15.3.3. I/O Devices and System Bus

The processor cannot work alone. We have to connect the Main Memory which is the storage device because the processor works on von Neumann stored program principle. We need some I/O devices Input Output devices.

I/O devices, or Input/Output devices, are essential for a computer system's operation. While the processor (CPU) processes data and instructions stored in the main memory, it needs a way to interact with the outside world - that’s where I/O devices come in. For example, keyboards and mice are I/O devices that allow users to input data, while monitors and printers are used to output data.

Imagine the processor as a chef in a kitchen (the main memory). The chef needs ingredients (data) from the pantry (main memory) and needs to serve the finished dish (output) to the table (users). The I/O devices act as the waitstaff, bringing ingredients to the chef and taking the finished meals to customers.

To control the Input Output devices we have this I/O Module. The Main Memory and I/O Module will be connected to the processor through this system bus.

The system bus acts as the communication highway for data transfer among the CPU, main memory, and I/O devices. It consists of three types of lines: the address bus, data bus, and control bus. The address bus carries the memory addresses from the processor to other components, indicating where data should be sent or fetched. The data bus carries the actual data that is being transferred. The control bus carries control signals from the CPU, coordinating actions of all components.

Think of the system bus like a network of roads in a city. Different types of roads (buses) serve different purposes: some roads are designated for cars (data), others for delivery trucks (control signals), and some simply allow you to access different areas (addresses). This network ensures that every part of the city (computer) is connected and can efficiently communicate and transport necessary materials.

We know that while executing an Instruction, the Instruction Cycle has 2 phases: Fetch and Execute. In the Fetch Cycle, we fetch the Instruction from the Memory and bring it to the processor.

The instruction cycle is how a CPU processes instructions. In the Fetch phase, the CPU retrieves an instruction from memory based on the current program counter (PC) value, which points to the memory address of the instruction. After fetching, the CPU increments the PC to point to the next instruction. In the Execute phase, the CPU performs the operation defined by the instruction. This cycle repeats until the program completes.

Imagine a librarian (CPU) fetching books (instructions) from shelves (memory). The librarian first checks the index (PC) for the location of the next book to fetch. Once the book is obtained, the librarian reads it (executes) and then looks up the index again for the next book. This process continues, one book at a time.

If you simply look into those particular figures, I think it is difficult to understand what we are going to do. Now, we can see what does it mean again.

When programming, it’s essential to understand memory addressing. Memory locations are often denoted in hexadecimal notation. For example, if a memory location is described as '300' in hexadecimal, it corresponds to a specific point in memory that the CPU can access using its address bus. The number of addressable locations depends on the size of the address bus, which dictates how much memory the CPU can directly access.

Consider a large library again - imagine each book (memory location) is assigned a unique identifier (memory address). If the library uses a more extensive numbering system (wider address bus), it can accommodate a larger collection of books compared to a library with a simpler system. This complexity allows the librarian (CPU) to find and utilize more resources efficiently.

Now what does this Instruction mean? 1940 says that after performing this particular Instruction then what will happen the 3 is loaded into this particular Register Accumulator.

Each instruction contains operation codes (opcodes) and operand addresses. The opcode '1940', for example, can instruct the CPU to load a value from memory into the accumulator. The '3' stored at the specified memory address is fetched and placed into the accumulator, where it can be used in subsequent operations. Understanding these steps is crucial for programming and debugging.

Think about this like putting a specific ingredient from your pantry (memory) into a mixing bowl (accumulator) before cooking. For instance, if you want to make a cake, the recipe might say to first get 3 eggs. You open the fridge and fetch those eggs (execute the instruction), placing them into the bowl to mix later with other ingredients.

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