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Enforcing Modularity

Enforcing Modularity. Junehwa Song CS KAIST. How to run multiple modules?. X server. Emacs. Mail Reader. File Server. Designing a Virtual Memory System. Can multiple Modules share MM?. Partition MM space? Intentional/unintentional intrusion LOAD STORE. …. Emacs. Mail. ……. ….

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Enforcing Modularity

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  1. Enforcing Modularity Junehwa Song CS KAIST

  2. How to run multiple modules? X server Emacs Mail Reader File Server Network Computing Lab.

  3. Designing a Virtual Memory System

  4. Can multiple Modules share MM? • Partition MM space? • Intentional/unintentional intrusion • LOAD • STORE … Emacs Mail …… … … X server Network Computing Lab.

  5. Then, What Can We Do??? Address Main Memory PROCESSOR Read/Write Data Virtual Address Physical Address Main Memory Virtual Memory Manager PROCESSOR Read/Write Read/Write Data Network Computing Lab.

  6. Address Translation • Page Map (Page Table) • Virtual Address Space • Segmentation • int PageTable[size] • int translate(int virtual) /* HW */ Network Computing Lab.

  7. Virtual address Address Translation Page # Byte Offset Block # Page Map (Page Table) Block # Byte Offset BTW, where do you want to put Page Map? Physical address

  8. Can multiple Modules share MM? 0 Page 10 (VM1) Physical Address 100 Virtual Address Page 10 (VM1) Page # Offset Block # Offset Virtual Memory Manager Page 12 (VM2) PROCESSOR 200 Read/Write Read/Write Unused block 300 300 10 100 Data Page Map of VM1 Page Map Address Register 11 400 12 0 400 Page 11 (VM1) Thus, a Page Map defines an address space 500 Page Map of VM2 12 100 13 800 Network Computing Lab.

  9. What do we further need to do? • Things to do • Create/Delete an address space • Grow an address space • Map a device to an address space • Switch an address space to another • Create-AS() { 1. identify an unused memory block; 2. generate a new address space id, ID ; 3. make a page map, PAGEMAP(ID) and initialize a page map ; 4. return (ID); } • Delete-AS(ID) { for each block of each entry in PAGEMAP(ID) free(block) ; free(PAGEMAP(ID)); } • Add-page(ID, page#) { 1. search for an unused block. Let’ the block be NEWBLOCK ; 2. make an entry (page#, NEWBLOCK) in PageMap(ID); } • Delete-page(ID, page#) • MAP(ID, page#, block) • Swich-AS() Network Computing Lab.

  10. Things to do • Create-AS() { } • Delete-AS(ID) { } • Add-page(ID, page#) { } • Delete-page(ID, page#) { 1. Search for the entry (page #, block) in PAGEMAP(ID) ; 2. Free(block); 3. Remove the entry from PAGEMAP(ID); } • MAP(ID, page#, block) { insert a new entry (page#, block) to PAGEMAP(ID); } • Swich-AS(ID) { 1. Change the address space to ID; 2. Load page map address register with the address of PAGEMAP(ID); /* we will come back to this later again*/ } Network Computing Lab.

  11. Virtual Memory System • Now, multiple programs can share a main memory • Each module has its own virtual memory space • Memory operations, e.g., LOAD, STORE, or control sequence instructions such as JMP are localized to its virtual address space. Network Computing Lab.

  12. Do we need a protected area in memory? • What if a module accidentally change a page map or page map address register? • Separate, special address space, called KERNEL address space • Put all page maps as well as the virtual memory manager programs in KERNEL • Define a flag bit (in a flag register) to designate if we are currently in KERNEL MODE or in user mode • In user mode, nobody can change the value the page map address register Kernel Mail Reader • Create-AS • Delete-AS • Add-page • Delete-page • Switch-AS • Map Text Editor Network Computing Lab.

  13. Interfaces to Kernel • Different ways to get a service from Kernel • A device signals an Interrupt • A user module executes an illegal instruction (exception), e.g., divide by zero • A user module executes a special instruction (system-call) • Create-AS, Delete-AS, Add-page, Delete-page, Switch-AS, Map 등등이 user module에게 주어지려면 system-call로 주어져야 함. Network Computing Lab.

  14. Exceptions and Interrupts • Interrupt • Maskable Interrupt • Non-maskable interrupt • Exceptions • Processor-detected exceptions: Deferin the addresses stored in STACK • Fault : the address of the instruction which generated Fault is saved • E.g., page fault exception handler • Trap : that of the next instruction • Generated when there is no need to re-execute the instruction • Mainly used for debugging, i.e., to notify the debugger that a specific instruction has been executed • Abort : cannot save anything • Process will be terminated • Programmed Exceptions (Software Interrupt) • E.g., in linux, generated by INT, INT3, and conditionally by INT0 and BOUND • Handled as a trapby CPU • For System Call and for notification to debugger of a specific event Network Computing Lab.

