comp. org Chapter 1

Chapter 1. Basic
Structure of Computers

Computer Organization
Lecture 5

Overview
 The basic structure of a computer
 Machine instructions and their execution

 System software that enables the preparation
and execution of programs
 Performance issues in computer systems

 The history of computer development

Computer Types
 What is a digital computer, or computer?
 A contemporary computer is a fast electronic calculating machine
that accepts digitized input information, process it according to a list
of internally stored instructions, and produces the resulting output
information.
 Many types of computers exist that differ widely in
size, cost, computational power, and intended use:
 Personal computers – desktop, notebook
 Workstations – high-resolution graphics input/output capability,
more computational power, reliability…
 Enterprise systems (mainframes) and servers – much more
computing power and storage capacity, accessible via internet
 Supercomputers – large-scale numerical calculations

Functional Units
Arithmetic
Input and
logic

Memory

Output Control

I/O Processor

Figure 1.1. Basic functional units of a computer.

Information Handled by a
Computer
 Instructions/machine instructions
 Govern the transfer of information within a computer as well as
between the computer and its I/O devices
 Specify the arithmetic and logic operations to be performed
 Program
 Data
 Used as operands by the instructions
 Source program
 Encoded in binary code – 0 and 1
 ASCII and EBCDIC

Memory Unit
 Storeprograms and data
 Two classes of storage
 Primary storage
 Fast
 Programs must be stored in memory while they are being executed
 Large number of semiconductor storage cells
 Processed in words
 Address
 RAM and memory access time
 Memory hierarchy – cache, main memory
 Secondary storage – larger and cheaper

Arithmetic and Logic Unit
(ALU)
 Most computer operations are executed in
ALU of the processor.
 Load the operands into memory – bring them
to the processor – perform operation in ALU
– store the result back to memory or retain in
the processor.
 Registers

 Fast control of ALU

Control Unit
 All computer operations are controlled by the control
unit.
 The timing signals that govern the I/O transfers are
also generated by the control unit.
 Control unit is usually distributed throughout the
machine instead of standing alone.
 Operations of a computer:
 Accept information in the form of programs and data through an
input unit and store it in the memory
 Fetch the information stored in the memory, under program control,
into an ALU, where the information is processed
 Output the processed information through an output unit
 Control all activities inside the machine through a control unit

Basic Operational
Concepts

Review
 Activity in a computer is governed by instructions.
 To perform a task, an appropriate program
consisting of a list of instructions is stored in the
memory.
 Individual instructions are brought from the memory
into the processor, which executes the specified
operations.
 Data to be used as operands are also stored in the
memory.

A Typical Instruction
 Add LOCA, R0
 Add the operand at memory location LOCA to the
operand in a register R0 in the processor.
 Place the sum into register R0.
 The original contents of LOCA are preserved.
 The original contents of R0 is overwritten.
 Instruction is fetched from the memory into the
processor – the operand at LOCA is fetched and
added to the contents of R0 – the resulting sum is
stored in register R0.

Separate Memory Access and
ALU Operation
 Load LOCA, R1
 Add R1, R0

 Whose contents will be overwritten?

Connection Between the
Processor and the Memory
Memory

MAR MDR
Control

PC R0

R1
Processor
IR

ALU
Rn 1

n general purpose
registers

Figure 1.2. Connections between the processor and the memory.

Registers
 Instruction
register (IR)
 Program counter (PC)

 General-purpose register (R – R )
0 n-1

 Memory address register (MAR)
 Memory data register (MDR)

Typical Operating Steps
 Programs reside in the memory through input
devices
 PC is set to point to the first instruction
 The contents of PC are transferred to MAR
 A Read signal is sent to the memory
 The first instruction is read out and loaded
into MDR
 The contents of MDR are transferred to IR
 Decode and execute the instruction

Typical Operating Steps
(Cont’)
 Get operands for ALU
 General-purpose register
 Memory (address to MAR – Read – MDR to ALU)
 Perform operation in ALU
 Store the result back
 To general-purpose register
 To memory (address to MAR, result to MDR – Write)
 During the execution, PC is incremented to
the next instruction

Interrupt
 Normal execution of programs may be preempted if
some device requires urgent servicing.
 The normal execution of the current program must
be interrupted – the device raises an interrupt
signal.
 Interrupt-service routine
 Current system information backup and restore (PC,
general-purpose registers, control information,
specific information)

Bus Structures
 There are many ways to connect different
parts inside a computer together.
 A group of lines that serves as a connecting
path for several devices is called a bus.
 Address/data/control

Bus Structure
 Single-bus

Input Output Memory Processor

Figure 1.3. Singlebus structure.

