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Laboratorio di Calcolo I
• Docente: prof. Berardi (Dip. Informatica)
• Lezione introduttiva su Computing ed
esercitazioni di Unix/Linux tenute da
Fabrizio Bianchi (saro’ il vostro docente di
Laboratorio di Calcolo II)
• Lezioni “teoriche” tutti insieme
• Esercitazioni individuali al computer
Textbook & Slides
Trasparenze in:
www.to.infn.it/~bianchi/LaboratorioCalcoloI/
*.ppt versione power point
*.pdf versione Acrobate Reader
Per la stampa usare la versione pdf
What is Programming?
A sequence of statements that instruct a
computer in how to solve a problem is
called a program.
The act of designing, writing and maintaining
a program is called programming.
People who write programs are called
programmers.
What kinds of statements
do computers understand?
A computer only understands
machine language statements.
A machine language statement is a
sequence of ones and zeros that cause
the computer to perform a particular
action, such as add, subtract, multiply, ...
Machine Language (ML)
ML statements are stored in a computer’s
memory, which is a sequence of switches.
For convenience of representation,
an “on” switch is represented by 1,
and an “off” switch is represented by 0.
ML thus appears to be binary (base-2):
0010111010110101
Early Computers
... required a programmer to write in ML...
– Easy to make mistakes!
– Such mistakes are hard to find!
– Not portable -- only runs on one kind of
machine!
Programming was very difficult!
A Bright Idea
Devise a set of abbreviations (mnemonics)
corresponding to the ML statements, plus a
program to translate them into ML.
ADD
Assembler
0010111010110101
The abbreviations are an assembly language,
and the program is called an assembler.
Assembly Languages
Allowed a programmer to use mnemonics,
which were more natural than binary.
+ Much easier to read programs
+ Much easier to find and fix mistakes
– Still not portable to different machines
High Level Languages
Devise a set of statements that are close to
human language (if, while, do, ...),
plus a program to translate them into ML.
The set of statements is called a high level
language (HLL) and the program is called
a compiler.
HLL Compilers
Where an assembler translates one mnemonic
into one ML statement,
a HLL compiler translates one HLL statement
into multiple ML statements.
z = x + y;
Compiler
1010110011110101
0000000000010000
0010111010110101
0000000000010010
0010111011111101
0000000000010100
HLLs
High level languages (like C++) are
+ Much easier to read programs
+ Much easier to find and fix mistakes
+ Portable from one machine to another
(so
long as they keep to the language standard).
Objectives in Programming
A program should solve a problem:
+ correctly (it actually solves the problem)
+ efficiently (without wasting time or space)
+ readably (understandable by another person)
+ in a user-friendly fashion
(in a way that is easy for its user to use).
Summary
There are “levels” to computer languages:
– ML consists of “low” level binary statements,
that is hard to read, write, and not portable.
– Assembly uses “medium” level mnemonics:
easier to read/write, but not portable.
– C++ is a “high” level language that is even
easier to read/write, and portable.
Computer Organization
Hardware and Software
Computing Systems
Computers have two kinds of components:
• Hardware, consisting of its physical devices
(CPU, memory, bus, storage devices, ...)
• Software, consisting of the programs it has
(Operating system, applications, utilities, ...)
Hardware: CPU
Central Processing Unit (CPU):
– the “brain” of the machine
– location of circuitry that performs arithmetic and
logical ML statements
– measurement: speed (roughly) in megahertz
(millions of clock-ticks per second)
– examples: Intel Pentium, AMD K6, Motorola
PowerPC, Sun SPARC,
Hardware: RAM
Random Access Memory (RAM)
– “main” memory, which is fast, but volatile...
– analogous to a person’s short-term memory.
– many tiny “on-off” switches: for convenience
• “on” is represented by 1, “off” by 0.
– each switch is called a binary digit, or bit.
• 8 bits is called a byte.
• 210 bytes =1024 bytes is called a kilobyte (1K)
• 220 bytes is called a megabyte (1M).
Hardware (Disk)
Secondary Memory (Disk):
– Stable storage using magnetic or optical media.
– Analogous to a person’s long-term memory.
– Slower to access than RAM.
– Examples:
• floppy disk (measured in kilobytes)
• hard disk (measured in gigabytes (230 bytes))
• CD-ROM (measured in megabytes), ...
Hardware: the Bus
The Bus:
– Connects CPU to other hardware devices.
– Analogous to a person’s spinal cord.
– Speed measured in megahertz (like the CPU),
but typically much slower than the CPU...
– The bottleneck in most of today’s PCs.
Hardware: Cache
While accessing RAM is faster than accessing
secondary memory, it is still quite slow,
relative to the rate at which the CPU runs.
To circumvent this problem, most systems add
a fast cache memory to the CPU, to store
recently used instructions and data.
(Assumption: Since such instructions/data
were needed recently, they will be needed
again in the near future.)
Hardware: Summary
Putting the pieces together:
cache
CPU
Main
Memory
Secondary
Memory
Bus
Programs are stored (long-term) in secondary memory,
and loaded into main memory to run, from which
the CPU retrieves and executes their statements.
Software: OS
The operating system (OS) is loaded from
secondary memory into main memory when
the computer is turned on, and remains in
memory until the computer is turned off.
Cache
Bus
CPU
RAM
OS
Disk
Software: OS
The OS acts as the “manager” of the system,
making sure that each hardware device
interacts smoothly with the others.
It also provides the interface by which the
user interacts with the computer, and awaits
user input if no application is running.
Examples: MacOS, Windows-98, Windows-NT,
UNIX, Linux, Solaris, ...
Software: Applications
Applications are non-OS programs that
perform some useful task, including
word processors, spreadsheets, databases,
web browsers, C++ compilers, ...
Example C++ compilers/environments:
– CodeWarrior (MacOS, Win95, WinNT, Solaris)
– GNU C++ (UNIX, Linux)
– Turbo/Borland C++ (Win95, WinNT)
– Visual C++ (Win95, WinNT)
Software: User Programs
Programs that are neither OS programs nor
applications are called user programs.
User programs are what you’ll be writing in
this course.
Putting it all together
Programs and applications that are not running are
stored on disk.
Cache
Bus
CPU
RAM
OS
Disk
App
Putting it all together
When you launch a program, the OS controls the CPU
and loads the program from disk to RAM.
OS
Bus
Disk
RAM
OS
App
Cache
CPU
App
Putting it all together
The OS then relinquishes the CPU to the program,
which begins to run.
App
Bus
Disk
RAM
OS
App
Cache
CPU
App
The Fetch-Execute Cycle
As the program runs, it repeatedly fetches the next
instruction (from memory/cache), executes it,
and stores any results back to memory.
App
Disk
RAM
OS
App
Cache
CPU
App
Bus
That’s all a computer does: fetch-execute-store,
millions of times each second!
Summary
A computer has two kinds of components:
– Hardware: its CPU, RAM, Disk(s), ...
– Software, its OS, Applications, and User Programs.