1.2.1 Primary storage (memory)

What are the features of primary storage?

• very quick because it is directly accessed by the CPU

• typically smaller in storage size

• known as main memory

• includes RAM and ROM

Why do computers need primary storage?

• primary storage is low capacity internal storage that can be directly accessed by the CPU

• program instructions and data must be copied from the hard drive into RAM to be processed by the CPU as primary storage access speeds are faster than secondary storage devices - faster at storing and retrieving data

• used for temporary storage of running programs as unlike secondary, RAM is volatile

What are the differences between RAM and ROM?

RAM

• volatile - loses data when power is off

• CPU can access any part of the memory in the same amount of time

• used for loading programs to run temporarily:

  • the part of the operating system currently in use

  • the software or part of it currently in use

  • the data which the software is using

ROM

• non volatile

• stores perment instructions such as:

  • the bootstrap - the initial program that is run which gives the instructions of where to find the OS on the hard drive

  • the BIOS which controls the processor speed and system time

What is meant by virtual memory and how does it work?

• a part of the hard drive (HDD) used as an extension of RAM

• used when there is insufficient space in RAM

• program that has been idle for longest time is moved to VM to free up space in RAM so new data can be transferred to RAM

• program is reloaded from VM back to RAM when it needs to resume execution

What are the advantages and disadvantages of virtual memory?

advantages

• uses cheap secondary storage on the hard drive

• - prevents “out of memory” error message - files and programs will still open

• allows programs to run simultaneously

disadvantages

• accessing virtual memory is slow - existing data in RAM needs to be copied to the virtual memory and then the data in virtual memory needs to be copied to RAM when in use as CPU cannot access VM directly