SLR 1.2 Memory

SLR 1.2 Memory

1. RAM

   • Random Access Memory
   • Purpose is to temporarily store instructions and data for programs being executed by the computer. Also holds the Operating System (OS) when the computer is running.
   • Volatile: contents is lost when powered off
   • Read and write

2. ROM

   • A small piece of memory soldered to the motherboard.
   • In some systems, ROM chips can be changed to change the program being executed. e.g. Games cartridges in portable entertainment systems
   • Read Only Memory
   • Contains very first instructions: - To check hardware is installed correctly - To load Basic Input/Output System (BIOS) (These very first instructions are also known as the bootstrap.)
   • Non-volatile: contents remains when powered off.

3. Virtual memory

   • Virtual memory is memory stored on the computer’s hard disk, rather than the RAM. Hard disk space is cheaper, so we can have more virtual memory than physical RAM. Apps stored in the virtual memory are not executed from it - because the hard disk is too slow to execute programs from, when the user wants to execute a program that is in the virtual memory, it will be moved to the RAM. When the user wants to open another app when the RAM is already full, an app that is not currently being used will be moved to the virtual memory. We use part of the hard disk space as memory, to make room for all the programs that the user wants to have open at the same time.
   • Virtual memory is needed because RAM can sometimes be too small to hold the open programs. Because the computer can only execute one instruction at a time, some instructions are not likely to be fetched in the near future. These instructions can be stored in the hard disk as virtual memory, to make space for a new app that the user opens if the RAM is already full.
   • The problem with using virtual memory is that it is stored on the hard disk, which is much slower than the RAM. This is because the hard disk has mechanical parts: it has a rotating circular platter that is paired with magnetic heads arranged on a moving arm, which reads and writes to the disk to access the data. The speed of the rotating circular platter and arm movement limits how fast the hard disk can read and write data. The hard disk is also slower because it is physically further away from the CPU, at the southbridge. In comparison, RAM has no moving parts, and it is closer to the CPU, at the northbridge. Unlike with the hard drive, data items on the RAM can be read or written in almost the same time irrespective of the physical location of the data.

4. Flash memory

   • Non-volatile: contents remains when powered off.
   • Read and write
   • Flash memory is used in a typical computer when settings need to be retained after the computer is powered off, without using the hard disk or other secondary storage.
   • Purpose: It is used for the BIOS (Basic Input/Output System) - Checks installed hardware is functioning correctly, including the hard disk - Contains basic settings for those components that need to be retained when the computer is powered off.
   • Tablet computers Mobile phones Smartwatches Digital cameras The above use flash memory as an alternative to a hard disk, for permanent storage.
   • An increasing range of microcomputers use flash memory rather than a hard disk.
   • Solid State Drives
     • They are significantly quicker than hard disks, because they have no moving parts. They are also more reliable as a result.
     • They are also significantly more expensive than mechanic hard disks byte-for-byte, so they come in smaller capacities, and are therefore only slowly becoming a viable alternative to the mechanical hard disk.
     • An SSD in a computer behaves exactly like a hard disk, so you can still have virtual memory on an SSD.