Fetch Execute Cycle

The Cycle The instruction cycle can be broken down into a series of steps, synchronised by the system clock and controlled by the control unit. The steps are as follows: (Fetch phase) 1) The address of the next instruction to be executed (held in the PC) is copied to the MAR. 2) The instruction held at that address is placed in the MBR. 3) Simultaneously, the contents of the PC are incremented by 1 to get ready for the next instruction. 4) The contents of the MBR is copied to the IR. (This frees up the MBR for the execute phase). (Execute phase) The instruction held in the IR is decoded. The instruction is executed. (The sequence of micro-operations in the execute phase depends on the particular instruction being executed.) That’s it.

The instruction cycle can be broken down into a series of

steps, synchronised by the system clock and controlled by the

control unit. The steps are as follows:

(Fetch phase)

1) The address of the next instruction to be executed (held in the PC) is copied to the MAR.

2) The instruction held at that address is placed in the MBR.

3) Simultaneously, the contents of the PC are incremented by 1 to get ready for the next instruction.

4) The contents of the MBR is copied to the IR. (This frees up the MBR for the execute phase).

(Execute phase)

The instruction held in the IR is decoded.

The instruction is executed. (The sequence of micro-operations in the execute phase depends on the particular instruction being executed.)

That’s it.

The Picture

The Task The homework will unsurprisingly be the example questions from the back of the book. You may start them in lesson and ask questions as you work.

The homework will unsurprisingly be the example questions from the back of the book. You may start them in lesson and ask questions as you work.