Fetch, Execution and I/O - Computer Organisation Study Notes

The Central Processing Unit (CPU), also known as the Processor, is considered the "heart" of the computer.
The fundamental operations of a computer system are relatively simple, but it derives immense power by repeating these basic operations billions of times per second.
The basic operation of a computer involves several steps: * Programs and data are input into the system and stored in the memory. * Data is then fetched from memory to be processed by the Arithmetic and Logic Unit (ALU). * All operations within the computer are coordinated and controlled by the Control Unit. * Information that has been processed is finally sent to the output unit.

Data in a computer is stored exclusively as binary digits, known as bits.
A bit is the smallest unit of data and can have one of two values: $1$ or $0$ .
The physical representation of bits depends on the medium: * Electronic Systems: A $1$ bit is represented by the presence of voltage (e.g., $5\,\text{volts}$ ), while a $0$ bit is represented by the absence of voltage. * Fiber-optic Systems: A $1$ bit is represented by the presence of light, while a $0$ bit is represented by the absence of light.
A group of $8\,\text{bits}$ forms a single byte.
Each bit within a byte has a positional value based on powers of two: $1, 2, 4, 8, 16, 32, 64, 128$ .
The specific combination of bits in a byte is what determines its corresponding decimal value.

The ASCII table (American Standard Code for Information Interchange) is used to convert binary data into characters.
The process involves converting bits and bytes into a decimal value, which then maps to a character in the ASCII table.
Example of bit-to-character conversion: * Binary: $0100 0001$ * Decimal: $65$ * ASCII Character: $\text{A}$
The computer performs a translation process for key characters (e.g., typing the word "LISA"): * 1. The user types "LISA" on the keyboard. * 2. The keyboard converts every character into its specific binary code. * 3. This binary data is sent to the memory and the CPU for processing. * 4. The binary data is transmitted to the output device (e.g., a printer). * 5. The printer converts the binary code back into characters to print the output.

Primary Memory (Main Memory): * It is organized into $32\text{-bit}$ words. * Each word contains four $8\text{-bit}$ bytes. * It typically provides a capacity of several gigabytes. * Programs and data must be loaded into primary memory before they can be executed.
Cache Memory: * This is an adjunct to the main memory and is fabricated directly on the processor chip. * It is much smaller and significantly faster than the main memory. * Its purpose is to hold sections of the program and data that are currently or frequently being executed.
Processor Components: * Logic Circuits: These circuits perform arithmetic and logic operations on word-size data operands. * Timing and Control Circuits: These are responsible for fetching program instructions and data from memory in a sequential manner. * Registers: The processor typically contains $16$ or $32$ registers, each of which holds one word of operand data.

Arithmetic and Logic Unit (ALU): * The ALU is where most computer operations are executed. * It specifically performs arithmetic operations (like addition) or logic operations.
Control Unit: * The Control Unit is responsible for coordinating the operation of all other units: Memory, ALU, and I/O units. * It ensures that information is stored, processed, and moved between units correctly.
Programming and Instruction Storage: * An instruction specifies a specific operation and the locations of the data operands required. * A single encoded instruction is typically held in a $32\text{-bit}$ word. * A program consists of a sequence of these instructions executed one after another. * Both the program and its required data are stored in the main memory.

A single cycle of operation is known as the instruction cycle or the machine cycle.
It consists of four primary phases: Fetch, Decode, Execute, and Store.
The specific steps in the instruction cycle are: * 1. Fetch: The CPU retrieves the next instruction from memory. * 2. Decode: The CPU interprets the instruction to determine the required action. * 3. Execute: The instruction is carried out (e.g., performing a calculation or moving data). * 4. Store: Results are stored or written back to memory after execution. * 5. Interrupt Check: The CPU checks for interrupts and services them if present; if not, it fetches the next instruction.

Computer instructions generally fall into three categories: * Load: Read a data operand from memory or an input device into a processor register. * Store: Write a data operand from a processor register to memory or an output device. * Operate: Perform an arithmetic or logic operation on data operands already located in processor registers.
Example Program for the calculation $C = A + B$ : * In this context, A, B, and C are labels for memory word addresses, and Ri represents processor registers. * 1. $\text{Load R2, A}$ * 2. $\text{Load R3, B}$ * 3. $\text{Add R4, R2, R3}$ * 4. $\text{Store R4, C}$

The Program Counter (PC) is a specialized register in the processor that contains the address (location) of the instruction being executed at any given time.
Mechanisms of the PC: * As each instruction is fetched, the PC increases its stored value by $1$ . * After a fetch, the PC points to the next instruction in the sequence. * Upon computer restart or reset, the PC normally reverts to $0$ .
Step-by-step process for the instruction $\text{Load R2, LOC}$ : * 1. The control circuits send the address stored in the PC to memory and issue a "Read" command. * 2. The instruction is loaded from memory into the Instruction Register (IR). * 3. The PC is incremented to point to the next instruction. * 4. The address $\text{LOC}$ is sent to memory, and another "Read" command is issued. * 5. The word from memory is loaded into register $\text{R2}$ .

Program execution performance is influenced by four main factors: * 1. The speed of the electronic circuits within the processor. * 2. The access times to the cache and the main memory. * 3. The specific design of the instruction set. * 4. Parallelism: The number of operations that can be performed simultaneously.
Parallelism and Multicore Processors: * Multiple processing units can be fabricated on a single chip. * The term "core" refers to each of these individual processors. * The term "processor" is then used to describe the entire chip. * Common configurations include dual-core, quad-core, and octo-core processors.