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Operating System Concepts (8)

Module 8: Memory Management.• Background.• Logical versus Physical Address Space.• Swapping.• Contiguous Allocation.• Paging.• Segmentation.• Segmentation with Paging. 8.1 Silberschatz and Galvin 1999 Background.• Program must be brought into memory and placed within a. process for it to be executed• Input queue – collection of processes on the disk that are waiting. to be brought into memory for execution• User programs go through several steps before being executed 8.2 Silberschatz and Galvin 1999 Binding of Instructions and Data to Memory.Address binding of instructions and data to memory addresses can.happen at three different stages. • Compile time: If memory location known a priori, absolute code. can be generated; must recompile code if starting location. changes • Load time: Must generate relocatable code if memory location is. not known at compile time • Execution time: Binding delayed until run time if the process. can be moved during its execution from one memory segment to. another. Need hardware support for address maps (e.g., base. and limit registers) 8.3 Silberschatz and Galvin 1999 Dynamic Loading.• Routine is not loaded until it is called.• Better memory-space utilization; unused routine is never loaded• Useful when large amounts of code are needed to handle. infrequently occurring cases• No special support from the operating system is required. implemented through program design 8.4 Silberschatz and Galvin 1999 Dynamic Linking.• Linking postponed until execution time• Small piece of code, stub, used to locate the appropriate memory-. resident library routine• Stub replaces itself with the address of the routine, and executes. the routine• Operating system needed to check if routine is in processes’. memory address 8.5 Silberschatz and Galvin 1999 Overlays.• Keep in memory only those instructions and data that are needed. at any given time• Needed when process is larger than amount of memory allocated. to it• Implemented by user, no special support needed from operating. system, programming design of overlay structure is complex. 8.6 Silberschatz and Galvin 1999 Logical vs. Physical Address Space.• The concept of a logical address space that is bound to a. separate physical address space is central to proper memory. management – Logical address – generated by the CPU; also referred to as. virtual address – Physical address – address seen by the memory unit• 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 8.7 Silberschatz and Galvin 1999 Memory-Management Unit (MMU).• Hardware device that maps virtual to physical address• In MMU scheme, the value in the relocation register is added to. every address generated by a user process at the time it is sent to. memory• The user program deals with logical addresses; it never sees the. real physical addresses 8.8 Silberschatz and Galvin 1999 Swapping.• A process can be swapped temporarily out of memory to a. backing store, and then brought back into memory for continued. execution• Backing store – fast disk l