Chapter 2

Chapter 2
Computer System
Structures
1
Computer System
Architecture
2
Adapted from Silberschatz, Galvin and Gagne (2013), Operating System Concepts
Computer-System
Operation
 A general-purpose computer system consists of one or more
CPUs and a number of device controllers.
 Each device controller is in charge of a specific type of
device.
 Each device controller has a local buffer.
 The CPU and device controllers can execute concurrently.
 Device controller informs CPU that it has finished its
operation by causing an interrupt.
 Interrupt indicates the occurrence of an event from either
the hardware or software.
3
Interrupt Handling
 When the CPU is interrupted, it stops what it is doing and
immediately transfers execution to a fixed location.
o The fixed location contains the starting address of the interrupt

service routine.
 Interrupt vector provide the address of the interrupt service

routine for the interrupting device.
4
Interrupt Handling (Cont)
 Separate segments of code determine what action should
be taken for each type of interrupt.
 The OS preserves the state of the CPU by storing registers

and the program counter.
 After the interrupt is serviced, the saved return address is

loaded into the program counter.
o The interrupted computation resumes.
5
Interrupt Handling
6
Synchronous I/O
Structure
 After I/O starts, control returns to user program only
upon I/O completion.
o Wait instruction idles the CPU until the next interrupt
o Wait loop (contention for memory access).
o At most one I/O request is outstanding at a time, no

simultaneous I/O processing.
7
Asynchronous I/O
Structure
 After I/O starts, control returns to user program without
waiting for I/O completion.
o System call – request to the operating system to allow

user to wait for I/O completion.
o Device-status table contains entry for each I/O device

indicating its type, address, and state.
o Operating system indexes into I/O device table to

determine device status and to modify table entry to
include interrupt.
8
Two I/O Methods
Synchronous Asynchronous
9
Storage Structure
 Main memory – only large storage media that the CPU
can access directly.
o A volatile storage device that loses its contents when

power is turned off.
 Secondary storage – extension of main memory that

provides nonvolatile storage capacity.
o It is capable to hold large quantities of data permanently.
10
Storage Hierarchy
 Storage systems organized in hierarchy.
o Speed
o Cost
o Volatility
 Caching – copying information into faster storage system;

main memory can be viewed as a last cache for secondary
storage.
11
Storage Device Hierarchy
12
Caching
Migration of A From Disk to Register
13
Dual-Mode Operation
 In order to ensure the proper execution of the operating
system, the computer system must be able to distinguish
between the execution of operating system code and user-
defined code.
 Provide hardware support to differentiate between at least

two modes of operations.
14
Modes of Operation
User Mode Kernel Mode

• user program executes in • monitor executes in kernel
user mode mode
• certain areas of memory are • privileged instructions may
protected from user access be executed
• certain instructions may not • protected areas of memory
be executed may be accessed
15
Adapted from Silberschatz, Galvin and Gagne (2013),
Operating System Concepts
Dual-Mode Operation
 Mode bit is added to computer hardware to indicate the current mode:
kernel (0) or user (1).
 When an interrupt or fault occurs, hardware switches to kernel mode.
 Privileged instructions can be issued only in monitor mode.
 In the kernel mode, the software has complete control of the processor
and all instruction, registers and memory.
 This level of control is not necessary and for safety is not desirable for
user programs.
16
I/O Protection
 All I/O instructions are privileged instructions; user
program cannot issue I/O instructions directly.
o User program must executes a system call to request that

the operating system to perform I/O.
 Must ensure that a user program could never gain control

of the computer in monitor mode (i.e., a user program, as
part of its execution, stores a new address in the interrupt
vector).
17
Use of A System Call to
Perform I/O
18
Memory Protection
 The computer system must provide memory protection at
least for the interrupt vector and the interrupt service
routines.
 In order to have memory protection, two registers are used

to determine the range of legal addresses a program may
access:
o Base register – holds the smallest legal physical memory address.
o Limit register – contains the size of the range
 Memory outside the defined range is protected.

19
Use of A Base and Limit
Register
20
Hardware Address
Protection
21
Hardware Address
Protection
 When executing in kernel mode, the operating system has
unrestricted access to both monitor and user’s memory.
o Operating system is allowed to load users’ programs into

users’ memory, to access and modify parameters of
system calls.
 The load instructions for the base and limit registers are
privileged instructions.
22
CPU Protection
 Timer – interrupts computer after specified period to
ensure operating system maintains control over CPU.
o Every time the clock ticks, the counter is decremented.
o When timer reaches the value 0, an interrupt occurs.
 When timer interrupts, control transfers to the operating

system.
 Load-timer is a privileged instruction.
23
Summary
 Computer-system operation  Dual-mode operation
 Interrupt handling  I/O protection
 I/O structure  Memory protection
 Storage structure / Storage  CPU protection
hierarchy
24

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o The fixed location contains the starting address of the interrupt

 Interrupt vector provide the address of the interrupt service

 The OS preserves the state of the CPU by storing registers

 After the interrupt is serviced, the saved return address is

o Wait instruction idles the CPU until the next interrupt

o Wait loop (contention for memory access).

o At most one I/O request is outstanding at a time, no

o System call – request to the operating system to allow

o Device-status table contains entry for each I/O device

o Operating system indexes into I/O device table to

o A volatile storage device that loses its contents when

 Secondary storage – extension of main memory that

o It is capable to hold large quantities of data permanently.

 Caching – copying information into faster storage system;

Migration of A From Disk to Register

 Provide hardware support to differentiate between at least

User Mode Kernel Mode

 When an interrupt or fault occurs, hardware switches to kernel mode.

 Privileged instructions can be issued only in monitor mode.

o User program must executes a system call to request that

 Must ensure that a user program could never gain control

 In order to have memory protection, two registers are used

 Memory outside the defined range is protected.

o Operating system is allowed to load users’ programs into

 When timer interrupts, control transfers to the operating

 Load-timer is a privileged instruction.

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