Speed Issue
 Different devices have different
transfer/operate speed.
 If the speed of bus is bounded by the slowest
device connected to it, the efficiency will be
very low.
 How to solve this?

 A common approach – use buffers.

Software
 System software must be in the memory in order for
a user to enter and run an application program on a
computer
 Receiving and interpreting user commands
 Entering and editing application programs and storing them as files
in secondary storage devices
 Managing the storage and retrieval of files in secondary storage
devices
 Running standard application programs
 Controlling I/O units to receive input information and produce output
results
 Translating programs form source form prepared by the user into
object form consisting of machine instructions
 Linking and running user-written application programs with existing
standard library routines

Software
 Application programs are usually written in a
high-level programming language.
 Compiler translates the high-level language
program into a suitable machine language
program.
 Text editor

Operating System (OS)
A large program used to control the sharing
of and interaction among various computer
units as they execute application programs.
 Assign computer resources to individual
application programs
 Memory
 Disk space
 Move data
 Handle I/O

OS Routine Example
 Example: one processor, one disk, and one
printer.
 Program is stored on disk

 Transfer program into memory

 Execute program

 Need to read a data file on disk into memory

 Calculation

 Print results

OS Routine Example
Printer

Disk

OS
routines

Program

t0 t1 t2 t3 t4 t5
Time

Figure 1.4. Figure 1.4. User program and OS routine sharing of the processor. processor.
User program and OS routine sharing of the

Performance
 The most important measure of a computer is
how quickly it can execute programs.
 Three factors affect performance:
 Hardware design
 Instruction set
 Compiler

Performance
 Processor time to execute a program depends on the hardware
involved in the execution of individual machine instructions.

Main Cache
memory memory Processor

Bus

Figure 1.5. The processor cache.

Performance
 The processor and a relatively small cache
memory can be fabricated on a single
integrated circuit chip.
 Speed

 Cost

 Memory management

Processor Clock
 Clock, clock cycle, and clock rate
 The execution of each instruction is divided
into several steps, each of which completes
in one clock cycle.
 Hertz – cycles per second

Basic Performance Equation
 T – processor time required to execute a program than has been
prepared in high-level language
 N – number of actual machine language instructions needed to
complete the execution (note: loop)
 S – average number of basic steps needed to execute one
machine instruction. Each step completes in one clock cycle
 R – clock rate
 Note: these are not independent to each other

N×S
T=
R
How to improve T?

Pipeline and Superscalar
Operation
 Instructions are not necessarily executed one after
another.
 The value of S doesn’t have to be the number of
clock cycles to execute one instruction.
 Pipelining – overlapping the execution of successive
instructions.
 Add R1, R2, R3
 Superscalar operation – multiple instruction
pipelines are implemented in the processor.
 Goal – reduce S (could become <1!)

Clock Rate
 Increase clock rate
 Improve the integrated-circuit (IC) technology to make
the circuits faster
 Reduce the amount of processing done in one basic step
(however, this may increase the number of basic steps
needed)
 Increases in R that are entirely caused by
improvements in IC technology affect all
aspects of the processor’s operation equally
except the time to access the main memory.

CISC and RISC
 Tradeoffbetween N and S
 A key consideration is the use of pipelining
 S is close to 1 even though the number of basic steps
per instruction may be considerably larger
 It is much easier to implement efficient pipelining in
processor with simple instruction sets
 Reduced Instruction Set Computers (RISC)
 Complex Instruction Set Computers (CISC)

Compiler
 A compiler translates a high-level language program
into a sequence of machine instructions.
 To reduce N, we need a suitable machine
instruction set and a compiler that makes good use
of it.
 Goal – reduce N×S
 A compiler may not be designed for a specific
processor; however, a high-quality compiler is
usually designed for, and with, a specific processor.

Performance Measurement
 T is difficult to compute.
 Measure computer performance using benchmark programs.
 System Performance Evaluation Corporation (SPEC) selects and
publishes representative application programs for different application
domains, together with test results for many commercially available
computers.
 Compile and run (no simulation)
 Reference computer

Running time on the reference computer
SPEC rating =
Running time on the computer under test
n 1
SPEC rating = (∏SPECi ) n

i =1

Multiprocessors and
Multicomputers
 Multiprocessor computer
 Execute a number of different application tasks in parallel
 Execute subtasks of a single large task in parallel
 All processors have access to all of the memory – shared-memory
multiprocessor
 Cost – processors, memory units, complex interconnection networks
 Multicomputers
 Each computer only have access to its own memory
 Exchange message via a communication network – message-
passing multicomputers

Historical Perspective
 After-class reading…

comp. org Chapter 1

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