After studying this chapter, you should be able to: Discuss the principal requirements for memory management, understand the reason for memory partitioning and explain the various techniques that are used, understand and explain the concept of paging,...
Chapter Memory Management • Basic requirements of Memory Management • Memory Partitioning • Paging • Segmentation Memory Management A program must be loaded into main memory to be executed The principal operation of memory management is to bring processes into main memory for execution by the processor Memory Management Memory needs to be allocated to ensure a reasonable supply of ready processes to consume available processor time – Otherwise, for much of the time all of the processes will be waiting for I/O and the processor will be idle The need for memory management • Memory is cheap today, and getting cheaper – But applications are demanding more and more memory, there is never enough! • Memory Management involves swapping blocks of data from secondary storage • Memory I/O is slow compared to CPU – The OS must cleverly time the swapping to maximise the CPU’s efficiency Memory Management Requirements • Relocation • Protection • Sharing • Logical organisation • Physical organisation Relocation • The programmer does not know where the program will be placed in memory when it is executed, – it may be swapped to disk and return to main memory at a different location (relocated) • But, OS knows because it is managing memory and is responsible for bringing this process into main memory Relocation Addressing The processor and OS must be able to translate the memory references found in the code of the program into actual physical memory addresses (to be discussed) Protection • Processes should not be able to reference memory locations in another process without permission • Impossible to check absolute addresses at compile time because the location of a program in main memory is unpredictable • Must be checked at run time by the processor Sharing • Allow several processes to access the same portion of memory – Better to allow each process executing the same program access to the same copy of the program rather than have their own separate copy – Processes that are cooperating on some task may need to share access to the same data structure Logical Organization • Memory is organized linearly (usually) • In contrast, programs are organized into modules – Modules can be written and compiled independently – Different degrees of protection can be given to different modules (read-only, execute-only) – Modules can be shared among processes • Segmentation helps here 10 Relocation • The actual (absolute) memory locations are determined when program is loaded into memory • A process may occupy different partitions which means different absolute memory locations during execution – Swapping – Compaction 29 Addresses • Logical – Reference to a memory location independent of the current assignment of data to memory – A relative address is expressed as a location relative to some known point (typically the program origin) • Physical or Absolute – The absolute address or actual location in main memory A translation must be made from a logical address to a physical address before memory access can be achieved 30 Relocation Hardware Support Add the value in the base register to the relative address to produce an absolute address Initialize base and bounds registers when a process is assigned to the Running state Compare the resulting address to the value in the bounds register 31 Relocation Registers Used during Execution • Base register – Starting address for the process • Bounds register – Ending location of the process • These values are set when the process is loaded or when the process is swapped in 32 Relocation Registers Used during Execution • The value of the base register is added to a relative address to produce an absolute address • The resulting address is compared with the value in the bounds register – If the address is within bounds, instruction execution may proceed – Otherwise, an interrupt indicating error is generated to OS 33 Paging • Partition memory into small equal fixedsize chunks (frames) and divide each process into the same size chunks (pages) • Pages of process could be assigned to available frames of memory • No external fragmentation but little internal fragmentation consisting of only a fraction of the last page of a process 34 Paging Processes and Frames A.0 A.1 A.2 A.3 D.0 B.0 D.1 B.1 D.2 B.2 C.0 C.1 C.2 C.3 D.3 D.4 OS finds free frames and loads the pages of Process A, B & C Process B is suspended and is swapped out of main memory All of the processes in main memory are blocked, and OS brings in Process D 35 Paging • Operating system maintains a page table for each process which contains the frame location for each page in the process • Given a logical address (page number, offset), the processor uses the page table to produce a physical address 36 Paging Page Table page number frame number 37 Paging Logical Addresses • Using a page size that is a power of 2, a logical address (page no., offset) is identical to its relative address • Example • • • • • 16-bit address 210 =1024-byte page 10-bit offset 6-bit page number A maximum of 26 =64 pages 38 Paging Logical to Physical Address Translation Consider an address of n+m bits, where the leftmost n bits are the page no and the rightmost m bits are the offset Extract the page no Use the page no as an index into the process page table to find the frame number, k The physical address is constructed by appending the offset to k 39 Segmentation • A program can be subdivided into segments – Segments may vary in length – There is a maximum segment length • Segmentation is similar to dynamic partitioning – But, a program may occupy more than one partition • No internal fragmentation but suffers from external fragmentation (as does dynamic partitioning) 40 Segmentation • A logical address consists of two parts – a segment number and – an offset • There is a segment table for each process and a list of free blocks of main memory • Each segment table entry would have to give – the starting address in main memory of the corresponding segment – the length of the segment, to assure that invalid addresses are not used 41 Segmentation Logical Addresses • There is no simple relationship between a logical address (segment no., offset) and the physical address • Example • • • • 16-bit address 12-bit offset 4-bit segment number maximum segment size= 212=4096 42 Segmentation Logical to Physical Address Translation Consider an address of n + m bits, where the leftmost n bits are the segment no and the rightmost m bits are the offset 1.Extract the segment no 2.Use the segment no as an index into the process segment table to find the starting physical address of the segment If the offset the length of the segment, the address is invalid 4.The physical address is the sum of the starting physical address of the segment plus the offset 43 .. .Memory Management A program must be loaded into main memory to be executed The principal operation of memory management is to bring processes into main memory for execution by the processor Memory. .. be idle The need for memory management • Memory is cheap today, and getting cheaper – But applications are demanding more and more memory, there is never enough! • Memory Management involves swapping... memory should be a system responsibility 11 Partitioning • An early method of managing memory – Pre-virtual memory – Not used much now • But, it will clarify the later discussion of virtual memory