Introduction to Memory

1. Background

  • Program must be brought (from disk) to memory and placed within process for it to be run
  • Main memory and registers are only storage CPU can access directly
  • Memory unit only sees a stream of addresses + read requests
    • or address +data and write request
  • Register access in one CPU clock (or less)
  • Main memory can take many cycles (stalls)
  • Cache sit between main memory and CPU registers
  • Protection of memory required to ensure correct operation 
2. Address binding
  • Addresses represented in different ways at different stages of a program’s life
    • source code addresses usually symbolic
    • compile code addresses bind to relocated addresses
      • i.e., 14 bytes from beginning of this module
    • Linker or loader will bind relocatable addresses to absolute addresses
    • Each binding maps one address space to another
  • Address binding of instructions and data to memory addresses can happen at three different stages
    • compile time: if memory location known a priori, absolute code can be generated; must recompile code if starting location changes
    • load time: must generate relocatable code if memory location is not known at compile time
    • execution time: binding delayed until run time if the process can be moved during its execution from one memory segment to another
      • need hardware support for address maps (e..g, base and limit registers)
3. Multistep Processing of a user program
4. Logical v.s. Physical Address Space
  • The concept of a logical address space that is bound to a separate physical address space is central to proper memory management
    • logical address: generated by the CPU; also referred to as virtual address
    • physical address: address seen by the memory unit
  • Logical and physical addresses are the same in compile-time and load-time address-binding schemes; logical (virtual) and physical addresses differ in execution-time address-binding scheme.
  • Logical address space is the set of all logical addresses generated by a program
  • Physical address space is the set of all physical addresses generated by a program
5. Dynamic loading
  • routine is not loaded until it is called
  • better memory-space utilization; unused routine is never loaded
  • all routines kept on disk is relocatable load format
  • useful when large amounts of code are needed to handle infrequently occurring cases
  • no special support from the operation system is required
    • implemented through program design
    • OS can help by providing libraries to implement dynamic loading
6. Dynamic Linking
  • Static linking
    • system libraries and program code combined by the loader into the binary program image
  • Dynamic liking
    • linking postponed until execution time
      • small piece of code, stub, used to locate the appropriate memory-resident library rountine
  • Stub replaces itself with the address of the routine, and executes the routine
  • Operating system checks if routine is in processes’ memory address
    • if not in address space, add to address space
  • Dynamic linking is particularly useful for libraries
    • system also known as shared libraries
  • Consider applicability to patching system libraries
    • versioning may be needed
7. Base and Limit Registers
  • A pair of base and limit registers define the logical address space
8. Hardware address protection with base and limit registers
9. Memory Management Unit (MMU)
  • Hardware device that at run time maps virtual to physical address
  • The user program deals with logical address; it never sees the real physical addresses
    • execution-time binding occurs when reference is made to location in memory
    • logical address bound to physical address

10. Dynamic relocation using a relocation register

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