COMPUTER ABSTRACTION
AND TECHNOLOGY
COMPUTERS
“THIRD REVOLUTION FOR CIVILIZATION”
Computer revolution continues…
• Computers in automobiles
• Cell phones
• Human genome project
• World wide web
• Search Engines
……Today’s science fiction
suggests tomorrow’s killer
application……
Classes of Computing Applications
• Personal computers
• Servers
• Super computers
• Embedded computers
Computer Architecture
Set of rules and methods that describe the
functionality, organization and implementation of
computer systems
It has many aspects,
• Instruction set design
• Functional organization
• Logic design
• Implementation( IC design, packaging, power and
cooling)
8 Design Features
1. Design for Moore’s law
Moore’s law:
Moore's Law refers to Moore's perception that the
number of transistors on a microchip doubles every
two years, though the cost of computers is halved.
Computer architect must anticipate where the
technology will be when the design finishes rather
the design for where it starts.
2. Use abstractions to simplify the design
A major productivity technique for hardware
and software is to use abstractions to
represent the design at different levels of
representation.
3.Make the common case fast
Making the common case fast will tend to enhance
performance better than optimizing the rare
case.
This principle applies when determining how to
spend resources , since the impact of
improvement is higher if the occurrence is
frequent.
4.Performance via parallelism
Computer architects offer design that get more
performance by performing operations in parallel.
System level: Multiple processors/disks
Individual processor: Instruction level parallelism
Digital design: set associative cache/ multiple
banks of memory
5.Performance via pipelining
Pipelining is a technique where multiple
instructions are overlapped during execution
Pipeline is divided into stages and these stages are
connected with one another to form a pipe like
structure.
Pipelining increases the overall instruction
throughput.
6.Performance via prediction
In some cases it can be faster on average to
guess and start working rather than wait until
you know for sure, assuming that the
mechanism to recover from a misprediction is
not too expensive and your prediction is
relatively accurate.
7.Hierarchy of memories
Fastest , smallest and most expensive memory
per bit at the top of the hierarchy and the
slowest, largest and cheapest per bit at the
bottom.
8.Dependability via redundancy
Since any physical device can fail , we make
systems dependable by including redundant
components that can take over when a failure
occurs and to help detect failures
Layers of Abstraction
• Hardware in a computer can only execute
extremely simpler low level instructions
• To go from complex applications to simple
instructions involves several layers of software
that interpret/ translate high level operations
into simple computer instructions.
• These layers of software are organized in
hierarchical fashion , with applications being
the outermost ring and a variety of system
software sitting between the hardware and
application software.
Layers of Abstraction
System software:
• It is a type of computer program that is
designed to run computer’s hardware and
application programs.
• It provides a platform to other software.
• It is the interface between hardware and use
applications
Eg: Operating Systems, Compilers , Assemblers
Operating System
Supervising program that manages the
resources of a computer for the benefit of the
programs that run on that machine.
Compiler
A program that translates high level language
statements into assembly language
statements.
Assembly language
A symbolic representation of machine
instructions
Assembler
A program that translates a symbolic version of
instructions into binary version.
Five key components of a computer
Five key components of a computer
• INPUT
• OUTPUT
• MEMORY
• DATAPATH
• CONTROL
System Block Diagram
Chapter 7 CPU and Memory 7-23
DATAPATH
A data path (also written as datapath) is a set of
functional units ,such as arithmetic logic units
or multipliers that perform data processing
operations, registers, and buses.
Functional units carry out data processing
operations.
Datapath, along with a control unit, make up the
CPU (central processing unit) of a computer
system.
CPU
• ALU (arithmetic logic unit)
– Performs calculations and comparisons (data changes)
• CU (control unit): performs fetch/execute cycle
– Functions:
• Moves data to and from CPU registers and other hardware components
(no change in data)
• Accesses program instructions and issues commands to the ALU
– Subparts:
• Memory management unit: supervises fetching instructions and data
• I/O Interface: sometimes combined with memory management unit as
Bust Interface Unit
• Registers
– Example: Program counter (PC) or instruction pointer determines next
instruction for execution
Chapter 7 CPU and Memory 7-25
Arithmetic/Logic Unit
• actual computation (the manipulation of data to generate
“new” data) takes place in the ALU.
• Because data is represented in binary form (1’s and 0’s), the
ALU is mainly comprised of logic gates, circuits made from
transistors that take inputs (combinations of highs and lows)
and produce outputs (different combinations of highs and
lows).
• Logic in which the output depends solely on the input is
called combinatorial.
