Transcript Class Agenda - Seneca - School of Information & Communications

Class Agenda
 What does a computer consist of?
 Basic design of a PC
 The CPU
 How a CPU stores instructions
 How are Program Instructions Processed?
 How is memory organized?
 Why is different hardware used to store the
same type of information?
 Computer Languages
 Key Definitions you need to know
What does a computer consist of?
 A computer has 5 distinct properties.
It accepts input through some user controlled device
It stores information
It processes raw data according to a set of instructions
It makes coherent data out of raw data
It sends the result in a form acceptable to the user
 Why is a computer different from any other
consumer electronic device? It is Programmable
- or these processes can be changed!
Basic design of a PC
 If you were to design a computer what would
you pick as its major components?
 -- a memory, a central processing unit, a clock
and a set of ports:
the memory stores the information used by the
system
the central processing unit (CPU) processes the
raw data
the clock controls the rate at which the CPU works
the ports connect the CPU to external devices
(such as a keyboard, a mouse, a hard disk, a CDROM, a floppy, a printer, a joystick and speakers)
The CPU
 The CPU includes a control unit and an
arithmetic/logic unit.
 The control unit fetches instructions from
memory, decodes them and executes them.
 The arithmetic/logic unit (ALU) performs all
arithmetic operations and logical comparisons.
 The CPU also contains registers where data is
stored. The ALU cannot work directly on data in
memory. Data needs to be copied into the
registers before the ALU can use it.
How a CPU stores instructions
 In a modern computer - that is, a
stored program machine - the
CPU retrieves instructions and
data from memory, processes the
instructions and returns the
coherent data to memory.
 The control unit includes a
program counter. This counter
holds the address of the next
instruction to be executed.
How are Program Instructions
Processed?
 A program instruction is stored as a binary number. A binary number
consists solely of the digits 0 and 1. For example, 01010011. The
CPU can only process binary information. The binary number there
may be either raw data or a program instruction. Under the stored
program concept, there is no way of distinguishing data from a
program instruction just by examining the information itself.
 The manufacturer of the CPU determines the format in which
program instructions are stored. This format includes an operation
code followed by one or more operands, which are address fields.
The operation code is a special code that tells the control unit what
to do. The address fields identify the registers and memory
addresses that need to be accessed while executing the instruction.
 Each manufacturer publishes the instruction format and the full set
of operation codes for its own CPUs.
How is memory organized?
 Computer memory is organized as a linear collection of
bytes. Typically, each byte consists of 8 bits. A bit is a single binary
digit, which holds the smallest unit of information. Each byte has a
unique address. The addressing is sequential, starts at zero and
ends at the size of memory less 1. Note that each byte and not
each bit has its own address.
 For example, 128 bytes of memory consists of 1024 (= 128 * 8) bits
where information can be stored, starts at address 0, and ends at
address 127.
 The maximum size of usable memory depends upon the number of
bits in the smallest address register in the CPU: in a 32-bit CPU,
addresses can range from 0 to 232-1, which is 0 to 4,294,967,295 or
4 Giga. So, the maximum size of usable memory for a 32-bit CPU is
4 Gb (Gigabytes). The size qualifiers are
 Giga or G (=1024M): 1 Gigabyte = 1024 * 1024 * 1024 bytes
 Mega or M (=1024k): 1 Megabyte = 1024 * 1024 bytes
 Kilo or k (=1024): 1 Kilobyte = 1024 bytes
Why is different hardware used to store
the same type of information?
 Data can be stored in the CPU registers, the internal cache (L1), the
external cache (L2) and RAM (Random Access Memory). The closer the
data is to the CPU, the higher the transfer rate. The rate at which
information is transferred where the information is stored.
 In a relative sense,
 CPU registers process data at less than 10 nanoseconds
 internal cache can hold 4-16kb data and transfer at less than 10
nanoseconds
 external cache can hold 128-512kb data and transfer at about 20
nanoseconds
 RAM can hold 32-128Mb and transfer at 60 nanoseconds
 a hard disk can hold 1-20Gb data and transfer at 12,000,000
nanoseconds
 (A nanosecond is 1 * 10-9 seconds.) The ratio of the time it takes a CPU to
process data to the time it takes a hard disk to store it is the same as the
ratio of the time it takes me to cross this room to the time it takes me to walk
around the earth once at the same pace.
 Fast memory is expensive. Slow memory is inexpensive. The most
expensive storage is reserved for the CPU itself while the least expensive is
limited to the peripheral devices.
Computer Languages – First Generation
 Each machine instruction is a binary number. Even for a
popular CPU, machine language is extremely awkward
to write.
 Machine Languages are First Generation or low-level
languages.
 Machine Language is a series of codes, represented by
binary numbers, used to communicate directly with the
internal instructions of the PC’s microprocessor.
 Deciphering Machine Language code or writing it is a
complex task
 However, programs called interpreters and compilers
translate commands written in higher-level languages
into machine language.
Computer Languages – Second
Generation
 The first step towards readability came with Assembly
Language in 1951 (or just Assembly).
 An assembly language instruction might look like
MOV 10000001, BX
; move the contents of
10000001 into register BX
 where MOV is the operation code,
 10000001 and BX are the memory address and register
fields,
 ; marks the end of the assembly language statement and
 move the contents of 10000001 into register BX is
commentary for programmer reference.
Computer Languages –Second
Generation (cont.)
 In assembly languages, the operation codes are written as simple command
words to supply step-by-step instructions for the processor to carry out.
 Assembly Languages directly manipulates the values contained in those
memory scratch pads in the microprocessor called registers.
 Although more intelligible to human than machine language codes,
assembly is still more difficult to use that higher-level languages.
 However, Assembly remains popular among programmers because it
creates compact, fast code.
