Introduction to Computer Architecture

Introduction to Computer Architecture and Organization

• This lecture covers computer architecture and organization, including structure and function, and computer evolution and performance.

Organization and Architecture

• Computer architecture describes visible attributes of a system to a programmer, affecting program execution.
• Examples include the instruction set, bit representations for data types, and input/output mechanisms and memory addressing techniques.

Organization and Architecture (2)

• Computer organization concerns the operational units and interconnections that implement architectural specifications.
• Examples include hardware details (e.g., control signals), interfaces between the computer and peripherals, and memory technology.

Organization and Architecture (3)

• Architectural decisions (e.g., including a multiply instruction) affect design choices.
• Organizational decisions (e.g., implementing a multiply instruction using a dedicated unit or leveraging an existing add unit) depend on frequency of use, relative speed, and cost/size of specialized units.

Structure and Function

• Structure describes the relationships among components.
• Function details how individual components act within the structure.
• Computer processing generally involves four functions: data processing, data storage, data movement, and control.

Functional View of the Computer

• A computer system is generally composed of a data storage facility, program facilities, data movement apparatus, and control mechanisms.

Data Movement and Storage

• Data movement transfers data between peripherals or communication lines.
• Data Storage transfers data between external environment and computer storage to read/write data.
• Data processing uses data in storage and in the process of transferring in or out of the computer.

Computer Structure

• A computer consists of four main structural components: the Central Processing Unit (CPU), Main Memory, input/output and system interconnection.
• The CPU directs computer operations, the main memory stores data, I/O moves data between the system and the external environment, and system interconnection connects these components.

CPU Structure (3)

• The CPU is a complex component with a control unit (controlling CPU operation), arithmetic logic unit (performing data processing), registers (internal storage), and CPU interconnections (communication between units).

Von Neumann Machine

• The stored-program concept, attributed to John von Neumann, allows programs to share memory with data.
• The IAS computer, developed in 1946, was a prototype for future general-purpose computers.
• IAS computer is the prototype for all general-purpose computers

Performance and Evolution 3

• The IAS computer comprises main memory, arithmetic and logic unit (ALU), and a control unit that executes instructions in memory.
• Input/output (I/O) devices work under control of the control unit.

Performance and Evolution 5

• Most modern computers share the general structure and function of the von Neumann architecture.
• The IAS memory consists of 1000 locations (words), each holding 40 binary bits.
• Both data and instructions exist in memory.
• Data is represented in binary form and instructions are binary codes.

Performance and Evolution 6

• Memory words may also hold two 20-bit instructions. Instructions have an 8-bit opcode and 12-bit address.

Performance and Evolution 7

• The control unit executes instructions by fetching them from memory and executing one at a time.
• The ALU and control unit utilize registers (e.g., Memory Buffer Register (MBR), Memory Address Register (MAR), Instruction Register (IR), Instruction Buffer Register (IBR), Program Counter (PC), Accumulator (AC) and MQ) for temporary storage or processing.

Performance and Evolution 8

• Registers serve temporary storage or processing during instruction execution using specific bits of an instruction

Performance and Evolution 9

• The IAS operates through instruction cycles comprising fetch and execute subcycles.
• The fetch cycle loads the next instruction's opcode into the Instruction Register (IR) and the address into the Memory Address Register (MAR).
• The instruction may come from the IBR or from memory via the MBR, IR, and MAR.

Performance and Evolution 10

• The control unit interprets opcodes (during the execute cycle) to manage data movement and ALU operations.
• The IAS has 21 instructions, grouped for data transfer, unconditional branching, conditional branching, arithmetic, and address modification.

Performance and Evolution 11

• Unconditional branching alters the instruction sequence.
• Conditional branching depends on a condition.
• Arithmetic operations involve the ALU.
• Address modification allows adjusting addresses in instructions for flexibility.

Performance and Evolution 12

• Instructions conform to a specific format (using instruction opcodes and addresses) in memory.
• The multiplication procedure requires separate suboperations for each bit position (except the sign bit), contributing to the overall complexity.

Computer System - Program Concept

• Hardwired systems are inflexible, handling only defined tasks.
• General-purpose computer systems utilize programmable instructions, requiring no re-wiring and changing the control signals.

What is a Program?

• A program is a sequence of steps, each employing distinct control signals, leading to arithmetic and logical operations.

Function of Control Unit

• A unique code is assigned for each operation. (E.g., ADD or MOVE).
• A dedicated hardware (segment) decodes these codes and generates control signals.
• This results in a computer.

Computer Components

• The Control Unit (CU) and the Arithmetic Logic Unit (ALU) together form the Central Processing Unit (CPU).
• Input/output (I/O) is needed for data and instruction transfer to and from the system.
• Temporary storage for code and outcomes is also needed. Main memory serves this function.

Computer Components 33

• A computer system has several components working together. The CPU, which has components such as the PC, IR, MAR and MBR directs the computer operations. This communicates with the main memory which stores data and instructions. Input/Output (I/O) devices handle data transfer in and out of the system.

Instruction Cycle

• The instruction cycle involves two steps: fetching an instruction and executing it.

Fetch Cycle

• The program counter (PC) holds the address for the next instruction to be fetched.
• The processor fetches the instruction from the memory location.
• The program counter is incremented to point to the next instruction.
• Instructions are loaded into the Instruction Register (IR).
• The processor interprets the instruction and performs the actions required.

Execute Cycle

• The processor-memory unit transfers data between the CPU and the main memory.
• The CPU communicates with the I/O module.
• Arithmetic or logical operations are performed on data.
• The sequence of operations can be altered (e.g., using jumps).
• Several or a combination of above may take place during the execute cycle

Example of Program Execution

Instruction Cycle State Diagram

• The instruction cycle consists of instruction fetch, operand fetch, operand store and returns for string or vector data activities.

Interrupts

• Interrupts allow other modules (e.g., I/O) to interrupt the normal processing sequence.
• These can be caused by program errors (e.g., overflow, division by zero), timers, I/O operations, or hardware failures.

Program Flow Control

• Program flow control differs based on interruptions.
• During no interruptions, the program progresses sequentially.
• With short I/O wait times, interruptions minimally disrupt the program's flow.
• With long wait times, interruptions significantly affect the program flow.

Interrupt Cycle

• The processor checks for interrupts throughout the instruction cycle.
• If no interrupts, it fetches the next instruction.
• If an interrupt is pending, the current program's execution is suspended and its context is saved.
• The processor sets the program counter (PC) to the address of the interrupt handler routine.
• Interrupt processing is performed and control returns to the interrupted program after the interruption has been handled.

Transfer of Control via Interrupts

• Interrupts interrupt a program at a specified point and transfer control to an interrupt handler.

Instruction Cycle with Interrupts

• The instruction cycle is extended to include an interrupt check at different points.
• Interrupts that are enabled are processed, and those that are disabled are ignored.

Program Timing: Short I/O Wait

• Without interrupts, a CPU waits for an I/O operation to complete; with short waits, interrupts allow the CPU to continue working on something else while waiting for I/O

Program Timing: Long I/O Wait

• Without interrupts, data transfer between the CPU and External Devices can cause the CPU to wait for a considerable period; with interrupts, the CPU can do other operations

Instruction Cycle (with Interrupts) - State Diagram

• Instructions fetch, address calculation, operation decoding, operand address calculation, operand store and multiple calculations are part of the instruction cycle. Interruption checks are integrated.