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CS552

CS552. Overview. Generalized O/S structure. Basic interrupt handler Determines cause Uses branch table to specific handler I/O manager Performs I/O for the user’s requests Directly Via drivers Paging Basic error handling Scheduler Dispatcher.

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CS552

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  1. CS552 Overview

  2. Generalized O/S structure • Basic interrupt handler • Determines cause • Uses branch table to specific handler • I/O manager • Performs I/O for the user’s requests • Directly • Via drivers • Paging • Basic error handling • Scheduler • Dispatcher

  3. Generalized O/S Operation“Booting” & login • Load self into RAM • Display logon screen • Enter h/w wait state - NO CPU cycles occur • Display pressed keys on screen • Call logon manager • Create a Process Control Block for user • Create an address space for user • Load GUI code into user’s address space • Load “user mode” state with address of GUI

  4. Generalized GUI Operation • Create windows and buttons • Wait for user interaction • User presses keys or uses mouse • I/O interrupt occurs (hardware gate flips) • O/S • handles interrupt (in kernel mode) • posts a flag for GUI to see • Value of key pressed • position of mouse • Returns to GUI in user mode (always) • GUI performs required action on the screen

  5. Kernel Basics • States • State vectors • Interrupts • Contexts • Context switching • Reserved RAM

  6. Remainder of Course • Paging • Scheduling & Dispatching • Virtualization • Multi-tasking • File & storage systems • Deadlock • Security & reliability • Distributed systems

  7. Basic Computer Architecture

  8. Components • Elementary instructions • Load • Add, subtract, etc • Store • Compare • Branch • Data access • Register (like a pointer) • Displacement • offset or “distance in bytes, from register contents

  9. Instruction types • Risc • Operation code and one operand • e.g. L x loads content of x into an accumulator • CISC • Operation code and TWO operands • e.g. L x,y loads contents of y into register x

  10. States • Define current machine capabilities • Interrupts allowed or not • Privileged instructions allowed or not • (ie; kernel mode vs. user mode) • Memory protection key or state (# and/or on/off) • Next sequential instruction to perform • Addressing mode • User-mode flag (div by 0, etc)

  11. Current State Vector • Contains the state information • Is NOT a memory location or data structure • Changeable by events (interrupts) • I/O completion or external signals • Machine failure, program failure (÷0), service call • Changeable by privileged instructions • Vector is loaded from data in RAM • Saved to RAM by hardware events

  12. Interrupts

  13. Interrupts - Generalized • Two types • Hardware (MAY be “turned off” by kernel) • I/O • Clock/interval timer • Program exception (e.g.; div by 0) • Paging • Addressing (32/64 bit) • Software (Service call - ALWAYS allowed) • Program requests for kernel service • Machine language instruction causes hardware “trap” • Int 0x80 PC style (‘n’ is on stack) • SVC n z/390 style

  14. Instruction Processing with Interrupts No No Interruptsallowed? Interrupt pending? yes fetch execute previousinst yes processinterrupt

  15. Trap or System CallInstruction • Atomic operation • Causes an interrupt (type=service request) • Kernel processes normally • Common service request handler • Uses code to select address in trap table • Trap table contains addresses of specific programs for specific request

  16. Traps or Kernel Calls • Examples • Cout << x; • Seek (device, position); • X=ftime(); • User functions expand into assembly code for a "trap" or "svc" instruction • "trap" causes a H/W switch to the kernel • Kernel performs op and returns to user

  17. fork (My_fork_loc); { ● ● ● trap (K_FORK, *My_fork_loc); } My_fork_loc:…; System call example User space Kernel space Traptable Do_fork(loc) { ● ● start_process (loc); mode=0; return; } *Do_fork *Do_fork Kernel space K_fork is entry # for "FORK"

  18. x86 specific Interrupts

  19. Allowing x86 interrupts • The x86 has an interrupt flag (IF) in the FLAGS register. Only for hardware interrupts. • cli sets this flag to 0 - disabled • sti sets it to 1 - enabled • Instructions that load values into the FLAGS register (such as popf and iret) may also modify this flag.

  20. X86 specific - 2 • int arg • int 0x04 • Calls interrupt handler #4, IF overflow flag is set • int 0x80 • Calls the service-call handler

  21. Example: the setuid system call • Coded as: _syscall1(int,setuid, uid_t, uid); expands to: _setuid: subl $4,%exp pushl %ebx movzwl 12(%esp),%eax movl %eax,4(%esp) movl $23,%eax movl 4(%esp),%ebx int $0x80 --->trap into kernel movl %eax,%edx --return from kernel testl %edx,%edx jge L2 negl %edx movl %edx,_errno movl $-1,%eax popl %ebx addl $4,%esp ret L2: movl %edx,%eax popl %ebx addl $4,%esp ret

  22. z/390 specific Interrupts

  23. Allowing z/390 interrupts • Program Status Word controls hardware interrupts • Bits 0-7 • Bits 20-23 • (fixedpoint overflow, decimal overflow, exp underflow, significance) • LPSW instruction loads all 64 bits of status • SSM instruction sets individual bits 0-7 only • See slides on setting Machine State

  24. Reserved space in RAM • Allows software/hardware interaction • Different for every machine architecture • Key to understanding of machine control • Key to understanding of Operating Systems

  25. Contexts

  26. User Mode • Normal programs: • (payroll, taxes, compilers, etc.) • Cannot perform ANY privileged instructions • Cannot branch or jump into kernel • Kernel does not branch or jump to user code • MUST use a “state switch” instruction • Must use “exposed” functions via Service Calls

  27. Kernel Mode • Can access ANY memory • Can use ANY instructions • NOT for doing “problem solving” • Manages users • Pages • Access to CPU • Access to devices (disk, monitor, etc)

  28. Context SwitchingFrom Kernel To User • Set up values for new state vector • Save any kernel registers and stack data • Atomic state change • Interrupts on • Privilege off • Memory protect on • Set IC

  29. Context SwitchingFrom User to Kernel • Set up values for service call • Issue service call (assembler instruction) • Atomic state change occurs • Interrupts off • Privilege on • Memory protect off • Set IC to predefined interrupt handler in kernel • Save any user’s registers and stack data

  30. Reserved RAM • Defined in hardware • Used by kernel only • Same for ANY O/S on that type of machine: • IBM-compatible PC • z/390 • MAC • powerPC • Sun

  31. paired paired

  32. PC-bootable disk layout 0x00-0x02 jump inst to 0x1e 0x03-0x0a PC manufacturer name 0x0b-0x0c sectors/cluster 0x0d-0x0f reserved for boot record 0x10-0x10 # of FAT's 0x11-0x12 # root directory entries 0x13-0x14 # logical sectors 0x15-0x15 media descriptor 0x16-0x17 sectors/FAT 0x18-0x19 sectors/track 0x1a-0x10b # surfaces (heads) 0x1c-0x1d # hidden sectors 0x1e-… boot program

  33. Preparing for Interrupts (PC) • BIOS loads the initial address of the IDT table into the idtr register • Linux init moves & re-inits the table • setup_idt( ) – an assembly language function • fills all of idt_table with ignore(int) • 2nd pass – fills in true handlers • Enable interrupts

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