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Applications, Address Spaces, and Processes

Applications, Address Spaces, and Processes. Separating Units of Computation. Definitions. User mode when the system is executing with the privileged bit off Kernel mode when the system is executing with the privileged bit on Address space

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Applications, Address Spaces, and Processes

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  1. Applications, Address Spaces, and Processes Separating Units of Computation CSE-451 Processes

  2. Definitions • User mode • when the system is executing with the privileged bit off • Kernel mode • when the system is executing with the privileged bit on • Address space • the range of addresses available for a program to use • Legal address space • the range of addresses that a program can use right now CSE-451 Processes

  3. User and Kernel Memory • When the mode bit is set to PRIVILEGED, the kernel can see all of memory • user program, arguments, etc • User memory is like a big data structure for the kernel • But, when the mode bit is off, the user program can only see its own memory • the kernel’s address space is OFF LIMITS • what happens if the user tries? • Good for the OS, and good for the user program CSE-451 Processes

  4. Syscall dispatch OS/User Protection Address Space 0x00000000 main() { int fd = open(“/tmp/foo”); close(fd); } User Program 0x7fffffff /* Syscall Dispatcher */ // determine requested routine // transfer control to requested routine // return result 0x80000000 File system VM system The Operating System 0xffffffff CSE-451 Processes

  5. Address low bits high bits Mode bit To memory Protection Fault C = AB Privileged Memory Protection CSE-451 Processes

  6. Syscall dispatch Inside the User Program User Program _open: load $v0, #SyscallOpen syscall cmp $v0, 0 jne Error move $a0, $v0 ret Error: …. The code we wrote Some code we didn’t write syscall_ent: cmp $v0, #0 // check if good system call jlt Error cmp $v0, #MaxSysCall jgt Error jsr SaveAllNonLinkRegisters load $v0, $v0(SyscallTable) // if so, get api entry point jsr $v0 // go there move $v0, $a0 // result in $a0 load $v0, #0 RestoreAllNonLinkRegisters retsys Error: ... CSE-451 Processes

  7. What Happens on Syscall? • Automatic • Hardware MODE bit flips (go from nonpriv to priv) • Minimal save and restore of context • SP <- Kernel Syscall SP • PC <- Kernel Syscall PC • *SP++ <- User SP • *SP++ <- User PC • What happens on retsys? CSE-451 Processes

  8. And then we pick it up... • Sycall handler checks to make sure we’re asking for a good service • Control is transferred to the service • Result is passed back CSE-451 Processes

  9. Understanding the Stack User stack Old stuff main int fd main+12 0x40040 SP New stuff 0x83000000 USP=0x40040 UPC=_open+12 Kernel stack stack at syscall SP $a0 $a1 $a2… syscall_ent+24 stack at entry to open SP CSE-451 Processes

  10. Concepts So Far • User programs operate out of a different portion of the address space than the kernel • There is a context switch that occurs every time we enter the kernel • Inside the kernel we have expanded privileges • A combination of hardware and software is responsible for this behavior CSE-451 Processes

  11. 0x00000000 0x00000000 0x00000000 0x7fffffff 0x7fffffff 0x7fffffff Multiple Address Spaces • Nearly all operating systems support the abstraction of multiple address spaces Emacs CC Mail User mode 0x80000000 Kernel mode 0xffffffff CSE-451 Processes

  12. A Process • Each address space contains a process • a bunch of text & data • a “thread” in execution • A thread represents the flow of control that is active inside a program • deterministic change of state prescribed by the current state and the PC (which is actually part of the current state) CSE-451 Processes

  13. A Process is a Program in Execution Process In Memory static int z = 5; main(int argc, char **argv) { int x = foo(); printf(“%d\n”, x); } int foo() { return z=23;} Source Code File start PC 0x00000000 thread text 1st instruction z=5 static data cc stack header: “size, start PC” heap Executable File (Program) 0x7fffffff text Create Process a.out CSE-451 Processes

  14. The Thread Of Control argc, argv are on the stack call main call foo set z to 23 return 23 set x to 23 push x push “%d\n” call printf return static int z = 5; main(int argc, char **argv) { int x = foo(); printf(“%d\n”, x); } int foo() { return z=23;} argc argv _exit main+4 23 argc argv _exit 23 23 “%d\n” main+16 stack Thread CSE-451 Processes

  15. Where do Processes Come From? • Remember, a process is an address space with some stuff in it and a thread of control • All operating systems have facilities for creating new processes • Some of them (eg, NT) are quite simple: • CreateAddressSpace, WriteAddressSpace, CreateThreadInAddressSpace, StartThread • Others (eg, UNIX) are more subtle, but quite elegant CSE-451 Processes

  16. Processes Under UNIX • In Unix, the fork() system call is the only way to create a new process • int fork() does many things at once: • creates a new address space (called the child) • copies the parent’s address space into the child’s • starts a new thread of control in the child’s address space • parent and child are equivalent -- almost • in parent, fork() returns a non-zero integer • in child, fork() returns a zero. • difference allows parent and child to distinguish • int fork() returns TWICE! CSE-451 Processes

  17. Example main(int argc, char **argv) { char *myName = argv[1]; int cpid = fork(); if (cpid == 0) { printf(“The child of %s is %d\n”, myName, getpid()); exit(0); } else { printf(“My child is %d\n”, cpid); exit(0); } } What does this program print? CSE-451 Processes

  18. Bizarre But Real lace:tmp<15> cc a.c lace:tmp<16> ./a.out foobar The child of foobar is 23874 My child is 23874 Parent Child fork() retsys v0=23874 v0=0 Operating System CSE-451 Processes

  19. Even More Bizarre lace:tmp<15> cc a.c lace:tmp<16> ./a.out foobar The child of foobar is 23874 My child is 23874 lace:tmp<17> ./a.out foobar My child is 24266 The child of foobar is 24266 lace:tmp<18> Parent Child fork() retsys Why do we get a different answer?? v0=24266 v0=0 Operating System CSE-451 Processes

  20. Fork is half the story • Fork() gets us a new address space, but not one that’s all that different. • parent and child share EVERYTHING • memory, operating system state • int exec(char *programName) completes the picture • throws away the contents of the calling address space • replaces it with the program named by programName • starts executing at header.startPC CSE-451 Processes

  21. Starting a new program main(int argc, char **argv) { char *myName = argv[1]; char *progName = argv[2]; int cpid = fork(); if (cpid == 0) { printf(“The child of %s is %d\n”, myName, getpid()); execl(progName, // executable name progName, 0); // null terminated argv printf(“OH NO. THEY LIED TO ME!!!\n”); } else { printf(“My child is %d\n”, cpid); exit(0); } } CSE-451 Processes

  22. Extra Credit for Friday • Write a simple UNIX program to simulate the UNIX shell in a “read/fork/exec” loop • don’t bother with path searches. All commands can be fully qualified CSE451Shell%/bin/cat /etc/motd DEC OSF/1 V3.2 (Rev. 214); Thu Feb 22 08:48:40 PST 1996 DEC OSF/1 V3.2 Worksystem Software (Rev. 214) This is an AFS fileserver. Please run long running jobs (hours) or memory intensive jobs elsewhere. CSE451Shell%/bin/date Sun Apr 5 22:51:50 PDT 1998 CSE-451 Processes

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