  15. 잠깐만! Our Interpreter Model • “4-register with Interrupt” Interpreter Model • 4 registers: • IC (Instruction Counter) • SP (Stack Pointer) • this means that each process (or thread) is given a stack • Flag (Flag Register) • Interrupt bit, kernel mode bit 포함 • PMAR (Page Map Address Register) • Interrupt is provided • Software interrupt is also provided Network Computing Lab.

  16. Things to do to enter/leave Kernel • The following can be common to three different kernel access methods • Let’s assume 4-register with Interrupt model • Change-mode-enter-Kernel (SYSCALL instruction) • Change mode from user to kernel : set kernel mode flag on • Save Page map address register (stack) and load that of the kernel • Save flags register (Stack) • Save Instruction counter (stack) and reload that of the kernel /* handling SP will be considered later….. E.g., ThreadTable entry에 …*/ • Change-mode-leave-Kernel (RTE instruction) • Change mode from kernel to user : set kernel mode flag off • Reload Page map address register (stack) • Pop flags register (stack) • Reload Instruction counter (stack) • Should be atomic actions • If not? • Processors implement the sequences differently • Can be completely or partly done in HW • E.g. ??? Network Computing Lab.

  17. Mode-Change Operations • Very expensive operation • Number of instructions • Invalidate or Clean up things (e.g., pipeline, cache, etc) Network Computing Lab.

  18. Switching address spaces • Switch-AS(ID) { • Change the address space to ID; • Load page map address register with the address of PAGEMAP(ID); } only kernel can change page map address register, thus • Switch-AS(ID) { • Change to KERNEL; /* change-mode-enter-kernel */ • Load page map address register to PAGEMAP(ID); • Change to the address space, ID; /*change-mode-leave-kernel */ } Network Computing Lab.

  19. Q/A • Isn’t it too slow doing address translation? • MMU • TLB • Do we want to see the details of a PageMap? Network Computing Lab.

  20. Designing a Virtual Processor System

  21. Running multiple modules in a processor? Abstraction of a Module in Execution Module A Module B Start execution Temporarily stop Resume execution Temporarily stop Network Computing Lab.

  22. Abstraction of a Module in Execution • An abstraction of a running program and its state • so that we can stop and resume an execution of a program at any time • Then, we can simulate a virtual processor for each module. • What do we need? Network Computing Lab.

  23. Thread • An abstraction of a running program and its state • We should be able to save and load a thread state, including • Next step of a thread • Environment of a thread • Registers • general purpose registers, • stack pointer, • flag register, etc • Pointer to address space: page map address register Network Computing Lab.

  24. First Trial !!! • Let’s make it simple • A very simple Virtual Thread Manager • Let’s assume that • the state of each thread is stored in its own stack • A function called yield() • ThreadTable[] yield() { • Save the current value of SP (current stack pointer) to ThreadTable[] • Select next thread to run // this part is a “scheduling” task • Load the stack pointer of the next thread to SP } Network Computing Lab.

  25. First Trial !!! (example) Thread 6 in execution  yield Thread 0 resumes Thread Table 100 0 Stack for Thread 6 1 204 …. 1042 1042 200 200 6 200 …. Next thread 0 0 6 Stack for Thread 0 200 204 200 100 Stack Pointer 3 104 100 Network Computing Lab.

  26. First Trial!!! (we still need a bit more) • Creat-Thread() • Exit-Thread() • Destroy-Thread() Network Computing Lab.

  27. First Trial!!! • Create-Thread (module_address) • Allocate space for a new stack • Initialize the stack • Push the address of “exit-thread()” • Push the address of (start of) module • Initialize the entry in the Thread-Table[] Network Computing Lab.

  28. 2048 Address of “exit-thread()” 396 Stack for Thread 7 392 388 Thread Table …. 100 0 300 1 …. Stack for Thread 6 …. 200 200 6 204 1042 1042 7 388 200 …. Next thread 6 0 0 204 200 100 Stack Pointer Stack for Thread 0 3 104 100 Network Computing Lab.

  29. First Trial!!! • Exit-Thread () • De-allocate space for a stack • De-allocate the entry in the Thread-Table[] Network Computing Lab.

  30. Design with Sequence Coordination • Sequence coordination with simple version <Editor module> ... While (input_byte_count <= processed_byte_count) { yield() } … <Keyboard module> … While a char is ready { input_byte_count++; do something; } … • Problems: • The shared variable input_byte_count should be carefully used • Editor module should repeatedly call yield() Network Computing Lab.

  31. Design with Sequence Coordination • Can we do any better? • What about making something similar to “interrupt”? • Wait() and notify() • Notify(eventcount) • Wait(eventcount, value) <Editor Module> … Eventcount input_byte_count; While (input_byte_count <= processed_byte_count) { wait(input_byte_count, processed_byte_count); } … <Keyboard module> … Input_byte_count++; notify(input_byte_count); … Network Computing Lab.

  32. Design with Sequence Coordination • What do we need to do to implement “virtual processor” with wait() and notify()? • We are making a thread to be a wait state, and ready (to run) state. Thus, a thread can be in a {running, ready, waiting} Create-thread() Notify() Schedule() waiting ready running Yield() Exit-thread() Wait() Network Computing Lab.