CSIT 301 (Blum) 26
The Little Man Computer
Chapter 7 CPU and Memory 7-27
Concept of Registers
• Small, permanent storage locations within the CPU
used for a particular purpose
• Manipulated directly by the Control Unit
• Wired for specific function
• Size in bits or bytes (not MB like memory)
• Can hold data, an address or an instruction
Chapter 7 CPU and Memory 7-28
Registers
• Use of Registers
– Scratchpad for currently executing program
• Holds data needed quickly or frequently
– Stores information about status of CPU and currently executing
program
• Address of next program instruction
• Signals from external devices
• General Purpose Registers
– User-visible registers
– Hold intermediate results or data values, e.g., loop counters
– Typically several dozen in current CPUs
Chapter 7 CPU and Memory 7-29
Special-Purpose Registers
• Program Count Register (PC)
– Also called instruction pointer
• Instruction Register (IR)
– Stores instruction fetched from memory
• Memory Address Register (MAR)
• Memory Data Register (MDR)
• Status Registers
– Status of CPU and currently executing program
– Flags (one bit Boolean variable) to track condition like
arithmetic carry and overflow, power failure, internal
computer error
Chapter 7 CPU and Memory 7-30
Register Operations
• Stores values from other locations (registers
and memory)
• Addition and subtraction
• Shift or rotate data
• Test contents for conditions such as zero or
positive
Chapter 7 CPU and Memory 7-31
Control Unit
• Control is responsible for determining what
action is to be performed on what data.
• If the action is a calculation, then control
will deliver the necessary data to the ALU,
inform the ALU what particular action is to
be performed, and then directs the output
to the appropriate location.
CSIT 301 (Blum) 32
Control Cont.
• All of the other units of a computer have “control
inputs” that determine whether or not they are
active and what particular action they will perform
(if they can do more than one thing).
• Control takes code down to this lowest level of
making the appropriate control inputs high or low,
active or inactive.
– Not all devices are active when the control input is high,
some devices are “active low.”
CSIT 301 (Blum) 33
ROM - Read Only Memory
• Non-volatile memory to hold software that is not
expected to change over the life of the system
• Magnetic core memory
• EEPROM
– Electrically Erasable Programmable ROM
– Slower and less flexible than Flash ROM
• Flash ROM
– Faster than disks but more expensive
– Uses
• BIOS: initial boot instructions and diagnostics
• Digital cameras
Chapter 7 CPU and Memory 7-34
Control is ROM
• At a high level, instructions may be arranged in new
and different orders (this after all is what makes the
computer programmable). But at the low level the
control sequences for a given instruction do not
change.
• Control sequences are thus made of Read Only
Memory (ROM) which are reprogrammed rarely if at
all.
CSIT 301 (Blum) 35
Memory
• Memory consists of circuits whose primary
purpose is to hold information, but only
temporarily.
• Memory holds the data that the processor has
just acted on, is acting on or will soon act on.
• When we use the term memory, we usually
mean Random Access Memory (RAM).
CSIT 301 (Blum) 36
RAM: Random Access Memory
• DRAM (Dynamic RAM)
– Most common, cheap
– Volatile: must be refreshed (recharged with power) 1000’s
of times each second
• SRAM (static RAM)
– Faster than DRAM and more expensive than DRAM
– Volatile
– Frequently small amount used in cache memory for high-
speed access used
Chapter 7 CPU and Memory 7-37
Random Access
• The data in any holding cell can be accessed immediately
for purposes of reading or writing by supplying its
address.
– Any random site can be “immediately” accessed.
– As opposed to sequential access in which one must pass
through all intermediate data between one’s current location
and the next desired location.
• VCR tapes are sequential;
• DVDs are random access (they support scene selection and don’t have
to be rewound).
CSIT 301 (Blum) 38
Operation of Memory
• Each memory location has a unique address
• Address from an instruction is copied to the MAR
which finds the location in memory
• CPU determines if it is a store or retrieval
• Transfer takes place between the MDR and memory
Chapter 7 CPU and Memory 7-39
Memory
Memory is collection of holding cells (like registers). Each cell has an address.
Address Value
0000 10010100
0001 01010010
0010 00101100
0011 01011100
0100 01100110
…
CSIT 301 (Blum) 40
Relationship between MAR,
MDR and Memory
Address Data
Chapter 7 CPU and Memory 7-41
MAR-MDR Example
Chapter 7 CPU and Memory 7-42
Visual Analogy of Memory
Chapter 7 CPU and Memory 7-43
