✅ Introduction
In a computer system, registers are small, fast memory locations used by the CPU to temporarily store and manipulate data.
To transfer data between these registers, we use a common bus system.
🧠 What is a Common Bus?
A bus is a set of shared lines (wires) that can carry data, addresses, or control signals among the different components of a computer.
The common bus allows multiple registers to connect through a single shared data path, reducing hardware complexity.
🧩 Why Use a Bus for Register Transfer?
Without a bus:
- Each register would need direct lines to every other register
- This would require many connections and increase circuit complexity
With a bus:
- A single shared pathway is used
- Data transfer is simpler and more efficient
⚙️ Basic Components Involved
| Component | Description |
|---|---|
| Registers | Small memory locations in the CPU (AC, DR, IR, etc.) |
| Bus (16-bit) | A common 16-bit data path shared by all registers |
| Multiplexer (MUX) | Selects one register to place data on the bus |
| Control Signals | Determine which register sends or receives data |
🔁 Steps of Register-to-Register Transfer Using the Bus
Let’s consider an example:
👉 Transfer data from Register A (source) to Register B (destination).
🔹 Step 1: Select Source Register
- A multiplexer (MUX) chooses Register A as the data source
- The control signal Enable_A is activated
🔹 Step 2: Data Placed on the Bus
- The content of Register A is placed on the common bus
- The bus now holds A’s data temporarily
🔹 Step 3: Select Destination Register
- The control unit activates Load_B signal
- Register B reads data from the bus and stores it
📘 Micro-operation Format
In micro-operations, the data transfer can be written as:
B ← A
Translated to control steps:
- Enable output of A
- Place data on bus
- Enable input of B
🧠 Example: DR → AC
If the Data Register (DR) has a value and we want to copy it to the Accumulator (AC):
- Activate
Enable_DR(DR outputs to bus) - Bus carries the 16-bit value
- Activate
Load_AC(AC reads from bus)
Result:
AC ← DR
📊 Diagram (Visual Summary)
[Register A] --┐
[Register B] --┤
[Register C] --┤→ [MUX] → [16-bit BUS] → [Input Load Control] → [Register D, etc.]
Control signals determine which register sends/receives.
✅ Advantages of Using Bus for Register Transfer
- ✅ Reduces hardware connections
- ✅ Easier to control data flow
- ✅ Efficient for small-scale CPU designs
- ✅ Enables modular system design
⚠️ Limitations
- ❌ Only one transfer at a time (serial operation)
- ❌ May become a bottleneck in high-performance systems
🎯 Conclusion
The common bus system is a fundamental concept in CPU design.
It allows efficient data transfer among registers using a single shared bus, controlled by multiplexers and control signals.
Understanding this helps students grasp how micro-operations work inside the CPU.