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Today, we are going to dive into the world of I/O Command Execution, specifically focusing on programmed I/O. Can anyone tell me what programmed I/O means?
I think it means that the CPU directly controls the I/O operations, and it has to check when the device is ready.
Exactly! That's a great start. In programmed I/O, the CPU must periodically check the status of I/O devices to see if they're ready. This process can lead to wasted CPU time, as the CPU can't perform other tasks while waiting. This is called polling.
So, what’s the first thing the CPU does when it wants to perform I/O?
The CPU first requests the I/O operation from the I/O module. After that, it checks the status to see if the operation can be executed.
How does the CPU know when the device is ready?
Good question! The I/O module sets a status bit that the CPU checks periodically. If the bit indicates that the device is ready, then the data transfer can occur.
To sum up, programmed I/O involves a series of steps starting from CPU requesting an I/O operation to checking the status bit until it can perform the operation.
Let's shift our focus to the addressing scheme used in I/O devices. Why do you think it’s important to uniquely identify each I/O device?
So that the CPU knows which device to communicate with, right?
Correct! Addressing is crucial. Each I/O device must have a unique address, just like memory locations, to ensure proper communication. There are two methods: memory-mapped I/O and isolated I/O. Can anyone differentiate between the two?
In memory-mapped I/O, I/O devices share the address space with memory, right?
Yes! And how about isolated I/O?
Isolated I/O keeps separate address spaces for I/O and memory devices.
Exactly! This method allows a larger number of devices since both memory and I/O can have their own address space. Now, remember the control signal I/O/M. It tells the system whether the address is for memory or an I/O device.
To wrap up, addressing for I/O operations is vital, and understanding the difference between memory-mapped and isolated I/O can help us optimize our systems.
Now that we understand how I/O devices are addressed, let's talk about the commands that we need to perform I/O operations. What types of commands do you think are necessary?
I assume there are commands for reading and writing data!
Absolutely! There are three main categories of commands: control commands, test commands, and read/write commands. Can someone give an example of a control command?
A command to initiate a printer, like setting the print head position, would be a control command.
Exactly! The control commands are crucial as they ensure devices are set up correctly before data transfer. What about test commands?
Those could be commands that check if a device is powered on or ready to operate.
Right! And finally, we have the read and write commands that actually transfer data between the CPU and I/O devices. For instance, the IN command fetches data from a device, and the OUT command sends data to a device.
In summary, effective I/O operation relies on using the appropriate commands for control, testing, and data transfer.
We've covered commands and addressing, but how does actual data transfer happen between a CPU and an I/O device using programmed I/O?
Is it like running a program that keeps checking if the device is ready?
Exactly! We write a device service routine, which involves a loop that checks the status bit. Once the device is ready, the routine executes the remaining logic to transfer data.
So, the program controls both the check for readiness and the actual data transfer?
Yes! That’s how programmed I/O works. It's significant because it allows us to automate the process while continuously monitoring the I/O status.
Are there any drawbacks to this method?
Yes, the main drawback is inefficiency due to CPU polling. The CPU is either waiting for the device or performing the operations sequentially, which can lead to delays.
To summarize, programmed I/O enables data transfer through iterative checking, but it can lead to CPU inefficiencies.
A technique for executing I/O commands where the CPU is responsible for polling device status.
The CPU frequently checks the device status, leading to potential inefficiency.
A binary flag set by the I/O module to indicate readiness for data transfer.
A scheme where I/O devices share the same address space as memory.
A method where separate address spaces are dedicated to I/O devices and memory.
When the CPU wants to read data from a keyboard, it continuously checks the status bit to decide when the keyboard is ready to send data.
In a memory-mapped I/O scheme, the address range 0xFFFF to 0xFFFF could be reserved for devices like printers, allowing load and store operations to treat them as if they were memory addresses.
In I/O device land, the CPU does stand; polling away, its time slips through sand.
Imagine a device shop where each device has a special badge. The CPU asks each device if it's ready, checking them one by one, losing time while it waits.
To remember the steps in Programmed I/O: R - Request, S - Status check, T - Transfer data.
P.I.O. - Polling I/O Operations: 'Polling' for readiness, 'Input/Output' commands processed.
Term
What is Polling?
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Term
What is the Status Bit?
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What differentiates Memory-Mapped I/O from Isolated I/O?
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A method where the CPU directly controls I/O devices and checks their status to perform data transfers.
The continuous checking of the status of an I/O device by the CPU.
A binary indicator set by the I/O module to signal if it is ready for a data transfer.
An I/O addressing scheme where I/O devices share the same address space as memory.
An I/O addressing scheme where memory and I/O devices have separate address spaces.
Instructions sent to I/O devices to prepare them for data operations.
Commands that facilitate data transfer from and to I/O devices.
This section covers the essentials of I/O command execution, focusing on programmed I/O techniques, addressing schemes, and the differences between memory-mapped and isolated I/O.
In this section, the I/O command execution process is outlined, detailing the interactions between the CPU and I/O devices via programmed I/O. The significance of unique addressing for each I/O device is emphasized, alongside the distinctions between memory-mapped I/O and isolated I/O systems.
This section delves into the processes involved in I/O command execution, primarily focusing on programmed I/O methodology. When the CPU initiates an I/O operation, it must first request this operation and then monitor the associated status register to determine whether the device is ready for data transfer. This constant polling can lead to CPU time wastage, as the CPU cannot perform other tasks during this wait.
Programmed I/O Steps: The steps involved in programmed I/O include:
Although programmed I/O allows simple device communication, it has limitations such as requiring manual polling and delaying CPU tasks.
I/O Addressing Schemes: Additionally, for successful data transmission, each I/O device must be uniquely identified via an addressing scheme. There are two main types of addressing: memory-mapped and isolated I/O. In memory-mapped I/O, both memory and I/O devices share the same address space, whereas in isolated I/O, dedicated separate address spaces are utilized for the two categories, enhancing the number of devices that can be connected. The control signal I/O/M distinguishes between memory and I/O addresses.
By placing I/O operations alongside memory operations in the case of memory-mapped I/O, a consistent instruction set can be used; however, isolated I/O requires specific instructions for device operations. This section concludes with an overview of how data transfer occurs using programmed I/O, illustrating its architecture and how addressing is integral to these operations.