Question

Draw detailed flowchart of the instruction cycle. Indicate the conditions in

which register-reference / memory-reference and input-output instructions are

executed. Also include the interrupt cycle micro-operations in the flowchart.​

Answer 1

Answer:

Registers Involved In Each Instruction Cycle:

Memory address registers(MAR) : It is connected to the address lines of the system bus. It specifies the address in memory for a read or write operation.

Memory Buffer Register(MBR) : It is connected to the data lines of the system bus. It contains the value to be stored in memory or the last value read from the memory.

Program Counter(PC) : Holds the address of the next instruction to be fetched.

Instruction Register(IR) : Holds the last instruction fetched.

The Instruction Cycle –

Each phase of Instruction Cycle can be decomposed into a sequence of elementary micro-operations. In the above examples, there is one sequence each for the Fetch, Indirect, Execute and Interrupt Cycles.The Indirect Cycle is always followed by the Execute Cycle. The Interrupt Cycle is always followed by the Fetch Cycle. For both fetch and execute cycles, the next cycle depends on the state of the system.We assumed a new 2-bit register called Instruction Cycle Code (ICC). The ICC designates the state of processor in terms of which portion of the cycle it is in:-

00 : Fetch Cycle

01 : Indirect Cycle

10 : Execute Cycle

11 : Interrupt Cycle

At the end of the each cycles, the ICC is set appropriately.The above flowchart of Instruction Cycle describes the complete sequence of micro-operations, depending only on the instruction sequence and the interrupt pattern(this is a simplified example). The operation of the processor is described as the performance of a sequence of micro-operation.

Different Instruction Cycles:

The Fetch Cycle –

At the beginning of the fetch cycle, the address of the next instruction to be executed is in the Program Counter(PC).

Step 1: The address in the program counter is moved to the memory address register(MAR), as this is the only register which is connected to address lines of the system bus.

Step 2: The address in MAR is placed on the address bus, now the control unit issues a READ command on the control bus, and the result appears on the data bus and is then copied into the memory buffer register(MBR). Program counter is incremented by one, to get ready for the next instruction.(These two action can be performed simultaneously to save time)

Step 3: The content of the MBR is moved to the instruction register(IR).