Virtual Addressing

Question 1

Concepts - why memory management

Answer 1

• Multiprogramming system = multiplex and allocate and protect memory
• Multiplexing CPU requires multiplexing memory
• Need to keep many processes in memory simultaneously
• Each process requires memory for : instructions, static data, dynamic data ie stack and heap
  And
• OS needs memory to store its own instructions and data

Question 2

Relocation in general

Answer 2

• MM Shared between processes .: programmer cannot know address occupied by process at runtime
• Swapping processes in/out of MM to maximise CPU utilisation (how/why????) and silly to require swapped in process to always go to same place in memory
• Processes may include addressing info (branches, pointers etc) and OS must sanity check - translation of logical addresses to physical addresses.

Question 3

Memory allocation in general

Answer 3

OS chooses addresses to load programs data to make linking , relocation, sharing easier

Question 4

Protection

Answer 4

• OS / MMU protect one process’s memory from another
• May want RWX protection on small memory units
• A process should not modify its own code (why?????)
• Dynamically computed addresses (eg array subscripts) should be sanity checked.