Computer Architecture and
the Fetch-Execute Cycle
The Fetch-Decode-ExecuteReset Cycle
Learning Objectives
Describe in simple terms the fetch /
decode / execute / reset cycle and the
effects of the stages of the cycle on
specific registers.
The Fetch-Decode-ExecuteReset Cycle
The following is an algorithm in
diagrammatic form that shows the steps in
the cycle.
It is the control unit which controls and
synchronises this cycle.

Loads / copies / places / passes, decodes and
executes.
At the end the cycle is reset and the
algorithm repeated.
Key for following slides:
PC / SQR

Program Counter / Sequence Control Register
MAR

Memory Address Register
MDR / MBR

Memory Data Register / Memory Buffer Register
CIR

Current Instruction Register
Fetch
Memory
CPU
PC incremented
by 1
of address of
PC Copy
next instruction
MAR
Copy of
instruction in
memory
address held in
MAR
MDR
Instruction
CIR
Decode
PC
CPU
MAR
MDR
CIR
Split instruction into operation code & address if
present. Then decode operation code.
Execute
CPU
PC
Execute instruction
(What is involved in this
depends on the
instruction being
executed - demonstrated
on the following slides).
MAR
MDR
CIR
Click an instruction or move on to see each instruction in turn.
Jump
Input / Load (number directly)
Input / Load (from memory)
Store
Add (a number directly)
Add (a number from memory)
Output (directly from accumulator)
Output (from memory)
Jump instruction
Execute Diagram
Execute
Jump
PC
CPU
MAR
MDR
CIR
Execute
Jump
CPU
Copy of address part instruction
(address to jump to).
PC
MAR
MDR
CIR
Back to list of instructions
Input / Load (number directly)
into accumulator instruction
Execute Diagram
Execute
Input / Load (number directly) into accumulator
CPU
PC
MAR
MDR
Accumulator
Number inputted
/ to be loaded.
CIR
Back to list of instructions
Reason for the CIR & MDR
As you can see the MDR is used to store the
number inputted / to be loaded during the
execution of this Input / Load instruction.
Therefore, if there was no CIR register to hold
the Input / Load instruction and as no register
can hold more than one “thing” at a time the
control unit would “lose” the Input / Load
instruction.

i.e. It would no longer “know” what it was supposed to
do.
You will find that the contents of the MDR may
be modified for similar reasons during other later
instructions.
Back to list of instructions
Load (from memory)
instruction
Execute Diagram
Execute
Load (from memory)
PC
CPU
MAR
Memory
Copy of data
in address
held in MAR
MDR
Accumulator
CIR
Back to list of instructions
Reason for the PC & MAR
As you can see the MAR is now used to store
the address part of instruction during the
execution of this Load (from memory)
instruction.
Therefore if there was no MAR register the PC
would be used to hold this address so the
control unit would no longer know the correct
address of the next instruction.
You will find that the contents of the MAR
may be modified for similar reasons during
other later instructions.
Back to list of instructions
Store instruction
Execute Diagram
Assume data has either been inputted, loaded
(directly or from memory) or a calculation has been
performed.
Any of the above will mean there is data in the
accumulator and it is this data that will be stored.
Execute
CPU
Store
PC
MAR
Memory
Copy of data in
MDR stored in
memory address
held in MAR
MDR
Accumulator
CIR
Back to list of instructions
Add (a number directly)
instruction
Execute Diagram
Assume a number has already been
inputted or loaded (directly or from memory)
into the accumulator.
Execute
Add (a number directly)
CPU
PC
MAR
MDR
ALU
Accumulator
NB. The ALU now does the arithmetic.
Accumulator value is now the result of the addition.
i.e. Accumulator = Accumulator + contents of MDR
Number to
be added.
CIR
Back to list of instructions
Add (a number from
memory) instruction
Execute Diagram
(Assume a number has already been
inputted or loaded into the
accumulator.)
Execute
Add (from memory)
PC
CPU
MAR
Memory
Copy of number
in memory
address held in
MAR
MDR
ALU
Accumulator
NB. The ALU now does the arithmetic.
Accumulator value is now the result of the addition.
i.e. Accumulator = Accumulator + contents of MDR
CIR
Back to list of instructions
Output (directly from
accumulator) instruction
Execute Diagram
Execute
Output (directly from accumulator)
CPU
PC
MAR
Output data in
accumulator
Accumulator
MDR
CIR
Back to list of instructions
Output (from memory)
instruction
Execute Diagram
Execute
Output (from memory)
Memory
CPU
MAR
PC
Output data in
accumulator
Accumulator
Copy of data in
memory address
held in MAR
MDR
CIR
Back to list of instructions
Reset
CPU
PC
Cycle is reset (restarted) by passing
control back to the PC.
Fetch – Decode - Execute – Reset
Cycle in writing
The following slides describe the cycle in
writing.
1. Load the address of next instruction in the PC into
the MAR.

So that the control unit can fetch the instruction from the
right part of the memory.
2. Copy the instruction/data that is in the memory
address given by the MAR into the MDR.
Fetch

MDR is used whenever anything is to go from the CPU to
main memory, or vice versa.
3. Increment the PC by 1.

So that it contains the address of the next instruction,
assuming that the instructions are in consecutive locations.
4. Load the instruction/data that is now in the MDR
into the CIR.

Thus the next instruction is copied from memory -> MDR > CIR.
5. Contents of CIR split into operation code and
address if present e.g. store, add or jump
Decode
instructions.
6. Decode the instruction that is in the CIR.
6. Execute the instruction but what is involved in this
depends on the instruction being executed (there
are several different instructions you need to know
about).
If the instruction is a jump instruction then

Execute
Load the address part of the instruction in the CIR into the
PC.
If the instruction is an input / load (directly)
instruction then take data input and place in
accumulator.
If the instruction is a load (from memory) instruction.



Copy address part of the instruction (to load from) in the
CIR into MAR.
Copy data from memory address held in MAR to MDR.
Copy data in MDR into accumulator.
If the instruction is a store instruction then:



Copy address part of the instruction (to store in) in the
CIR into MAR.
Copy data in accumulator to MDR.
Copy data in MDR into memory address held in MAR.
If the instruction is an add instruction then:

Execute


Copy address part of the instruction (of number to add) in
the CIR into MAR.
Copy number from memory address held in MAR into
MDR.
Add number in MDR to number in accumulator
(accumulator will now hold the result).
If the instruction is an output (directly from
accumulator) then output number in accumulator.
If the instruction is an output (from memory)
instruction then:
Execute


Reset
7.
Copy address part of part of the instruction (of data to
output) in CIR into MAR.
Output contents of MDR.
Cycle is reset (restarted) by passing control back
to the PC (step 1).
Plenary
Contents of PC loaded into MAR
PC is incremented
Contents of address stored in MAR loaded into MDR
Contents of MDR loaded into CIR
Instruction in CIR is decoded.
PC (program counter) stores the address of the next
instruction to be executed.
MAR (memory address register) holds the address in
memory that is currently being used
MDR (memory data register) holds the data (or
instruction) that is being stored in the address
accessed by the MAR.
CIR (current instruction register) holds the instruction
which is currently being executed.