Question

1. Explain the steps in the fetch–decode–execute cycle. Your explanation should include what is happening in the various registers.

2. Explain why, in MARIE, the MAR is only 12 bits wide and the AC is 16 bits wide. (Hint: Consider the difference between data and addresses.)

Answer 1

1) Steps in the fetch–decode–execute cycle

A) Fetch :-

The first step the CPU carries out is to fetch some data and instructions (program) from main memory then store them in its own internal temporary memory areas. These memory areas are called 'registers'.

This is called the 'fetch' part of the cycle.

For this to happen, the CPU makes use of a vital hardware path called the 'address bus'.

The CPU places the address of the next item to be fetched on to the address bus.

Data from this address then moves from main memory into the CPU by travelling along another hardware path called the 'data bus'.

B) Decode :-

The next step is for the CPU to make sense of the instruction it has just fetched.

This process is called 'decode'.

The CPU is designed to understand a specific set of commands. These are called the 'instruction set' of the CPU. Each make of CPU has a different instruction set.

The CPU decodes the instruction and prepares various areas within the chip in readiness of the next step.

C) Execution:-

This is the part of the cycle when data processing actually takes place. The instruction is carried out upon the data (executed). The result of this processing is stored in yet another register.

Once the execute stage is complete, the CPU sets itself up to begin another cycle once more.

2)

MARIE can handle 16-bit data, so the AC must be 16 bits wide. However, MARIE's memory is limited to 4096 address locations, so the MAR only needs to be 12 bits wide to hold the largest address.