 An assembly language program, to be executable on a computer, must be
translated into machine language code. A separate program called an
assembler does this translation.
 However, since each manufacturer creates its own operation codes,
program instructions vary from machine to machine and new assembly
language programs are written for each new CPU. For example, even for
the same instruction, the order of the operands may vary.
High Level Languages
 The third generation of programming languages broke the relation to
machine language. In high-level languages, the instructions are
English-like as opposed to machine-like.
 Languages such as C and Java allow programmers to write in terms
and words that more closely parallel English
 Java is the current rising star in language since a program written in
Java will run on any computer no matter what operating system it is
using.
 At the highest level are languages such as BASIC (Beginners Allpurpose Symbolic Instruction Code), Visual Basic, the DOS batch
language and the macro languages used to automate applications
such as MS Office
 However - the only language that a computer can interpret is its
native machine language. To convert a program written in a highlevel language to machine language, pass the high-level source
through a separate program called a compiler.
Some of the Languages Developed since
1950
 Over the development history of programming languages, the first three
generations were procedural. These languages execute instructions
sequentially. The fourth generation languages are non-procedural.
Key Definitions
(see White, Ron. (2004) How Computers Work. 7th Edition. Que Publishing, New York. Pages 8-9)
 BIOS (basic input / output system)
A collection of software codes built into a PC that handle
some of the fundamental tasks of sending data from one
part of the computer to another
 Boot or boot-up
The process that takes place when a PC is turned on
and performs the routines necessary to get all the
components functioning properly and the operating
system loaded. The term comes from lifting yourself by
your bootstraps
Key Definitions (continued)
 Circuit Board
Originally, wires ran form and to any component in any
electrical device, not just computers. A circuit board
replaces the need for separate wiring with metallic
traces printed on the board – sometimes also on the
bottom of the board and in the hidden middle layer.
The traces lead to connections for processors, resistors,
capacitors and other electrical components.
 The importance of the circuit board is that its entire
creation can be automated, and the board packs more
components into an ever smaller space.
Key Definitions (continued)
 Clock
 A microchip that regulates the timing and speed of all of the
computers functions. The chip includes a crystal that vibrates at
a certain frequency when electricity is applied to it.
 The shortest length of time in which a computer can perform
some operation is one clock, or one vibration of the clock chip.
 The speed of clocks – and therefore computers themselves – is
expressed in megahertz (MHz). One megahertz is 1 million
cycles, or vibrations, a second. Thus, a PC can be described as
having a 200 or 300 MHz processor, which means that the
processor has been designed to work with a clock chip running
at that speed.
 NOTE: posted clock speed is an ideal maximum – the
actual speed will always be slower.
Key Definitions (continued)
 CMOS
An acronym for complementary metal-oxide
semiconductor – a term that describes how a CMOS
microchip is manufactured. Powered by a small battery,
the CMOS chip retains crucial information about what
hardware a PC is comprised of even when the power is
turned off.
 CPU
An acronym for central processing unit, it is used to
mean the microprocessor – also, processor - which is
the microchip that processes the information and the
code (instructions) used by a computer. The ‘brains’ of a
computer.
Key Definitions (continued)
 Expansion slot
Most PCs have unused slots into which the owner can
plug circuit boards and hardware to add to the
computers capabilities. Most slots today are personal
computer interface (PCI).
One other slot, the accelerated graphics port (AGP),
accepts a video card designed to move images out of
memory quickly.
 Motherboard
A sheet of plastic onto which metallic circuits have been
printed and to which slots for other components wait to
receive daughterboards, smaller circuits that add to the
motherboard capabilities.
Key Definitions (continued)
 Operating System
 Software that exists to control the operations of
hardware.
 Essentially, the operating system directs any
operations, such as writing data to memory or to
disk, and regulates the use of hardware among
several application programs that are running at
the same time. This frees program developers
from having to write their own code for these
most basic operations.
Key Definitions (continued)
 ROM and RAM
 Acronyms for Read Only Memory and Random Access Memory.
 ROM is memory chips or data stored on disks that can be read
by the computer’s processor. The PC cannot write new data to
those chips or disk drives.
 RAM is memory or disks that can be both read and written to.
 NOTE: RAM is really a misnomer because even ROM can be
accessed randomly. The term originally was used to distinguish
RAM from data and software that was stored on magnetic tape,
and which could be accessed only sequentially. That is, to get
the last chunk of data or code on a tape, a computer must read
through all the information contained in the tape until it finds the
location where it stored the data code or code for which it is
looking. In contrast, a computer can jump directly to any
information stored in random locations in RAM chips or on a
disk.
Key Definitions (continued)
 System Files
 Small files that contain software code that are the first files a
computer reads from a disk when it is booted.
 On DOS (disk operating system) and Windows systems, the files
are named IO.SYS and MSDOS.SYS and are hidden so that
ordinarily you cannot see them in a listing of files on a disk. The
systems files contain the information needed, following the initial
hardware boot, to load the rest of an operating system. In DOS, one
other system file is COMMAND.COM, which contains the operating
system’s basic functions, such as displaying a list of files (a
directory).
 A boot disk must contain all three files for a PC to start up.
 System files can also include CONFIG.SYS, which makes some
initial setting of hardware, and AUOTEXEC.BAT, a collection of
commands that are executed when all other boot functions are
finished. In Windows 95, 98 and Me, the Registry – consisting of
two hidden files USER.DAT and SYSTEM DAT – is also necessary
for Windows to run and can be considered a system file.
Key Definitions (continued)
WRITE and READ
Writing is the process by which a computer
stores data in either RAM chips or on a disk
drive.
Reading is the process by which a computer
transfers data or software code from a drive to
RAM or from RAM to the microprocessor.
SCSI HardDisk