Imagine your computer as a busy chef in a kitchen.
The CPU is that chef — fast, smart, and in charge of everything.
The peripheral devices (like keyboard, printer, or mouse) are the kitchen tools — each working at its own speed.
Now, the chef (CPU) has to talk to these tools to get things done — but the question is, how does it talk to them?
One of the simplest methods is called Program Controlled I/O.
⚙️ What is Program Controlled I/O?
Program Controlled I/O (also known as Programmed I/O) is the simplest way the CPU and I/O devices exchange data.
In this method, the CPU controls everything step by step using instructions from a program.
The CPU sends data to the device or receives data from it only when the device is ready — but here’s the catch: the CPU keeps checking again and again to see if the device is ready.
This is called polling or busy waiting.
🧩 Let’s make it relatable
Think of a student waiting for an email from their teacher.
Instead of relaxing, the student keeps refreshing the inbox every few seconds 😅 —
That’s exactly what the CPU does in Program Controlled I/O!
It keeps asking the device:
“Are you ready now?”
“How about now?”
“Ready yet?”
Only when the device finally says “Yes, ready!” — the CPU transfers the data.
💡 How It Works (Step-by-Step)
Let’s break it down into simple steps:
- CPU gives command:
The CPU sends an instruction to the I/O interface saying, “Hey, get ready to send or receive data.” - CPU checks status:
The CPU keeps reading a status flag (a small indicator) from the device to know if it’s ready. - Wait until ready:
The CPU keeps looping — checking the flag repeatedly — this is the polling process. - Transfer data:
Once the device signals that it’s ready, the CPU either sends data to it (for output) or reads data from it (for input). - Repeat or stop:
The CPU continues this for all data items or stops when the transfer is done.
🖼️ Diagram: Program Controlled I/O
+----------------------+
| CPU |
| (Executes Program) |
+----------+-----------+
|
| Control & Data Lines
|
+----------+-----------+
| I/O Interface |
+----------+-----------+
|
| Data Transfer
|
+----------+-----------+
| Peripheral Device |
+----------------------+Explanation of Diagram:
- The CPU sends control signals to the I/O interface.
- The I/O interface communicates with the peripheral device (like a printer or keyboard).
- The CPU keeps checking the device status before every transfer.
🧮 Example
Suppose your CPU wants to print a document:
- CPU tells the printer to get ready.
- Printer takes time to warm up and get in position.
- CPU keeps checking: “Ready yet?”
- Printer signals “Ready.”
- CPU sends one line of text.
- CPU checks again for the next line.
The CPU is busy the whole time — not doing anything else!
⚖️ Advantages of Program Controlled I/O
✅ Simple to design and easy to understand
✅ No extra hardware (like interrupt controllers or DMA units) needed
✅ Useful for small or low-speed devices
⚠️ Disadvantages
❌ Wastes CPU time: The CPU stays in a loop waiting for the device.
❌ Slow for large data: Since the CPU waits for every single byte, it’s inefficient.
❌ Not suitable for multitasking: CPU can’t perform other tasks while waiting.
🔁 Analogy
Let’s think of it like this:
If the CPU were a person, Program Controlled I/O would be like standing in front of the microwave watching your popcorn pop — you’re doing nothing else, just waiting for it to finish 🍿.
Wouldn’t it be better if you could go do something else and get notified when it’s ready?
(That’s what interrupts and DMA do — but we’ll get to that later.)
🧭 Summary Table
| Step | Action | Description |
|---|---|---|
| 1 | Command | CPU tells device to prepare for I/O |
| 2 | Polling | CPU keeps checking device status |
| 3 | Data Transfer | CPU sends/receives data |
| 4 | Repeat | Continue until all data transferred |
About the Author
