Memory Management

• The CPU fetches instructions and data of a program from memory; therefore, both the program and its data must reside in the main (RAM and ROM) memory
• What is memory Huge linear array of storage
• malloc library call
  • used to allocate and free 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
• Example of allocation (see the Fig. 3)

  char *ptr=malloc(4096); //char* is address of a single byte
  

  Figure 3: Allocating Memory.

  
• Modern multiprogramming systems are capable of storing more than one program, together with the data they access, in the main memory
• A fundamental task of the memory management component of an operating system is to ensure safe execution of programs by providing:
  • Sharing of memory; issues are
    • Transparency; Several processes may co-exist, unaware of each other, in the main memory and run regardless of the number and location of processes.
    • Efficiency; CPU utilization must be preserved and memory must be fairly allocated. Want low overheads for memory management.
    • Relocation Ability of a program to run in different memory locations.
  • Memory protection; processes must not corrupt each other (nor the OS!)
• Information stored in main memory can be classified in a variety of ways:
  • Program (code) and data (variables, constants).
  • Read-only (code, constants) and read-write (variables).
  • Address (e.g., pointers) or data (other variables); binding (when memory is allocated for the object): static or dynamic
  • The compiler, linker, loader and run-time libraries all
    cooperate to manage this information.
• Before a program can be executed by the CPU, it must go through several steps:
  • Compiling (translating)-generates the object code.
  • Linking-combines the object code into a single self-sufficient executable code.
  • Loading-copies the executable code into memory. May include run-time linking with libraries.
  • Execution-dynamic memory allocation.

  Figure 4: From source to executable code.

  
• The process of associating program instructions and data (addresses) to physical memory addresses is called address binding, or relocation
  • Static-new locations are determined before execution
    • Compile time: The compiler or assembler translates symbolic addresses (e.g., variables) to absolute addresses.
    • Load time: The compiler translates symbolic addresses to relative (relocatable) addresses. The loader translates these to absolute addresses.
  • Dynamic-new locations are determined during execution
    • Run time: The program retains its relative addresses. The absolute addresses are generated by hardware.