Rensselaer Polytechnic Institute CSC 432 – Operating Systems David Goldschmidt, Ph.D.

very small very large very fast very slow volatile non-volatile

 Based on the von Neumann architecture, data and program instructions exist in physical memory  Repeatedly perform fetch-decode-execute cycles  The execute part often results in data fetch and store operations physical memory

 Locations in memory are identified by memory addresses  When compiled, programs consist of relocatable code  Other compiled modules also consist of relocatable code symbolic addresses in source code relative addresses in object code

 At load time, any additional libraries also consist of relocatable code physical addresses generated by loader

 At run time, memory addresses of all object files are mapped to a single memory space in physical memory

 Using dynamic loading, external libraries are not loaded when a process starts  Libraries are stored on disk in relocatable form  Libraries loaded into memory only when needed  Using dynamic linking, external libraries can be preloaded into shared memory  When a process calls a library function, the corresponding physical address is determined

 Main memory is partitioned and allocated to resident operating system and user processes fixed partitioning scheme

 A pair of base and limit registers define the logical address space  Also known as relocation registers

 The CPU generates logical memory addresses  A Memory-Management Unit (MMU) maps logical memory addresses to the physical address space  User programs never see physical memory addresses

 Hardware protects against memory access outside of a process’s valid memory space

 Variable-length or dynamic partitions:  When a new process enters the system, the process is allocated to a single contiguous block  The operating system maintains a list of allocated partitions and free partitions OS Process 5 Process 8 Process 2 OS Process 5 Process 2 OS Process 5 Process 2 Process 9 OS Process 5 Process 9 Process 2 Process 1

 How can we place new process P i in memory?  First-fit algorithm: allocate the first free block that’s large enough to accommodate P i  Best-fit algorithm: allocate the smallest free block that’s large enough to accommodate P i  Next-fit algorithm: allocate the next free block, searching from last allocated block  Worst-fit algorithm: allocate the largest free block that’s large enough to accommodate P i

 Memory is wasted due to fragmentation, which can cause performance issues  Internal fragmentation is wasted memory within a partition or process memory  External fragmentation can reduce the number of runnable processes ▪ Total memory space exists to satisfy a memory request, but memory is not contiguous OS Process 5 Process 8 Process 2 Process 3 Process 6 Process 12 Process 7 Process 9

Process 3 Process 6 Process 12 Process 7  Reduce external fragmentation by compaction or defragmentation  Rearrange memory contents to organize all free memory blocks together into one large contiguous block  Compaction is possible only if relocation is dynamic and is done at execution time  Compaction is expensive OS Process 5 Process 8 Process 2 Process 9 Process 3 Process 6 Process 12 Process 7 Process 3 Process 6 Process 12 Process 7 Process 9