Ch2: Computer System Structure (OS)

Silberschatz, Galvin and Gagne ©20022.1
Chapter 2: Computer-System Structures
Computer System Operation
I/O Structure
Storage Structure
Storage Hierarchy
Hardware Protection
General System Architecture

Computer-System Architecture

Computer-System Operation
I/O devices and the CPU can execute concurrently.
Each device controller is in charge of a particular device
type.
Each device controller has a local buffer.
CPU moves data from/to main memory to/from local
buffers
I/O is from the device to local buffer of controller.
Device controller informs CPU that it has finished its
operation by causing an interrupt.

Common Functions of Interrupts
Interrupt transfers control to the interrupt service routine
generally, through the interrupt vector, which contains the
addresses of all the service routines.
Interrupt architecture must save the address of the
interrupted instruction.
Incoming interrupts are disabled while another interrupt is
being processed to prevent a lost interrupt.
A trap is a software-generated interrupt caused either by
an error or a user request.
An operating system is interrupt driven.

Interrupt Handling
The operating system preserves the state of the CPU by
storing registers and the program counter.
Determines which type of interrupt has occurred:
polling
vectored interrupt system
Separate segments of code determine what action should
be taken for each type of interrupt

Interrupt Time Line For a Single Process Doing
Output

I/O Structure
After I/O starts, control returns to user program only upon
I/O completion.
Wait instruction idles the CPU until the next interrupt
Wait loop (contention for memory access).
At most one I/O request is outstanding at a time, no
simultaneous I/O processing.
After I/O starts, control returns to user program without
waiting for I/O completion.
System call – request to the operating system to allow user
to wait for I/O completion.
Device-status table contains entry for each I/O device
indicating its type, address, and state.
Operating system indexes into I/O device table to determine
device status and to modify table entry to include interrupt.

Two I/O Methods
Synchronous Asynchronous

Device-Status Table

Direct Memory Access Structure
Used for high-speed I/O devices able to transmit
information at close to memory speeds.
Device controller transfers blocks of data from buffer
storage directly to main memory without CPU
intervention.
Only on interrupt is generated per block, rather than the
one interrupt per byte.

Storage Structure
Main memory – only large storage media that the CPU
can access directly.
Secondary storage – extension of main memory that
provides large nonvolatile storage capacity.
Magnetic disks – rigid metal or glass platters covered with
magnetic recording material
Disk surface is logically divided into tracks, which are
subdivided into sectors.
The disk controller determines the logical interaction
between the device and the computer.

Moving-Head Disk Mechanism

Storage Hierarchy
Storage systems organized in hierarchy.
Speed
Cost
Volatility
Caching – copying information into faster storage system;
main memory can be viewed as a last cache for
secondary storage.

Storage-Device Hierarchy

Caching
Use of high-speed memory to hold recently-accessed
data.
Requires a cache management policy.
Caching introduces another level in storage hierarchy.
This requires data that is simultaneously stored in more
than one level to be consistent.

Migration of A From Disk to
Register

Hardware Protection
Dual-Mode Operation
I/O Protection
Memory Protection
CPU Protection

Dual-Mode Operation
Sharing system resources requires operating system to
ensure that an incorrect program cannot cause other
programs to execute incorrectly.
Provide hardware support to differentiate between at least
two modes of operations.
1. User mode – execution done on behalf of a user.
2. Monitor mode (also kernel mode or system mode) –
execution done on behalf of operating system.

Dual-Mode Operation (Cont.)
Mode bit added to computer hardware to indicate the
current mode: monitor (0) or user (1).
When an interrupt or fault occurs hardware switches to
monitor mode.
Privileged instructions can be issued only in monitor mode.
monitor user
Interrupt/fault
set user mode

I/O Protection
All I/O instructions are privileged instructions.
Must ensure that a user program could never gain control
of the computer in monitor mode (I.e., a user program
that, as part of its execution, stores a new address in the
interrupt vector).

Use of A System Call to Perform
I/O

Memory Protection
Must provide memory protection at least for the interrupt
vector and the interrupt service routines.
In order to have memory protection, add two registers
that determine the range of legal addresses a program
may access:
Base register – holds the smallest legal physical memory
address.
Limit register – contains the size of the range
Memory outside the defined range is protected.

Use of A Base and Limit
Register

Hardware Address Protection

Hardware Protection
When executing in monitor mode, the operating system
has unrestricted access to both monitor and user’s
memory.
The load instructions for the base and limit registers are
privileged instructions.

CPU Protection
Timer – interrupts computer after specified period to
ensure operating system maintains control.
Timer is decremented every clock tick.
When timer reaches the value 0, an interrupt occurs.
Timer commonly used to implement time sharing.
Time also used to compute the current time.
Load-timer is a privileged instruction.

Network Structure
Local Area Networks (LAN)
Wide Area Networks (WAN)

Local Area Network Structure

Wide Area Network Structure

Ch2: Computer System Structure (OS)

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Ch2: Computer System Structure (OS)