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COMPUTER ORGANIZATION
AND ASSEMBLY LANGUAGE
Lecture 1 & 2
Introduction and Basics
Course Instructor: Aisha Danish
Lecture Overview
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Course Information
Marking Scheme
Recommended Books
Why study Computer Organization?
What is a Microcomputer ?
Fetch, Decode and Execute
Three-Bus System Architecture
Course Information
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Name: Computer Organization and Assembly
Language
Course Code: CSC-395
Credit Hours: 2+1
Marking Scheme
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3 quizzes
2 Assignments
1 Class Presentation
1 Lab Quiz
1 Lab Project
Mid-term Exam
Final Exam
10 marks
5 marks
5 marks
5 marks
5 marks
20 marks
50 marks
Recommended Books
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Computer Organization and Architecture: Designing
for Performance, 8/E, William Stallings
Assembly Language for Intel Based Processors, Kip
R. Irvine
Why study computer organization and
architecture?
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Computer organization
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Design better programs, including system software such as compilers,
operating systems, and device drivers.
Optimize program behavior.
Evaluate (benchmark) computer system performance.
Understand time, space, and price tradeoffs.
Encompasses all physical aspects of computer systems.
E.g., circuit design, control signals, memory types.
How does a computer work?
Computer architecture
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Logical aspects of system implementation as seen by the designer.
E.g., instruction sets, instruction formats, data types, addressing modes.
How do I design a computer?
Microcomputer
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A microcomputer is an electronic device with a
microprocessor as its central processing unit (CPU),
a memory, and input/output (I/O) facilities
Most of today’s computer systems are based on a
design principle proposed by Dr. John Von
Neumann (1946)
Program Concept
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Hardwired systems are inflexible
General purpose hardware can do different tasks,
given correct control signals
Instead of re-wiring, supply a new set of control
signals
What is a program?
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A sequence of steps
For each step, an arithmetic or logical operation is
done
For each operation, a different set of control signals
is needed
Function of Control Unit
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For each operation a unique code is provided
 e.g.
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ADD, MOVE
A hardware segment accepts the code and issues
the control signals
We have a computer!
Components
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The Control Unit and the Arithmetic and Logic Unit
constitute the Central Processing Unit
Data and instructions need to get into the system
and results out
 Input/output
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Temporary storage of code and results is needed
 Main
memory
Computer Components:
Top Level View
Instruction Cycle
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Two steps:
 Fetch
 Execute
Fetch Cycle
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Program Counter (PC) holds address of next
instruction to fetch
Processor fetches instruction from memory location
pointed to by PC
Increment PC
 Unless
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told otherwise
Instruction loaded into Instruction Register (IR)
Processor interprets instruction and performs
required actions
Execute Cycle
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Processor-memory
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Processor I/O
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Data transfer between CPU and I/O module
Data processing
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data transfer between CPU and main memory
Some arithmetic or logical operation on data
Control
Alteration of sequence of operations
 e.g. jump
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Combination of above
The Instruction Set
− The job of the Instruction Decoder (ID) is to recognize
and activate appropriate controls in the CPU needed to
execute the instruction.
− The list of all instructions recognized by the ID is called
the instruction set
− Microprocessors are classified based on the
specification of the instruction sets into two categories:
(1) Complex Instruction Set Computers (CISC) and
(2) Reduced Instruction Set Computers (RISC)
Bus Interface Unit (BIU) and
Execution Unit (EU)
Modern CPUs:
− Most microprocessors today are designed to allow the fetch and
execute cycles to overlap.
− This is done by dividing the CPU into two units:
(1) a Bus Interface Unit (BIU) and
(2) an Execution Unit (EU).
− The job of the BIU is to fetch instructions from memory and store them
in a special instruction queue.
− The EU then fetches instructions from this queue (not from memory).
− Some processors have a pipelined execution unit that allows the
decoding and execution of instructions to overlap.
Buses
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There are a number of possible interconnection
systems
Single and multiple BUS structures are most common
e.g. Control/Address/Data bus (PC)
e.g. Unibus (DEC-PDP)
What is a Bus?
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A communication pathway connecting two or more
devices
Usually broadcast
Often grouped
A
number of channels in one bus
 e.g. 32 bit data bus is 32 separate single bit channels
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Power lines may not be shown
Data Bus
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Carries data
 Remember
that there is no difference between “data”
and “instruction” at this level
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Width is a key determinant of performance
 8,
16, 32, 64 bit
Address bus
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Identify the source or destination of data
e.g. CPU needs to read an instruction (data) from a
given location in memory
Bus width determines maximum memory capacity of
system
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8080 has 16 bit address bus giving 64k address
space
Control Bus
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Control and timing information
 Memory
read/write signal
 Interrupt request
 Clock signals
Three-Bus System Architecture
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A bus is a collection of electronic signal lines all
dedicated to a particular task
The architecture considered in the previous slides
consists of three types of buses: address, data, and
control buses
Three-Bus System Architecture
The Data Bus:
−The data bus consists of internal and external data
buses.
−The internal data bus connects the internal components
of the CPU (e.g. Registers, ALU, etc.) to the data I/O pins
of the CPU.
− The external data bus connects the data I/O pins of the
CPU to the memory and I/O devices (e.g. printer,
monitor, etc).
−The width of the internal data bus in bits is usually used
to classify a microprocessor (e.g. 8-bit, 16-bit, 32-bi
microprocessors)
Three-Bus System Architecture
The Data Bus:
−The width of the internal data bus is usually the
same as the external data bust – but not always.
− The 80386 processor has 32-bit internal and 32bit external data buses.
− The Pentium processor has 32-bit internal data
bus and 64-bit external data bus
Three-Bus System Architecture
The address Bus:
−It is used to identify the memory location or I/O device
(also called I/O port) to be accessed by the CPU
−The width of this bus in the 80x86 family varies from
one processor to the other for example:
 The 8086/8088 processors have 20-bit address bus.
 The 80286 processor has 24-bit address bus.
 The 80386/80486/Pentium processors have 32-bit
address bus.
 The Pentium Pro processor has 36-bit address bus.
Three-Bus System Architecture
The Control Bus:
−How can we tell if the address on the address bus is a
memory address or an I/O port ?
− How can we tell if the memory or I/O access is a read or
write operation ?
− These questions are answered by the control bus
--The control bus carries commands from the CPU and returns status
signals from the devices
− Each time the processor outputs an address, it also
activates one of 4 control signals
(1) Memory Read
(2) Memory Write
(3) I/O Read
(4) I/O Write
Instruction Cycle State Diagram
Interrupts
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Mechanism by which other modules (e.g. I/O) may
interrupt normal sequence of processing
Program
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e.g. overflow, division by zero
Timer
Generated by internal processor timer
 Used in pre-emptive multi-tasking
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I/O
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from I/O controller
Hardware failure
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e.g. memory parity error
Interrupt Cycle
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Added to instruction cycle
Processor checks for interrupt
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Indicated by an interrupt signal
If no interrupt, fetch next instruction
If interrupt pending:
Suspend execution of current program
 Save context
 Set PC to start address of interrupt handler routine
 Process interrupt
 Restore context and continue interrupted program
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Transfer of Control via Interrupts
Instruction Cycle with Interrupts
Instruction Cycle (with Interrupts) State Diagram