Operating system 32 logical versus physical address
- 1. Operating System 32 Logical versus physical address Prof Neeraj Bhargava Vaibhav Khanna Department of Computer Science School of Engineering and Systems Sciences Maharshi Dayanand Saraswati University Ajmer
- 2. Memory Management âą How to utilize memory optimally by manipulating objects in the memory is referred to as memory management. âą Program must be brought (from disk) into memory and placed within a process for it to be run âą Main memory and registers are only storage CPU can access directly âą Memory unit only sees a stream of addresses + read requests, or address + data and write requests âą Register access in one CPU clock (or less) âą Main memory can take many cycles, causing a stall âą Cache sits between main memory and CPU registers âą Protection of memory required to ensure correct operation
- 3. Base and Limit Registers âą A pair of base and limit registers define the logical address space âą CPU must check every memory access generated in user mode to be sure it is between base and limit for that user
- 5. Binding âą Assigning an address to an instruction or data/variable is known as binding. âą The addresses assigned to high level language program components residing on a secondary storage device are symbolic addresses, viz. variable names, function names etc. âą When this program is compiled, the addresses are translated into relative addresses in the object program. âą Another translation of the relative addresses into relocatable addresses (relocatable address is an address that can be reassigned to a different physical location) takes place when this object code is linked and loaded into the memory. âą Thus each binding is a mapping from one address space to another address space. âą The responsibility of these address translations is basic responsibility of operating systems.
- 6. Address Binding âą Programs on disk, ready to be brought into memory to execute form an input queue â Without support, must be loaded into address 0000 âą Inconvenient to have first user process physical address always at 0000 â How can it not be? âą Further, addresses represented in different ways at different stages of a programâs life â Source code addresses usually symbolic â Compiled code addresses bind to relocatable addresses âą i.e. â14 bytes from beginning of this moduleâ â Linker or loader will bind relocatable addresses to absolute addresses âą i.e. 74014 â Each binding maps one address space to another
- 7. Binding of Instructions and Data to Memory âą Address binding of instructions and data to memory addresses can happen at three different stages â Compile time: If memory location known a priori, absolute code can be generated; must recompile code if starting location changes â Load time: Must generate relocatable code if memory location is not known at compile time â Execution time: Binding delayed until run time if the process can be moved during its execution from one memory segment to another âą Need hardware support for address maps (e.g., base and limit registers)
- 8. Multistep Processing of a User Program
- 9. Logical vs. Physical Address Space âą The concept of a logical address space that is bound to a separate physical address space is central to proper memory management â Logical address â generated by the CPU; also referred to as virtual address â Physical address â address seen by the memory unit âą 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 âą Logical address space is the set of all logical addresses generated by a program âą Physical address space is the set of all physical addresses generated by a program
- 10. Memory-Management Unit (MMU) âą Hardware device that at run time maps virtual to physical address âą Many methods possible, covered in the rest of this chapter âą To start, consider simple scheme where the value in the relocation register is added to every address generated by a user process at the time it is sent to memory â Base register now called relocation register â MS-DOS on Intel 80x86 used 4 relocation registers âą The user program deals with logical addresses; it never sees the real physical addresses â Execution-time binding occurs when reference is made to location in memory â Logical address bound to physical addresses
- 11. Dynamic relocation using a relocation register ïź Routine is not loaded until it is called ïź Better memory-space utilization; unused routine is never loaded ïź All routines kept on disk in relocatable load format ïź Useful when large amounts of code are needed to handle infrequently occurring cases ïź No special support from the operating system is required ïŹ Implemented through program design ïŹ OS can help by providing libraries to implement dynamic loading
- 12. Dynamic relocation using a relocation register
- 13. Dynamic Linking âą Static linking â system libraries and program code combined by the loader into the binary program image âą Dynamic linking âlinking postponed until execution time âą Small piece of code, stub, used to locate the appropriate memory-resident library routine âą Stub replaces itself with the address of the routine, and executes the routine âą Operating system checks if routine is in processesâ memory address â If not in address space, add to address space âą Dynamic linking is particularly useful for libraries âą System also known as shared libraries âą Consider applicability to patching system libraries â Versioning may be needed
- 14. Assignment âą Explain the concept and need of Memory Management âą Explain Address Binding âą What Logical vs. Physical Address Space and explain role of Memory Management Unit.