Basic Structure of
Computers
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
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.
User program and OS routine sharing of the processor.
Performance
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…
