Explain how Unix and Windows manage memory and processors. Compare between the approaches of the two operating systems .
1. Windows uses tree data structure and each node of tree is called virtual address descriptors. In Unix, uses list data structure and It maintains a list of vm_area_structs.
2. Windows on 32 bit x86 systems can access up to 4GB of physical memory. and its own 4GB logical address space by using paging. The upper 2GB is kept for windows kernel mode and the lower 2GB of the address space is reserved for user mode. In Unix 3GB of memory space is reserved for user mode and 1GB is kept for Kernel mode.
3. Windows use cluster demand paging. Unix uses demand paging with no pre paging.
4. Windows use two level of paging. Unix uses three level paging.
5. Windows use FIFO for page replacement and Unix use LRU ["lease recently used"].
6. Windows address is device into two part : page number and page offset. Unix address is device into four parts: global directory, middle directory, page table and offset.
7. In Windows number of page fault is 155. and In Unix number of page fault is 12.
- Memory management is the process of managing the computer memory which consists of primary memory and secondary memory.
- The main goal for memory management is to keep track of which parts of memory are in use and which parts are not in use, to allocate memory to processes when they need it and de-allocate it when they are done.
In Unix System
- UNIX memory management scheme includes swapping and demand paging.
- Swapping is a useful technique that enables a computer to execute programs and manipulate data files larger than main memory.
- swapping is use in which a process in main memory is copied to the pre-configured space on the hard disk, called swap space, to free up that space of memory.
- The combined sizes of the physical memory and the swap space is the amount of virtual memory available. Kernel allocates contiguous space on the swap space and main memory. It maintains free space of the swap space in an internal table, called map. As kernel allocates and frees resources, it updates the map accordingly.
- when the system requires more memory than is physically available, the kernel swaps out less used pages and gives memory to the current application (process) that needs the memory immediately.
- Kernel swaps out when memory is needed and Kernel swaps in When a new process needs to be executed When a previous process which was swapped out wants to continue its execution
- demand paging is a type of swapping in which pages of data are not copied from disk to RAM until they are needed.
- Bring a page into memory only when it is needed: Less I/O needed, Less memory needed, Faster response, More users
- Not all pages of a process reside in memory. Pages should only be brought into memory if the executing process demands them. This is often referred to as lazy evaluation as only those pages demanded by the process are swapped from backing store to main memory.
In Windows System
- In Windows, each process on 32-bit Microsoft Windows has its own virtual address space and it's enables addressing up to 4 gigabytes of memory. Each process on 64-bit Windows has a virtual address space of 8 terabytes.
- The 32-bit Windows Operating System addressable memory space is shared between active applications and the kernel.
- The kernel address space includes a System Page Table Entry area , Paged Pool, System Cache, and a Non Paged Pool.
- The default Operating System (OS) configuration provides up to 4 GB of addressable memory space divided equally between the kernel and the user applications. With 4 GB of physical memory available, 2 GB will be allocated to the kernel and 2 GB to application memory.
- The kernel mode address space is shared across processes and the application mode access space is allocated for each user process (each process has it’s own space).
- Below memory management function:
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