CS 162 Operating Systems and Systems Programming 0x000000 Professor Anthony D Joseph Spring 2003 Application Lecture 12 Protection Kernel and Address Spaces Physical Memory 12 0 Main Points Kernel vs user mode What is an address space How is it implemented Operating System 0xFFFFFF Physical memory Abstraction virtual memory No protection Each program isolated from all others and from the OS Illusion of infinite memory Transparent can t tell if memory is shared Ability to share code data Limited size Sharing visible to programs Easy to share data between programs 12 1 2 Multiprogramming without protection Linker loader Can multiple programs share physical memory without hardware translation Yes when a program is copied into memory its addresses are changed loads stores jumps to use the addresses of where the program lands in memory This is done by a linker loader Used to be very common 12 1 Operating system organizations 0x00000 Application1 12 1 1 Uniprogramming without protection Early personal computer operating systems application always runs at the same place in physical memory because each application runs one at a time application given illusion of dedicated machine by giving it reality of a dedicated machine For example load application into low memory operating 0x20000 Application2 Physical Memory Operating System system into high memory Application can address any physical memory location 0xFFFFFF CS 162 Spring 2003 Lecture 12 1 13 CS 162 Spring 2003 Lecture 12 2 13 12 2 Address translation UNIX ld does the linking portion of this despite its name deriving from loading compiler generates each o file with code that starts at location 0 Address space literally all the addresses a program can touch All the state that a program can affect or be affected by How do you create an executable from this Scan through each o changing addresses to point to where each module goes in larger program requires help from compiler to say where all the relocatable addresses are stored Restrict what a program can do by restricting what it can touch Hardware translates every memory reference from virtual addresses to physical addresses software sets up and manages the mapping in the translation box 12 1 3 Multiprogrammed OS with protection Virtual Address Goal of protection Keep user programs from crashing corrupting OS Keep user programs from crashing corrupting each other CPU Translation Box MMU Physical Address Physical Memory How is protection implemented Hardware support Address translation Dual mode operation kernel vs user mode Data read or write untranslated Address Translation in Modern Architectures Two views of memory View from the CPU what program sees virtual memory View from memory physical memory Application User mode Application library Translation box converts between the two views Kernel mode Portable OS layer Translation helps implement protection because there s no way for programs to even talk about other programs addresses no way for them to touch operating system code or data Machine dependent OS layer Hardware Translation can be implemented in any number of ways typically by some form of table lookup we ll discuss various options for implementing the translation box later Separate table for each user address space Typical Operating System Structure CS 162 Spring 2003 Lecture 12 3 13 CS 162 Spring 2003 Lecture 12 4 13 12 3 Dual mode operation Can application modify its own translation tables If it could could get access to all of physical memory Has to be restricted somehow Dual mode operation When in the OS can do anything kernel mode When in a user program restricted to only touching that program s memory user mode Hardware requires CPU to be in kernel mode to modify address translation tables 12 3 2 User kernel How does the user program get back into the kernel Voluntarily user kernel System call special instruction to jump to a specific operating system handler Just like doing a procedure call into the operating system kernel program asks OS kernel please do something on procedure s behalf Can the user program call any routine in the OS No Just specific ones the OS says are OK Always start running handler at same place otherwise problems In Nachos as well as most OS s OS runs in kernel mode untranslated addresses User programs run in user mode translated addresses Want to isolate each address space so its behavior can t do any harm except to itself A couple issues 1 How to share CPU between kernel and user programs 2 How do programs interact 3 How does one switch between kernel and user modes when the CPU gets shared between the OS and a user program OS user kernel user mode User OS user mode kernel mode 12 3 1 Kernel user To run a user program create a thread to Allocate and initialize address space control block Read program off disk and store in memory Allocate and initialize translation table point to program memory Run program or to return to user level after calling the OS with a system call Set machine registers CS 162 Spring 2003 Lecture 12 Set hardware pointer to translation table Set processor status word from kernel mode to user mode Jump to start of program 5 13 How does OS know that system call arguments are as expected It can t OS kernel has to check all arguments otherwise bug in user program can crash kernel Involuntarily user kernel Hardware interrupt also program exception Examples of program exceptions Bus error bad address e g unaligned access Segmentation fault out of range address Page fault important for providing illusion of infinite memory On system call interrupt or exception hardware atomically Sets processor status to kernel mode Changes execution stack to an OS kernel stack Saves current program counter Jumps to handler routine in OS kernel Handler saves previous state of any registers it uses CS 162 Spring 2003 Lecture 12 6 13 Context switching between programs same as with threads except now also save and restore pointer to translation table To resume a program re load registers change PSL hardware pointer to translation table and jump to old PC How does the system call pass arguments Two choices a Use registers Can t pass very much that way b Write into user memory kernel copies into its memory Except User addresses translated Kernel addresses untranslated Addresses the kernel sees are not the same addresses as what the user sees We ll explore this riddle in a later lecture 12 3 3 Communication between address spaces How do two
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