Memory Questions? ! What is main memory? CSCI [4|6]730 ! How does - PDF document

Memory Questions? ! What is main memory? CSCI [4|6]730 ! How does multiple processes share memory Operating Systems space? » Key is how do they refer to memory addresses? ! What is static and dynamic allocation? Main Memory ! What is segmentation ? Maria Hybinette, UGA Maria Hybinette, UGA Review: Motivation for Multiprogramming Multiprogramming Goals Uniprocessing: One process runs at a time High Address (0x7fffffff) 2 n -1 ! Sharing Physical » Several processes coexist in main memory Memory OS Stack » Cooperating processes can share portions of address space ! Transparency » Processes are not aware that memory is shared » Works regardless of number and/or location of processes User Address ! Protection Space Heap Process » Cannot corrupt OS or other processes » Privacy: Cannot read data of other processes Code 0 ! Efficiency ! Disadvantages: Low Address » Do not waste CPU or memory resources (0x00000000) » Only one process runs at a time » Keep fragmentation low (later) » Process can destroy OS Maria Hybinette, UGA Maria Hybinette, UGA Memory Addresses Static Relocation (after loading) System Memory ! Address space ! Goal: Allow transparent sharing - Each address space may be » What we go so far: Initial P 0 placed anywhere in memory – Physical addresses Process P 1 » OS finds free space for new process » Modify addresses statically (similar Process P 2 to linker) when loading the process » Fixed addresses. Process P 3 ! Advantages: » Allows multiple processes to run » Requires no hardware support Maria Hybinette, UGA Maria Hybinette, UGA

Static Reallocation Dynamic Relocation System Memory ! Disadvantages: ! Goal: Protect processes from one another » No protection ! Requires hardware support Initial P 0 – Process can destroy OS or other » Memory Management Unit (MMU) processes ! MMU dynamically changes process address at every Process P 1 – No privacy memory reference (compute address on-the-fly) » Address space must be allocated Process P 2 » Process generates logical or virtual addresses contiguously » Memory hardware uses physical or real addresses – Allocate space for worst-case stack and heap Process P 3 Process runs here – Processes may not grow OS can control MMU » Cannot move process after they are CPU MMU placed or loaded (static addresses) Memory Process P 4 » Fragmentation (later) Logical address Physical address Maria Hybinette, UGA Maria Hybinette, UGA Hardware Support for Implementation of Dynamic Relocation Dynamic Relocation ! Translation on every memory access of user process ! Two operating modes » MMU compares logical address to bounds register » Privileged (protected, kernel) mode: OS runs – if logical address is greater, then generate error – When enter OS (trap, system calls, interrupts, exceptions) » MMU adds base register to logical address to form physical address – Allows certain instructions to be executed ! Can manipulate contents of MMU – Allows OS to access all of physical memory logical » User mode: User processes run mode ! no " physical = ! address user? " address – Perform translation of logical address to physical address Registers: " ! MMU contains base and bounds registers mode ! 1 bit " yes " » base: start location for address space (physical address) » bounds: size limit of address space (memory span) bounds ! no " + ! 32 < " error bounds? " bits " base ! base ! 10 Maria Hybinette, UGA Maria Hybinette, UGA Managing Processes Example of Dynamic Relocation with Base and Bounds ! What are the physical addresses for the following 16- ! Context-switch bit logical addresses (HEX: highest F:1111)? » Add base and bounds registers to PCB ! Process 1: base: 0x4320 , bounds: 0x2220 (in HEX) � » Steps: » 0x0000: � 1. Change to privileged mode 2. Save base and bounds registers of old process » 0x1110: � 3. Load base and bounds registers of new process » 0x3000: � 4. Change to user mode and jump to new process ! Process 2: base: 0x8540 , bounds: 0x3330 � ! What if don ’ t change base and bounds registers when » 0x0000: � switch? » 0x1110: � ! Protection requirement » 0x3000: � » User process cannot change base and bounds registers ! Operating System » User process cannot change to privileged mode » 0x0000: � » 0x5FFF: � 12 Maria Hybinette, UGA Maria Hybinette, UGA

Base and Bounds Discussion Segmentation 0 " ! Advantages heap " ! Divide address space into Operating » Provides protection (both read and System logical segments write) across address spaces stacj " » Each segment » Supports dynamic relocation 30004 " 30004 " symbol " corresponds to logical subroutine " – Can move address spaces Process base " table " entity in address space – Why might you want to do this? 42094 " 12090 " – code, stack, heap » Simple, inexpensive: Few registers, bound " little logic in MMU Process ! Each segment can symbol " subroutine " table " » Fast: Add and compare can be done in independently: main " table " program " parallel » be placed separately in ! Disadvantages physical memory stack " » Each process must be allocated » grow and shrink main " contiguously in physical memory Process program " » be protected (separate – Must allocate memory that may not be heap " read/write/execute used by process protection bits) » No partial sharing: Cannot share limited parts of address space Logical Address Physical Address 13 14 Space Space Maria Hybinette, UGA Maria Hybinette, UGA Segmented Addressing Segmentation Implementation ! MMU contains Segment Table (per process) ! How does process designate a particular » Each segment has own base and bounds, protection bits » Example: 14 bit logical address, 4 segments segment? Segment Base Bounds R W » Use part of logical address – Top bits of logical address select segment 0 0x2000 0x06ff 1 0 – Low bits of logical address select offset within 1 0x0000 0x04ff 1 0 segment 2 0x3000 0x0fff 1 1 3 0x1000 0x0fff 0 0 ! Translate logical addresses ! physical addresses: » 0x0240: 0 th segment 240 internal address within segment ! what address? � » 0x1108: � » 0x265c: � » 0x3002: � 15 16 Maria Hybinette, UGA Maria Hybinette, UGA When to Bind Discussion of Segmentation Physical & Logical Addresses Source " Program " ! Compile time: If memory location ! Advantages known a priori, absolute code can be » Enables sparse allocation of address space Compiler or generated; must recompile code if Assembler – Stack and heap can grow independently starting location changes – Heap: If no data on free list, dynamic memory allocator requests more from OS (e.g., UNIX: malloc calls sbrk()) ! Load time: Must generate Object " Other " – Stack: OS recognizes reference outside legal segment, Module " relocatable code if memory location Object " extends stack implicitly Modules " is not known at compile time » Different protection for different segments Linkage ! Execution time: Binding delayed editor – Read-only status for code until run time if the process can be System " » Enables sharing of selected segments Library " Load " moved during its execution from » Supports dynamic relocation of each segment module " one memory segment to another. ! Disadvantages Need hardware support for address » Each segment must be allocated contiguously loader System " maps (e.g., base and limit registers) Library " – May not have sufficient physical memory for large segments In-memory Binary 17 18 Memory image Maria Hybinette, UGA Maria Hybinette, UGA