5.2 Memory

5.2 Memory

1. Physical memory sits on the bus of a processor

   • Each byte of memory has a unique address

2. Memory management is a hardware mechanism that seeks to make reliable and efficient use of memory

   • When management is used programs do not use the physical addresses, instead they use virtual memory
     • Virtual Memory can be used to implement a protection mechanism that prevents unauthorised access to physical memory
       • Virtual Addresses
         • 2 parts, base address and offset address
       • Protection keeps tasks from overwriting the instructions of another task
         • if 1 program makes a reference to the wrong space, protection mechanisms can block and report these occurrences
         • Guard against possible software failures

3. Segmentation

   • Provides multiple independent address spaces
     • Segments can be different sizes, this can lead to poor utilisation of physical memory
   • Programs can be loaded in 1 segment and files containing data in another segment
     • Protection requirements at an object level can easily be implemented on segments
   • Can be combined with paging

4. Paging

   • Uses RAM and disk space
   • Pages are fixed size, determined by hardware and OS
   • Single files may be stored in different pages or several files in 1 page
     • So not good for access control because the protection for the file needs to be maintained by each page
   • Pages are stored in secondary memory and only brought into main memory when required
   • Can be combined with segmentation