Operating Systems: Three Easy Pieces Ch. 15

• Limited Direct Execution (LDE)
  • Make the program run directly on the hardware
  • OS gets involved and makes sure the “right” thing happens
  • Efficient virtualization, interposing at that critical point
  • interposing at those critical point in the time, OS ensure that it maintains the control
• Flexibility
  • we like for program to be able to use their address spaces in whatever way they would like
• Address translation
  • Hardware-based address translation(address translation)
    • Virtual address provided by the instruction to a physical address where the desired location
    • OS get involved at key points to set up hardware
      • Manage memory
    • Illusion of program has own memory, own code and own data
• Assumption
  • Small space
  • less than size of physical memory, and all are in same size
• Address space starts at address 0 and grows to a maximum of 16 KB
• relocate and transparent?
• Dynamic relocation
  • base and bound(limit) register as dynamic relocation
  • Translation (Address translation)
    • Each memory reference generated by the process is a virtual address
    • Hardware in turn adds the contents of the base register to this address
    • Result is a physical address
    • Due to relocationof the address happens at runtime, we can move address spaces even after the process started running → dynamic relocation
  • Making sure memory reference is within bounds
  • Memory management unit
    • size of the address space
    • holds the physical address at the end of the address space
  • OS Issues
    • First, the OS must take actions when a process is created, finding space for its address space in memory
    • Os will have to search a data structure(free list) to find room for new address space
    • Taken actions when a process is terminated, reclaiming all of its memory for x
    • Third, the OS must also be taken action when a context switch occurs and needs to save and restore the base-and-bound pairs when it switches between processes
      • It is privileged operations
      • Required to access base-and-bounds registers; if a process, running in user mode, attempts to do so, the CPU will raise an exception and the OS will likely terminate the process
  • Process Structure or Process Control Block
  • Summary
    • Address translation
    • Key to the efficiency of this technique is hardware support
    • Transparent the process
    • Base and bound register
    • Protection