Explain the following terms: i) Stack organization ii) Bus structure and addressing modes iii) Fetching and execution cycle

i) Stack Organization

Stack organization in computer architecture refers to a specific way of managing data in a Last-In-First-Out (LIFO) manner. It is a linear data structure that serves as a collection of elements, with two principal operations: push, which adds an element to the collection, and pop, which removes the most recently added element.

How Stack Organization Works:

1. Initialization: A stack pointer (SP) is initialized to point to the top of the stack, which is the most recently added element.
2. Push Operation: When a new element is added, the stack pointer is decremented (in most architectures) and the element is placed at the location pointed to by the stack pointer.
3. Pop Operation: When an element is removed, it is taken from the location of the stack pointer, and then the stack pointer is incremented to point to the next available element.

Characteristics of Stack Organization:

• LIFO Structure: The last element pushed onto the stack will be the first one to be popped off.
• Stack Pointer (SP): A special register that always points to the top of the stack.
• Stack Overflow/Underflow: Occurs when trying to push an element onto a full stack or pop an element from an empty stack, respectively.

Example:

Consider a stack with a capacity of 3 elements:

Initial Stack (empty): |   | SP -> 3
                       |___|
                       |   |
                       |___|
                       |   |
                       |___|

Push 10: SP decrements to 2, 10 is placed at position 2.

Stack after push 10:   |   | SP -> 2
                       |___|
                       |10 |
                       |___|
                       |   |
                       |___|

Push 20: SP decrements to 1, 20 is placed at position 1.

Stack after push 20:   |   | SP -> 1
                       |___|
                       |20 |
                       |___|
                       |10 |
                       |___|

Pop: 20 is removed, SP increments to 2.

Stack after pop:       |   | SP -> 2
                       |___|
                       |   |
                       |___|
                       |10 |
                       |___|

ii) Bus Structure and Addressing Modes

Bus Structure:

A bus in computer architecture is a communication system that transfers data between components inside a computer or between computers. The bus structure typically includes data lines for data, address lines for addressing, and control lines for control signals.

Types of Buses:

• Data Bus: Carries the actual data being processed.
• Address Bus: Carries the addresses of the data (where the data is to be read from or written to).
• Control Bus: Carries control signals to manage the data and address buses.

Addressing Modes:

Addressing modes are the ways in which a microprocessor can access data stored in memory. Each mode provides different methods for accessing operands.

Addressing Mode	Description	Example
Immediate	Operand is part of the instruction	MOV AL, 5
Direct	Address of the operand is given explicitly	MOV AL, [1234h]
Indirect	Address of the operand is in a register	MOV AL, [BX]
Indexed	Base address plus an index	MOV AL, [BX+SI]
Register	Operand is in a register	MOV AL, BL
Register Indirect	Operand address is in a register pair	MOV AL, [BP+DI]

Example:

Consider an instruction MOV AL, [BX+SI] in an x86 architecture:

• MOV: Instruction to move data.
• AL: Destination register.
• [BX+SI]: Source operand in Indexed Addressing Mode, where BX is the base register and SI is the index register.

iii) Fetching and Execution Cycle

The fetching and execution cycle, also known as the instruction cycle, is the process by which a computer retrieves a program instruction from its memory, determines what actions the instruction dictates, and carries out those actions. This cycle is repeated continuously by the CPU from boot up to shut down.

Steps in the Fetch-Execute Cycle:

1. Fetch: The CPU reads an instruction from memory at the address pointed to by the program counter (PC).
2. Decode: The instruction is decoded to determine the operation to be performed and the operands to be used.
3. Execute: The CPU performs the operation on the operands.
4. Store: The result of the operation is written back to memory or a register if necessary.

Example of Fetch-Execute Cycle:

Consider a simple instruction: ADD A, B

1. Fetch: The CPU fetches the instruction ADD A, B from memory.
2. Decode: The CPU decodes the instruction and understands that it needs to add the contents of register B to register A.
3. Execute: The CPU adds the value in register B to the value in register A.
4. Store: The result is stored back in register A.

This cycle is fundamental to the operation of any CPU and is the basis for running all the programs on a computer.