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Lecture 2: Review of Computer Organization

Operating SystemSpring 2007

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Four Elements of a Computer System Processor (CPU)

Control the operation of the computer and its data processing functions.

Main Memory Stores data and programs RAM - random access memory

I/O Modules Auxiliary storage like disk drives, tape drives Printers, terminals, monitors

System Bus Provides for transfer of data among processors, main

memory, and I/O modules

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Computer Organization

CPU

PC

IR

ExecutionUnit

MAR

MBR

I/O AR

I/O BR

I/O module

…

DATA

DATA

…

Instruction

Instruction

Instruction

…

buffer

MAIN MEMORY

SystemBus

01

n-1n-2

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CPU (Processors) Registers A processor includes a set of registers

that provide a level of memory faster than main memory. User-visible Registers Control and Status Registers

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User-visible Registers Many instructions operate on data sitting on

working registers. Since registers are faster than main memory,

it is better that data be moved to registers before operating on them.

May be referenced by the machine language that the processor executes

available to all programs - application programs and system programs.

Types of registers: Data registers Address registers

For indirect addressing For index register For segment pointer For stack pointer

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Control and Status Registers

Program Controller (PC) – contains the address of an instruction to be fetched

Instruction Register (IR) – contains the instruction most recently fetched

Memory Address Register (MAR) Memory Buffer Register (MBR) I/O Address Register (I/O AR) I/O Buffer Register (I/O BR) Processor Status Word (PSW)

condition codes or flags interrupt enable/disable user/supervisor mode

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Instruction Cycle

StartFetch next instruction

Execute instruction

Halt

Fetch Cycle Execute Cycle

Opcode Address of OperandInstruction Format

Sign magnitudeInteger Format

exponent magnitudeFloating point Format

Sign

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Actions of CPU (Types of Instructions)

Processor-Memory Data Transfer Processor-I/O Data Transfer Data Processing

Arithmetic or logic operation on data Control

Alter sequence of execution

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Interrupts

An interruption of the normal sequence of execution

Improve processing efficiency Allows the processor to execute

other instructions while an I/O operation is in progress

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Interrupts - Classes of Interrupts

Program arithmetic overflow or underflow division by zero attempt to execute an illegal machine

instruction reference outside user’s memory space

Timer I/O Hardware Failure

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Interrupts –Interrupt Handler

A program that determines nature of the interrupt and performs whatever actions are needed

Control is transferred to this program

Generally part of the operating system

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Interrupt Timeline

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Interrupts and the Instruction Cycle

StartFetch next instruction

Execute instruction

Fetch Cycle Execute Cycle

InterruptsDisabled

Check for interrupt and process interrupt

InterruptsEnabled

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Interrupt Cycle Processor checks for interrupts If no interrupts, fetch the next

instruction for the current program If an interrupt is pending, suspend

execution of the current program, and execute the interrupt handler

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Program Flow of Control without and with Interrupts

User Program

WRITE

WRITE

WRITE

(1)

(2)

(3)

I/O Program

I/O Command

END

(4)

(5)

No interrupts

User Program

WRITE

WRITE

WRITE

(1)

(2)

(3)

I/O Program

I/O Command

END

(4)

(5)

Interrupt Handler

Interrupts

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Simple Interrupt Processing

Device controller or other Hardware generates an interrupt

Processor finished execution of current instruction

Processor signals acknowledgement of interrupt

Processor pushes PSW and PC onto control stack

Processor loads new PC value based on interrupt

Hardware

Save remainder of process state information

Process interrupt

Restore process state information

Restore old PSW and PC

Software

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Multiple Interrupts

Two methods: Disable other interrupts while

processing one interrupt Assign priorities to different

interrupts. Interrupts at higher priority can interrupt lower ones

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Memory Registers Cache Main memory Electronic disk Magnetic disk Optical disk Magnetic tapes

Decreasing cost per bit

Increasing capacity

Increasing access time

Decreasing frequency of access of the memory by the processor

volatile

nonvolatile

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Performance of Various Levels of Storage

Movement between levels of storage hierarchy can be explicit or implicit

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Caching Important principle, performed at many levels

in a computer (in hardware, operating system, software)

Information in use copied from slower to faster storage temporarily

Faster storage (cache) checked first to determine if information is there

If it is, information used directly from the cache (fast) If not, data copied to cache and used there

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Cache

Invisible to operating system Increase the speed of memory Processor speed is faster than

memory speed Contain a portion of main memory

CPU CacheMain

memory

WordTransfer

BlockTransfer

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Cache Design Cache size Block size Mapping function

Determine which cache location the block will occupy Replacement algorithm

Determines which block to replace Least-Recently-Used(LRU) algorithm

Write policy When the memory write operation takes place Can occur every time block is updated Can occur only when block is replaced

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I/O Communication Techniques

Programmed I/O Interrupt-Driven I/O Direct Memory Access (DMA)

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I/O Communication Techniques

Programmed I/O Interrupt-Driven I/O Direct Memory Access (DMA)

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Programmed I/O I/O module performs

the action, not the processor

Sets appropriate bits in the I/O status register

No interrupts occur Processor checks

status until operation is complete

Issue Read command

to I/O module

Read status ofI/O module

Check status

Read word fromI/O module

Write word intoMain memory

Done?

Notready

ready

yes

Next instruction

no

CPU->I/O

I/O -> CPU

Error condition

I/O -> CPU

CPU->memory

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I/O Communication Techniques

Programmed I/O Interrupt-Driven I/O Direct Memory Access (DMA)

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Interrupt-Driven I/O Processor is interrupt

when I/O module ready to exchange data

Processor is free to do other work

No needless waiting Consumes a lot of

processor time because every word read or written passes through the processor

Issue Read command

to I/O module

Read status ofI/O module

Check status

Read word fromI/O module

Write word intoMain memory

Done?

ready

yes

Next instruction

no

CPU->I/O

I/O -> CPU

Error condition

I/O -> CPU

CPU->memory

Do somethingelse

Interrupt

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I/O Communication Techniques

Programmed I/O Interrupt-Driven I/O Direct Memory Access (DMA)

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Direct Memory Access Transfer a block of

data directly to or from memory

An interrupt is sent when the task is complete

The processor is only involved at the beginning and end of the transfer

Issue Read command

to I/O module

Read status ofDMA module

Next instruction

CPU->DMA

DMA -> CPU

Do somethingelse

Interrupt

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End of lecture 2

Thank you!