1. Computer Organization and Assembly language
Lecture 1 & 2
Introduction and Basics
Course Instructor: Aisha Danish

2. Lecture Overview Course Information Marking Scheme Recommended Books
Why study Computer Organization?
What is a Microcomputer ?
Fetch, Decode and Execute
Three-Bus System Architecture

3. Course Information Name: Computer Organization and Assembly Language
Course Code: CSC-395
Credit Hours: 2+1

4. Marking Scheme 3 quizzes 10 marks 2 Assignments 5 marks
1 Class Presentation 5 marks
1 Lab Quiz marks
1 Lab Project 5 marks
Mid-term Exam 20 marks
Final Exam 50 marks

5. Recommended Books
Computer Organization and Architecture: Designing for Performance, 8/E, William Stallings
Assembly Language for Intel Based Processors, Kip R. Irvine

6. Why study computer organization and architecture?
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.
Computer organization
Encompasses all physical aspects of computer systems.
E.g., circuit design, control signals, memory types.
How does a computer work?
Computer architecture
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?

7. Microcomputer
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)

8. Program Concept 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

9. What is a program? 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

10. Function of Control Unit
For each operation a unique code is provided
e.g. ADD, MOVE
A hardware segment accepts the code and issues the control signals
We have a computer!

11. Components
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
Temporary storage of code and results is needed
Main memory

12. Computer Components: Top Level View

13. Instruction Cycle
Two steps:
Fetch
Execute

14. Fetch Cycle
Program Counter (PC) holds address of next instruction to fetch
Processor fetches instruction from memory location pointed to by PC
Increment PC
Unless told otherwise
Instruction loaded into Instruction Register (IR)
Processor interprets instruction and performs required actions

15. Execute Cycle Processor-memory Processor I/O Data processing Control
data transfer between CPU and main memory
Processor I/O
Data transfer between CPU and I/O module
Data processing
Some arithmetic or logical operation on data
Control
Alteration of sequence of operations
e.g. jump
Combination of above

16. 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)

17. 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.

18. Buses 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)

19. What is a Bus? 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
Power lines may not be shown

20. Data Bus Carries data Width is a key determinant of performance
Remember that there is no difference between “data” and “instruction” at this level
Width is a key determinant of performance
8, 16, 32, 64 bit

21. Address bus 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
e.g has 16 bit address bus giving 64k address space

22. Control Bus Control and timing information Memory read/write signal
Interrupt request
Clock signals

23. Three-Bus System Architecture
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

24. 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)

25. 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 processor has 32-bit internal and 32- bit external data buses. − The Pentium processor has 32-bit internal data bus and 64-bit external data bus

26. 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 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.

27. 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

28. Instruction Cycle State Diagram

29. Interrupts
Mechanism by which other modules (e.g. I/O) may interrupt normal sequence of processing
Program
e.g. overflow, division by zero
Timer
Generated by internal processor timer
Used in pre-emptive multi-tasking
I/O
from I/O controller
Hardware failure
e.g. memory parity error

30. Interrupt Cycle Added to instruction cycle
Processor checks for interrupt
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

31. Transfer of Control via Interrupts

32. Instruction Cycle with Interrupts

33. Instruction Cycle (with Interrupts) - State Diagram