Programmed I/O: Examples, Issues, and IO Transfer Modes | COA

Summary

In this video, the presenter explains programmed I/O and its challenges using a practical example involving a CPU operating at 1 GHz interacting with a keyboard. The CPU must wait for data from the slow I/O device, leading to inefficient idle time and performance degradation. As the CPU processes instructions in nanoseconds while the keyboard responds in milliseconds, this delay results in a million clock cycles of waiting. The video emphasizes the limitations of programmed I/O and hints at solutions like interrupt-driven I/O and direct memory access, which will be explored in future content.

Takeaways

• 😀 Programmed I/O functions involve direct communication between the CPU and I/O devices via a system bus.
• 🔄 The system bus consists of an address bus, data bus, and control signals, facilitating data transfer.
• ⏱️ A CPU operating at 1 GHz has a clock cycle of 1 nanosecond, affecting instruction execution speed.
• 📥 The instruction 'LDA io0' loads data from an I/O port (like a keyboard) into the CPU's accumulator.
• ⌨️ A keyboard typically operates at a speed of 1 kbps, taking 1 millisecond to respond to CPU requests.
• 🕒 The CPU experiences a significant wait time of approximately 1 million clock cycles due to the slow response of the keyboard.
• 🚫 This idle time results in CPU blocking, where it cannot perform other tasks while waiting for data.
• 📉 The inefficiency of programmed I/O leads to a decrease in overall CPU performance.
• ⚙️ Alternatives such as interrupt-driven I/O and Direct Memory Access (DMA) can help address the drawbacks of programmed I/O.
• 🔍 Future videos will explore these alternative methods for enhancing I/O operations and CPU efficiency.

Q & A

What is programmed I/O?

Programmed I/O is a method where the CPU directly controls the transfer of data between I/O devices and memory, executing instructions sequentially.

How does the system bus function in the context of CPU and I/O devices?

The system bus facilitates communication between the CPU and I/O devices, comprising an address bus, data bus, and control signals to manage data transfer.

What is the significance of the CPU clock frequency in programmed I/O?

The CPU clock frequency determines how quickly the CPU can execute instructions, with a higher frequency leading to shorter clock cycles and faster processing.

What example is given to illustrate the concept of programmed I/O?

The video uses a keyboard as an example of an I/O device operating at 1 kbps, demonstrating how its slower response time impacts CPU performance.

What is the time taken by the keyboard to send data to the CPU?

The keyboard takes 1 millisecond to send data to the CPU, which is significantly longer than the CPU's clock cycle time of 1 nanosecond.

What issue arises due to the slow response of I/O devices in programmed I/O?

The main issue is that the CPU has to wait for a long time (up to 1 million clock cycles) for data from slow I/O devices, leading to inefficiency and idle CPU time.

How many clock cycles does the CPU wait when using a slow I/O device like the keyboard?

The CPU waits for 1 million clock cycles when using a keyboard operating at 1 kbps, translating to a wait time of 1 millisecond.

What are the potential solutions to the issues caused by programmed I/O?

Two potential solutions include interrupt-driven I/O and direct memory access (DMA), which help improve CPU utilization and reduce idle time.

What will the next videos cover regarding I/O systems?

The next videos will explore interrupt-driven I/O and direct memory access (DMA) in detail, addressing how these methods can enhance system performance.

Why is understanding programmed I/O important for students of engineering?

Understanding programmed I/O is crucial because it highlights the interaction between the CPU and I/O devices, and informs students about performance issues and solutions in computer architecture.