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Memory Management

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Memory Management:
Memory management operations are one of the fundamental core components of an operating system. The memory management function is to organize and manage memory hierarchy of the random access memory, bulky address spaces, memory mapping, manage alternate memory devices, physical allocation, de-allocation of processes, and the computer’s hard drive. Physical address is the address perceived by the memory unit. Virtual memory is the necessity of memory by exploiting the hard drive as additional access memory.
What is Memory:
An exceptional concept of virtual memory is to present a software application program with the appearance of additional or a considerable amount of memory accessible for its programming use. The virtual memory management system will require mapping functions, which execute address translations converting virtual addresses to physical addresses. The virtual or logical address is the address used to communicate a memory location and physical address is the actual memory location passed to the local memory bus.
Paging:
In paging, the local address space for both virtual and physical memory is divided into fixed size blocks called pages. The pages can be individually located at different areas in the hard drive and physical memory. The address translation is carried out by the memory management system of the central processing unit.
Compare/Contrast:
Windows:
When running, Windows can access up to four gigabytes of physical memory. Each process can have up to four gigabytes of logical address space by using paging. This allocation is broken up into two sections. The upper two gigabytes is used for kernel code and the lower two gigabytes is reserved for the user access. Paging within a Windows system also has two levels. This occurs both as needed and simultaneously. Paging is completed using a set model based on the amount of memory currently assigned to the process. Windows employs three data structures. Each node of the “tree” is identified as Virtual Address Descriptors. The category nodes of these virtual address descriptors are reserved, free, or committed. These addresses are divided into either a page number or a page offset. When divided, each is then further divided into modified, stand-by, free, or zeroed pages. The modified pages stay in a modified page list. After writing to the disk, they are then moved to and stored in the stand-by list. Unmodified pages are automatically stored in the stand-by list. When a page needs to replaced, Windows uses a First In First out (FIFO) system and replaces the oldest pages first. The page fault rates increase because of the number of frames and a low performance rate.
Mac OS X:
Mac OS X is based off UNIX operating systems and sold by Apple Inc. The main concern is fragmentation or the allocation and de-allocation of memory using pointers leading to unusable small isolated areas of memory (Apple Inc., n.d.). To solve this problem, Apple engineers developed a handle or a memory reference, allowing the data to be moved. A handle is a non-re-locatable pointer that points to the data. Apple implemented two areas: the system heap used for the operating system and the application heap used for running applications. If the system heap was not protected, it would result in frequent system crashes or shut-downs and programming errors. Apple was forced to adopt a scheme allowing applications the ability to access certain areas of the available random access memory because the switcher between heaps did not allow the system to run multiple applications at the same time (Apple Inc., n.d.). This method was also inefficient since the application might use all the random access memory starving the other applications. In system 4.2, MultiFinder was the logical solution for Apple’s engineers. MultiFinder became the Process Manager in System 7 (Apple Inc., n.d.). Apple’s engineers experimented with virtual memory schemes utilizing a third party manager such as Optimem.
UNIX/Linux:
UNIX/Linux has four gigabytes of memory with three gigabytes for the kernel code and the other one gigabyte for the user access. UNIX/Linux has three levels of their paging operations that only allow necessary pages to be swapped into memory, decreasing the swap time and the amount of physical memory needed. The data structure of UNIX/Linux systems depend on the situation. There is a list that is preserved of each structure. This list is used if a page is not found when searched for and it records the range of address, protection, and direction of each movement a page has. UNIX/Linux will convert any entry to this list over 32 characters into a tree, thereby utilizing the protection mode. Virtual pages in this system are labeled as active, inactive-dirty, inactive-clean, and free when placed within any list. When pages have to be replaced, the Least Recently Use (LRU) method of algorithm is used. This is in the counter and stack design. During counters, pages with the shortest times are replaced, and during the stack, the least used are placed at the base of the stack.

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