Operational Flow Chart

🔄 📊 🖥️

Of the IAS Computer

The IAS Computer (and any Von Neumann type machine) runs instructions in a repeating cycle called the Instruction Cycle.

This cycle has three main phases:

  • Fetch Phase → Get the instruction from memory.
  • Decode Phase → Interpret what the instruction means.
  • Execute Phase → Perform the required operation.
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🔹 Step-by-Step Instruction Cycle

1. Fetch Phase

Step 1: Control Unit uses the Program Counter (PC) to find the instruction's memory address.

Step 2: Instruction is fetched from memory into the Instruction Register (IR).

Step 3: PC increments → points to the next instruction.

2. Decode Phase

Step 4: The Control Unit (CU) decodes the instruction in the IR.

Decoding determines:

  • What operation to perform (add, subtract, load, store, jump).
  • Which operands (data values) are needed.
3. Execute Phase

Step 5: CU sends control signals to the ALU or other components.

Step 6: ALU performs the operation (e.g., A + B).

Step 7: Result is stored in Accumulator (AC), registers, or memory.

4. Branching or Halt

Step 8: If instruction is a branch (jump), PC updates with new address.

Step 9: If it's a halt, execution stops. Otherwise, go back to Step 1.

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🔹 Flow Chart (Visual Representation)

Here's a simplified diagram of the operational flow of the IAS Computer:

Start/Reset
Fetch instruction from memory (PC→IR)
Increment PC
Decode instruction (CU interprets IR)
Execute instruction (ALU / Memory / I/O)
Store result (AC/Register/Mem)
Branch/Halt check
Yes
Update PC for Jump Address
No
Next instruction cycle (Repeat)
Halt (End)

💡 This flow chart represents the continuous cycle of operations in the IAS Computer, forming the foundation of modern CPU operation.

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🔹 Example (Simple Program Execution)

Let's say the IAS computer is given:

Instruction: ADD A, B → C

1

Fetch

PC points to the memory location of ADD. Instruction loaded into IR.

2

Decode

CU decodes → "Perform addition of operands A and B".

3

Execute

ALU adds values from memory location A and B.

4

Store

Result stored at memory location C.

5

Repeat/End

Next instruction is fetched unless HALT.

🔄 This process repeats for each instruction in the program, creating a continuous cycle of fetch, decode, and execute operations.

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✅ Summary

  • The IAS Computer instruction cycle is a repeating loop of Fetch → Decode → Execute, with possible branching or halting.
  • Fetch Phase: Retrieves the instruction from memory using the Program Counter.
  • Decode Phase: Interprets the instruction to determine the operation and operands.
  • Execute Phase: Performs the operation and stores the result.
  • Branching/Halting: Determines the next instruction or ends execution.
  • This operational flow is the foundation of modern CPUs today.
🔄

Continuous Cycle

Instructions processed in a continuous loop

🧠

CPU Foundation

Basis of modern processor operation

⚡

Efficiency

Streamlined process for execution

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