip in the motherboard. Features of ROM include:

• Read only - ROM is often made from flash memory. This means that it's contents cannot be changed easily. This means that the ROM only stores small programs that boots or starts up the computer. These programs are called the Basic Input Output System (BIOS). They usually:
  • Load an operating system.
  • Check the system for errors.
• Non-volatile - ROM is non-volatile. This means that ROM keeps its contents when power is lost.
• Fast but expensive - ROM is very fast to access. ROM is expensive, so tends to be only small.

Flash Memory

Flash memory is usually based on a technology called Electrically Erasable Programmable Read-Only Memory (EEPROM). EEPROM can be used to create many flash memory devices:

• Solid State Drives (SSD)
  • SSDs are often used in home computers because of their performance.
  • SSDs are a form of re-writable flash storage.
• ROM chips
  • The Read-Only Memory chips on board modern Central Processing Units (CPUs) are made from EEPROM.
  • This provides the software needed for the computer to start.
• Secure Digital (SD) cards
  • SD cards are used to extend the capacity of devices such as: cameras, smartphones, and tablets.
  • They provide high capacity for their physical size. This is useful in these smaller devices.

Properties of Flash Memory

Flash memory is a type of memory technology that can be used to create primary or secondary storage. It is commonly used to make Read-Only Memory (ROM), and Solid State Drives (SSDs).

• Robust
  • Flash memory is not easily damaged by shocks.
  • Flash memory is appropriate in scenarios where it might be exposed to lots of vibrations. For example: Industrial machinery and vehicles.
• Short lifetime
  • Flash memory can only be flashed (re-written) a certain number of times before it starts to degrade.
  • We may lose access to the drive once the flash memory has degraded past a point.
  • Re-writable devices often include several blocks of flash memory. This means that only one block must be flashed at a time and so the device will last longer.
• Non-volatile
  • Flash memory is non-volatile. This means that it will keep its contents once power is switched off.
• Fast access
  • Flash memory is significantly faster than many other storage media such as Hard Disk Drives (HDDs).
  • This is partly because flash memory does not have any moving parts, and so we do not need to wait for parts to move before we can access the drive.

Virtual Memory

Virtual Memory is a technique where we use secondary storage as if it were RAM.

• Virtual Memory can have negative impacts on performance.
• Virtual Memory can have negative impacts on the health of Solid State Disks (SSDs).
• We might have limited space in secondary storage.

How Virtual Memory Works

Virtual Memory allows us to access more memory than we have by setting aside some secondary storage space to act as extra memory.

• Paging
  • Memory can be split into sections called 'pages'.
  • Virtual Memory works by moving these pages between the Random Access Memory (RAM) and the secondary storage.
• Move pages to Hard Disk Drive (HDD)
  • If a page is not being used, then we might want to put it into Virtual Memory.
  • When a page is moved to the HDD, it is put into a special file called the pagefile.
  • Other processes can then use the memory that we just freed in the RAM.
• Move pages to RAM
  • When we want to use a page again, we first need to move it back to the RAM.
  • We do this by swapping the page that we want with one that is already in the RAM.