25.3.3. Memory-Mapped I/O vs Isolated I/O

So, basically this is now programmed I/O details now you just see these are the simple steps ; first one CPU requests to I/O operation that in processor we have shown it, I/O module performs operation, then by looking into the state of the devices I/O module sets the status bit, CPU checks status bits periodically, I/O module does not inform CPU directly.

To understand how programmed I/O works, we can break it down into simple steps: 1. The CPU initiates an I/O operation by sending a request to the I/O module. 2. The I/O module performs the requested operation based on the current state of the devices connected. 3. It then updates a status bit that indicates to the CPU whether the operation was successful or if the device is ready. 4. However, the I/O module does not send direct notifications to the CPU about this status; instead, the CPU must continuously check the status, also known as polling.

Think of this like a teacher asking students if they are ready to present their projects. Instead of raising hands when they're ready (interrupts), students must continuously look up at the teacher to see if they can start presenting. This keeps the teacher busy, as they can't help others in the meantime, just like the CPU gets stuck waiting.

Already I have mentioned that we need some commands I/O commands. So, first the issues for that I/O command is like that how to identify the module. So that means, we have to have device address so this is the one point how we are going to give the addresses of the I/O devices.

I/O commands are essential for performing operations with input/output devices. To use these commands, we first need to identify the I/O device through unique addresses. Each device connected to the system must have a specific address assigned to it, similar to how every house has a unique postal address. The CPU uses these addresses to interact with the correct devices during operations. Additional control commands may be issued to start or initialize processes on the I/O devices.

Imagine ordering pizza online. You need to specify your home address to ensure the delivery driver finds the right house. In the same way, the CPU uses device addresses to ensure data is sent to the correct I/O device.

In memory mapped I/O it says that same address space is paired by a, shared by my memory and input output devices. Now you consider that size of my address bus is your 16, then what is the memory capacity with my size of address bus is 16, you should know it now.

Memory-mapped I/O utilizes the same address space for both memory and I/O devices. This means that when the CPU sends an address, it can refer to a memory location or an I/O device. For instance, if the address bus is 16 bits wide, it allows addressing 2^16 (or 64K) memory locations. A portion of this memory capacity is reserved for addressing I/O devices, allowing for a straightforward mapping between memory and I/O device operations.

Consider this system like a multi-purpose community center that serves as both a library (memory) and a recreation area (I/O devices). The same physical space is used, but certain areas are designated for specific activities. When individuals enter the center (the CPU initiating addresses), they could either check out books (read from memory) or play games (interact with I/O devices).

In case of isolated I/O the memory space and I/O space are different. So, now, if you consider the same example then what will happen? We are having 16 as size of address bus is 16.

Isolated I/O distinguishes between memory and I/O device address spaces. With a 16-bit address bus, both the memory and the I/O devices can have their unique address spaces (e.g., 64K each). This means that the CPU can access up to 65,536 different devices via I/O addressing, providing a significant advantage when it comes to interfacing with many devices, as each type of space is independent of the other.

Picture a function hall where different events can occur simultaneously: one side hosts a corporate meeting (memory access), while the other side accommodates a wedding (I/O operations). The distinct areas prevent confusion over which event is taking place, just like how isolated I/O keeps memory operations separate from I/O operations.

To identify these things so, we are going to use one more control signal, there will several control signal will come through this particular control bus.

In isolated I/O systems, control signals are essential in distinguishing whether the address placed on the address bus refers to a memory location or an I/O device. A common control signal referred to is the IO/M signal. If this signal is low (0), the address refers to memory; if high (1), it indicates an I/O device. Hence, this control signal allows the CPU to know where to direct its operations correctly and helps in maintaining the separation between memory and I/O addressing.

Think of it like a smart phone that has different modes: one mode for calling (memory function) and another for taking photographs (I/O function). The mode switch signal ensures the phone knows which functionality to activate at any time, just as the control signal identifies the type of operation to perform.

In most of the cases we are talking about your LDA load accumulator or say SPA store accumulator; one is your read command, another is write command.

When using memory-mapped I/O, the same commands used for memory operations can apply to I/O devices. This means instructions to load (LDA) or store (SPA) data work for both memory locations and I/O devices if the address corresponds to an I/O device. In contrast, isolated I/O requires separate commands specifically designed for I/O operations, such as IN and OUT instructions, to handle data transfers.

Imagine writing a note to your friend (memory operation) and sending a message via text (I/O operation). With memory-mapped I/O, you can write the same note regardless of whether it's for your friend or is just a draft. However, with isolated I/O, different methods (note writing vs. texting) apply for sending communication to ensure your message reaches its destination effectively.

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