Fetch Decode Execute Cycle

Fetch Decode Execute Cycle

https://www.youtube.com/watch?v=yyHFI5juppA&index=118&list=PLCiOXwirraUDUYF_qDYcZV8Hce8dsE_Ho

Fetch

Fetch Transfer Notation

[PC] → MAR

[Memory_mar] → MBR

[PC] + 1 → PC

[MBR] → CIR

Explanation

The address of the next instruction to be executed is copied from the Program Counter to the Memory Address Register.

The instruction at that address (stored in the Memory Address Register) is then copied to the Memory Buffer Register.

Simultaneously, the contents of the Program Counter are incremented by 1.

The contents of the Memory Buffer Register are copied to the Current Instruction Register.

Decode

Decode Transfer Notation

[CIR_op-code] → CU

Explanation

The op code from the CIR is passed to the Control Unit

Execute

Execute Transfer Notation (LOAD)

[CIR_operand] → MAR

[Memory_mar] → MBR

[MBR] → A-reg

Explanation

The operand of the instruction in the Current Instruction Register is copied into the Memory Address Register.

The contents of the Memory address (stored in the Memory Address Register) are fetched into the Memory Buffer Register.

Because it is a LOAD the Memory Buffer Register is copied into the A-reg (Accumulator).

Execute Transfer Notation (ADD)

[CIR_operand] → MAR

[Memory_mar] → MBR

[MBR] + A-reg → A-reg

Explanation

The operand of the instruction in the Current Instruction Register is copied into the Memory Address Register.

The contents of the Memory address (stored in the Memory Address Register) are fetched into the Memory Buffer Register.

Because it is a ADD the Memory Buffer Register is added to the current value of the A-reg (Accumulator), and then copied into the A-reg (Accumulator).

Execute Transfer Notation (STORE)

[CIR_operand] → MAR

A-reg → MBR

[MBR] → Memory_mar

Explanation

The operand from the instruction currently in the Current Instruction Register is copied into the Memory Address Register.

The current value in the A-reg (Accumulator) is copied into the Memory Buffer Register.

The value in the Memory Buffer Register is copied to the memory address (stored in the Memory Address Register) in the Memory.