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

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

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

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

Recommended Books 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? 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?

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)

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

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

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!

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

Computer Components: Top Level View

Instruction Cycle Two steps: Fetch Execute

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

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

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 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? 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

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

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. 8080 has 16 bit address bus giving 64k address space

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

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

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 32- bit 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 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

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

Transfer of Control via Interrupts

Instruction Cycle with Interrupts

Instruction Cycle (with Interrupts) - State Diagram