CS552
Overview
© 2009, D. J. Foreman
1
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
© 2009, D. J. Foreman
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Generalized O/S Operation
“Booting” & login
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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
© 2009, D. J. Foreman
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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
© 2009, D. J. Foreman
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Kernel Basics
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States
State vectors
Interrupts
Contexts
Context switching
Reserved RAM
© 2009, D. J. Foreman
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Remainder of Course
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Paging
Scheduling & Dispatching
Virtualization
Multi-tasking
File & storage systems
Deadlock
Security & reliability
Distributed systems
© 2009, D. J. Foreman
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Basic Computer Architecture
© 2009, D. J. Foreman
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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
© 2009, D. J. Foreman
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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
© 2009, D. J. Foreman
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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)
© 2009, D. J. Foreman
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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
© 2009, D. J. Foreman
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Interrupts
© 2009, D. J. Foreman
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Interrupts - Generalized
• Two types
– Hardware
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(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
– SVC
© 2009, D. J. Foreman
0x80
n
PC style (‘n’ is on stack)
z/390 style
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Instruction Processing with Interrupts
No
No
previous
inst
fetch
execute
Interrupts
allowed?
yes
Interrupt
pending?
yes
process
interrupt
© 2009, D. J. Foreman
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Trap or System Call
Instruction
• 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
© 2009, D. J. Foreman
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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
© 2009, D. J. Foreman
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System call example
User space
Kernel space
fork (My_fork_loc);
{
●
●
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trap (K_FORK,
*My_fork_loc);
}
My_fork_loc:…;
Trap
table
Do_fork(loc)
*Do_fork
{ ●
●
start_process (loc);
mode=0;
return;
}
Kernel space
K_fork is entry # for "FORK"
© 2009, D. J. Foreman
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Interrupts
x86 specific
© 2009, D. J. Foreman
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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.
© 2009, D. J. Foreman
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X86 specific - 2
• int arg
• int 0x04
– Calls interrupt handler #4, IF overflow flag is set
• int 0x80
– Calls the service-call handler
© 2009, D. J. Foreman
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Example: the setuid system call
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Coded as: _syscall1(int,setuid, uid_t, uid); expands to:
_setuid:
subl
pushl
movzwl
movl
movl
movl
int
movl
testl
jge
negl
movl
movl
popl
addl
ret
L2: movl
popl
addl
© 2009, D. J. Foreman
ret
$4,%exp
%ebx
12(%esp),%eax
%eax,4(%esp)
$23,%eax
4(%esp),%ebx
$0x80
%eax,%edx
%edx,%edx
L2
%edx
%edx,_errno
$-1,%eax
%ebx
$4,%esp
--->trap into kernel
--return from kernel
%edx,%eax
%ebx
$4,%esp
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Interrupts
z/390 specific
© 2009, D. J. Foreman
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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
© 2009, D. J. Foreman
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Reserved space in RAM
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Allows software/hardware interaction
Different for every machine architecture
Key to understanding of machine control
Key to understanding of Operating Systems
© 2009, D. J. Foreman
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Contexts
© 2009, D. J. Foreman
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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
© 2009, D. J. Foreman
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Kernel Mode
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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)
© 2009, D. J. Foreman
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Context Switching
From 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
© 2009, D. J. Foreman
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Context Switching
From 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
© 2009, D. J. Foreman
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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
© 2009, D. J. Foreman
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Hex address
0
8
10
18
20
28
30
38
58
60
68
70
78
80
88 (4 bytes)
© 2009, D. J. Foreman
IBM z/390 memory content
IPL PSW or Restart PSW
IPL CCW1 or Restart old PSW
IPL CCW2
External Old PSW
Supervisor Call Old PSW
paired
Program Check Old PSW
Machine Check Old PSW
I/O Old PSW
External New PSW
Supervisor Call New PSW
Program Check New PSW
Machine Check New PSW
paired
I/O New PSW
External interrupt data
SVC interruption data: 13-14= ILC,
16-31= interruption code (SVC #)
31
PC-bootable disk layout
0x00-0x02
0x03-0x0a
0x0b-0x0c
0x0d-0x0f
0x10-0x10
0x11-0x12
0x13-0x14
0x15-0x15
0x16-0x17
0x18-0x19
0x1a-0x10b
0x1c-0x1d
0x1e-…
© 2009, D. J. Foreman
jump inst to 0x1e
PC manufacturer name
sectors/cluster
reserved for boot record
# of FAT's
# root directory entries
# logical sectors
media descriptor
sectors/FAT
sectors/track
# surfaces (heads)
# hidden sectors
boot program
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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
© 2009, D. J. Foreman
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