  33. Virtual Thread Manager with Wait and Notify • Struct thread { int SP ; //value of the stack pointer Int state ; //wait, ready, or run Int *event //if waiting, the eventcount we are waiting for } ThreadTable[] • Yield() { ThreadTable[me].SP = SP; //save my stack pointer scheduler() ; } • Scheduler() { do { // select a runnable thread next_thread = (next_thread +1) %7 ; //select a thread by a certain policy, here “round robin” } while (ThreadTable[next_thread].state == waiting) ; SP = ThreadTable[next_thread].SP ; //load SP register return ; // pop return address from stack } • Wait() { } • Notify() { } Network Computing Lab.

  34. Virtual Thread Manager with Wait and Notify • Struct thread { int SP ; //value of the stack pointer Int state ; //wait, ready, or run Int *event //if waiting, the eventcount we are waiting for } ThreadTable[] • Yield() { } • Scheduler() { } • Wait(eventcount, value) { ThreadTable[me].event = &eventcount; ThreadTable[me].state = waiting; if ( *eventcount > value) ThreadTable[me].state = ready ; scheduler() ; } • Notify(eventcount) { for ( i=0 ; i < size ; i++) { if ( (ThreadTable[i].state == WAITING) &&(ThreadTable[i].event==&eventcount) ) ThreadTable[i].state = ready ; } } Network Computing Lab.

  35. Virtual Thread Manager with Wait and Notify • Again, we should be cautious in using a shared variable, especially when it is updated. In this case, the updated shared variable is “ThreadTable[me].state” • What if Wait(eventcount, value) { ThreadTable[me].event = &eventcount; ThreadTable[me].state = waiting; if ( *eventcount > value) { ThreadTable[me].state = ready ; scheduler() ; } else ThreadTable[me].state = waiting; } Network Computing Lab.

  36. Virtual Thread Manager with Wait and Notify • Be cautious in using Wait() and Notify() • What if everybody waits and nobody notifies? • What can we do for the case? • Deadlock Network Computing Lab.

  37. Interface of our Simple Kernel Text Editor System Call Interrupt Mail Reader Exception • Create-AS • Delete-AS • Add-page • Delete-page • Switch-AS • Map • Create-thread • Exit-thread • Destroy-thread • yield • Register-gate • Transfer-to-gate We can say that the above is A simple Kernel model based on 4-register with interrupt interpreter model Network Computing Lab.

  38. Design of a Simple OS • Initialization Sequence • Reset • Boot • Kernel initialization • Initialization of initial applications • Reset • Turn on of computer • Load boot program from ROM • Virtual and physical address 0 • Boot • Read kernel program from disk and initialize • Kernel is located in a pre-agreed address of disk (or floppy disk) • Kernel is loaded into a pre-agreed address of the physical memory • CPU starts the first instruction of the kernel Network Computing Lab.

  39. Design of a simple OS • Kernel initialization • Sets on the supervisor bit (kernel mode bit) • Sets off the interrupt bit • Assume that interrupt does not occur in kernel mode • Simplification to make the kernel design simple • Allocate the Kernel stack • Preparation for Procedure call in kernel • Use add-page() • Make its own page table • Allocate several blocks • Start fromm a specific address, say KERNELPAGEMAP • Make the map • For simplification, use a simple mapping • 0  0, 11, etc • Fill up the PageMapAddress register • With KERNELPAGEMAP • Then, address translation starts Network Computing Lab.

  40. Design of a simple OS • Initialization of initial applications • Assume that initial applications are located in a pre-agreed places on DISK • Use Create-AS() for an application • Allocate several blocks • Use Add-Page() • Starts from a pre-agreed address, say, FIRSTUSER • Read program into the blocks • Make a Page Map • Allocate blocks for Page Map • Make the map, Assign FIRSTUSER virtual address 0 • Make a Stack • From the other end of the address space • Push 0 to the stack • To consider RTE instruction • Switch to the application • Switch-AS() Network Computing Lab.

  41. Kernel Organization: Monolithic Kernel Text Editor Mail Reader File Manager VM Thread Manager Window Manager Network Manager Network Computing Lab.

  42. Kernel Organization: Microkernel Text Editor Window Manager Mail Reader Network Manager File Manager Communication Manager VM Thread Manager Network Computing Lab.

  43. Kernel Organization • Which is better? • Monolithic .vs. Micro-kernel • Any other ways? • E.g., exokernel ???? Network Computing Lab.

  44. Design Project • So far, we designed a simple OS and Simple Kernel based on 4-register with interrupt interpreter model. Then, we want to design a different one. • Give specification of a simple artificial machine and have students design a simple OS for it • We may extend or restrict the “4 –register with interrupt” interpreter model • Eg. A machine • A program can be loaded into at maximum 3 blocks • 5 programs can run at the same time (5 threads) • DISK access operations are given • Give a simple vocabulary • No interrupt in a kernel mode • Size of the physical memory • Block size x number of blocks • Initial applications and the kernel program are located in specific addresses of the disk Network Computing Lab.

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