Memory Management (Linux Kernel)
The data related to the processes are to be stored in the memory somehow by the operating system. In the olden days, primary memory is a scarce resource. and the application demands increases, so store the data of a process, it is necessary to use other forms of memory, either we can use
- Physical address space
- virtual address space
- Mapping the virtual addresses onto the physical addresses is the responsibility of the operating system’s memory management system.
- Memory protection prevents two processes exchanging data by changing the areas of memory used by both.
- The program code of a process can be executed in different threads at the same time. In principle, these threads share a common virtual address space.
Architecture Independent Memory Model
- Access time is the primary factor used in Physical (main) memory. To minimize the access times, the cache memory versions like Level1 Cache (L1 cache) and level2 cache (L2 Cache) were used.
- Cache memory improves the performance of Personal Computers.
- The physical memory is divided into pages (page memory) and the size is determined by the PAGE_SIZE macro of the asm/page.h file.
- For the x86 architecture, the page size is usually 4kbytes and 8kbytes for the 64 bit architecture and 64kbytes for the IA64 architecture
- The memory management units (MMU) of various architecture has the support the page size of 4Mbytes, which is necessary when the mapping between the physical address and virtual address takes place.
Virtual Address Space
- The Code and the data of a process is scattered in the virtual address space.
- the size of the Virtual address space depends on the number of bits used in the memory address, for example for a 32 bit memory address, nearly 4GBytes is available and for 64 bit memory address 16exabytes are available.
- usually the process can be either in the user mode or system mode. Each process starts in the user mode initially and changed to system mode when there is a need to access the system resources.
- The standard procedure to change from the user mode to the system mode is to call the software interrupt 0x80. In the user mode, access to kernel memory is forbidden. Access to the memory page can be granted to system mode or user mode.
- The physical memory of the kernel maps to the virtual memory only after an offset (PAGE_OFFSET), but this memory space is permitted only under the system mode.
- For this reason, the user address space is always smaller than the theoretical maximum. For example, for x86 – Page Size (4Kbytes) – User Address Space (3GB), but the theoretical maximum is 4GB.