Background

• Memory is central to the operation of a modern computer system.
• The part of the OS that manages the memory hierarchy is called the memory manager.
  • to keep track of which parts of memory are in use and which parts are not in use,
  • to allocate memory to processes when they need it and deallocate it when they are done,
  • to manage swapping between main memory and disk when main memory is too small to hold all the processes.
• Memory consists of a large array of words or bytes, each with its own address.
  • malloc library call
    • used to allocate memory,
    • finds sufficient contiguous memory,
    • reserves that memory,
    • returns the address of the first byte of the memory.
  • free library call
    • give address of the first byte of memory to free,
    • memory becomes available for reallocation.
  • Both malloc and free are implemented using the brk system call.
• The CPU fetches instructions from memory according to the value of the program counter. These instructions may cause additional loading from and storing to specific memory addresses.
• The memory unit sees only a stream of memory addresses; it does not know how they are generated (by the instruction counter, indexing, indirection, literal addresses, and so on) or what they are for (instructions or data).
• Accordingly, we can ignore how a program generates a memory address. We are interested only in the sequence of memory addresses generated by the running program.
• Memory management systems can be divided into two classes:
  1. Those that move processes back and forth between main memory and disk during execution (swapping and paging), (Memory Abstraction)
  2. Those that do not. Simpler. (No Memory Abstraction)