chp9

• Program must be brought (from disk) into memory and placed within a process for it to be run
• Main memory and registers


  Only storage CPU can access directly
• Memory


  Consists of a large array of bytes
• Memory unit


  Only sees a stream of addresses + read requests, or address + data and write requests

  
• Register access


  Done in one CPU clock (or less)
• Main memory access


  Can take many cycles, causing a stall - memory does not have the data required to complete the instruction that it is executing
• Cache


  Sits between main memory and CPU registers
• Protection of memory is required to ensure correct operation
• Base and limit registers


  Define the logical address space of a process
• CPU must check every memory access generated in user mode to be sure it is between base and limit for that user

  
• The instructions to loading the base and limit registers are privileged
• Symbolic addresses


  Addresses represented in source code
• Relocatable addresses


  Addresses that bind to a specific location at a later stage
• Absolute addresses


  Final addresses that a program will execute at
• Address binding


  1. Compile time

  2. Load time

  3. Execution time
• Logical address is generated by the CPU; also referred to as virtual address
• Physical address is the 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 corresponding to these logical addresses
• Memory-Management Unit (MMU)


  Hardware device that at run time maps virtual to physical address

  
• Relocation register


  The value in the register is added to every address generated by a user process at the time it is sent to memory
• The user program deals with logical addresses; it never sees the real physical addresses
• Execution-time binding occurs when reference is made to location in memory
• Logical address bound to physical addresses
• Dynamic loading


  Routine is not loaded until it is called
• No special support from the operating system is required for dynamic loading
• Dynamic linking


  Linking postponed until execution time
• Stub


  Small piece of code used to locate the appropriate memory-resident library routine

  
• Operating system checks if routine is in processes' memory address, and if not, adds it to the address space
• Contiguous allocation


  Main memory is divided into two partitions: resident operating system in low memory, and user processes in high memory
• Relocation registers


  Used to protect user processes from each other, and from changing operating-system code and data
• Base register


  Contains value of smallest physical address
• Limit register


  Contains range of logical addresses - each logical address must be less than the limit register
• MMU maps logical address dynamically

  
• Variable partition


  Multiple-partition allocation with variable-sized partitions
• Hole


  Block of available memory; holes of various size are scattered throughout memory
• Operating system maintains information about allocated partitions and free partitions (holes)
• Dynamic storage-allocation algorithms


  • First-fit
  • Best-fit
  • Worst-fit
• External fragmentation


  Total memory space exists to satisfy a request, but it is not contiguous