THE

FETCH-EXECUTE

CYCLE

1

1

REGISTERS

ACC

MDR

MAR

CIR

PC

1 LDA 23

2 ADD 72

3 STA 53

THE PROGRAM

23 14

53 26

72 35

MEMORY

Location Contents

Fetch instruction from memory

Update Program counter

Execute instruction

Check for interrupts

The address of the first

instruction is copied from the PC to the MAR

LDA 23

1

1

REGISTERS

ACC

MDR

MAR

CIR

PC

1 LDA 23

2 ADD 72

3 STA 53

THE PROGRAM

23 14

53 26

72 35

MEMORY

Location Contents

Fetch instruction from memory

Update Program counter

Execute instruction

Check for interrupts

The first instruction is read into the

MDR

LDA 23

LDA 23

1

1

REGISTERS

ACC

MDR

MAR

CIR

PC

1 LDA 23

2 ADD 72

3 STA 53

THE PROGRAM

23 14

53 26

72 35

MEMORY

Location Contents

Fetch instruction from memory

Update Program counter

Execute instruction

Check for interrupts

The first instruction is

copied into the CIR

LDA 23

LDA 23

1

2

REGISTERS

ACC

MDR

MAR

CIR

PC

1 LDA 23

2 ADD 72

3 STA 53

THE PROGRAM

23 14

53 26

72 35

MEMORY

Location Contents

Fetch instruction from memory

Update Program counter

Execute instruction

Check for interrupts

The program counter is

updated to point to the next instruction

LDA 23

LDA 23

23

2

REGISTERS

ACC

MDR

MAR

CIR

PC

1 LDA 23

2 ADD 72

3 STA 53

THE PROGRAM

23 14

53 26

72 35

MEMORY

Location Contents

Fetch instruction from memory

Update Program counter

Execute instruction

Check for interrupts

The address part of the current instruction is placed in the

MAR

14

LDA 23

23

2

REGISTERS

ACC

MDR

MAR

CIR

PC

1 LDA 23

2 ADD 72

3 STA 53

THE PROGRAM

23 14

53 26

72 35

MEMORY

Location Contents

Fetch instruction from memory

Update Program counter

Execute instruction

Check for interrupts

The contents of the location

whose address is in the MAR are loaded into the

MDR

14

14

LDA 23

23

2

REGISTERS

ACC

MDR

MAR

CIR

PC

1 LDA 23

2 ADD 72

3 STA 53

THE PROGRAM

23 14

53 26

72 35

MEMORY

Location Contents

Fetch instruction from memory

Update Program counter

Execute instruction

Check for interrupts

The contents of the MDR are

copied into the ACC

14

14

LDA 23

23

2

REGISTERS

ACC

MDR

MAR

CIR

PC

1 LDA 23

2 ADD 72

3 STA 53

THE PROGRAM

23 14

53 26

72 35

MEMORY

Location Contents

Fetch instruction from memory

Update Program counter

Execute instruction

Check for interrupts

The Status Register is checked for interrupts

14

14

LDA 23

2

2

REGISTERS

ACC

MDR

MAR

CIR

PC

1 LDA 23

2 ADD 72

3 STA 53

THE PROGRAM

23 14

53 26

72 35

MEMORY

Location Contents

Fetch instruction from memory

Update Program counter

Execute instruction

Check for interrupts

The address of the second

instruction is copied from the PC to the MAR

14

ADD 72

LDA 23

2

2

REGISTERS

ACC

MDR

MAR

CIR

PC

1 LDA 23

2 ADD 72

3 STA 53

THE PROGRAM

23 14

53 26

72 35

MEMORY

Location Contents

Fetch instruction from memory

Update Program counter

Execute instruction

Check for interrupts

The second instruction is read into the

MDR

14

ADD 72

ADD 72

2

2

REGISTERS

ACC

MDR

MAR

CIR

PC

1 LDA 23

2 ADD 72

3 STA 53

THE PROGRAM

23 14

53 26

72 35

MEMORY

Location Contents

Fetch instruction from memory

Update Program counter

Execute instruction

Check for interrupts

The second instruction is

copied into the CIR

14

ADD 72

ADD 72

2

3

REGISTERS

ACC

MDR

MAR

CIR

PC

1 LDA 23

2 ADD 72

3 STA 53

THE PROGRAM

23 14

53 26

72 35

MEMORY

Location Contents

Fetch instruction from memory

Update Program counter

Execute instruction

Check for interrupts

The program counter is

updated to point to the next instruction

14

ADD 72

ADD 72

72

3

REGISTERS

ACC

MDR

MAR

CIR

PC

1 LDA 23

2 ADD 72

3 STA 53

THE PROGRAM

23 14

53 26

72 35

MEMORY

Location Contents

Fetch instruction from memory

Update Program counter

Execute instruction

Check for interrupts

The address part of the current instruction is placed in the

MAR

14

35

ADD 72

72

3

REGISTERS

ACC

MDR

MAR

CIR

PC

1 LDA 23

2 ADD 72

3 STA 53

THE PROGRAM

23 14

53 26

72 35

MEMORY

Location Contents

Fetch instruction from memory

Update Program counter

Execute instruction

Check for interrupts

The contents of the location

whose address is in the MAR are loaded into the

MDR

49

35

ADD 72

72

3

REGISTERS

ACC

MDR

MAR

CIR

PC

1 LDA 23

2 ADD 72

3 STA 53

THE PROGRAM

23 14

53 26

72 35

MEMORY

Location Contents

Fetch instruction from memory

Update Program counter

Execute instruction

Check for interrupts

The contents of the MDR are added to the

ACC

49

35

ADD 72

72

3

REGISTERS

ACC

MDR

MAR

CIR

PC

1 LDA 23

2 ADD 72

3 STA 53

THE PROGRAM

23 14

53 26

72 35

MEMORY

Location Contents

Fetch instruction from memory

Update Program counter

Execute instruction

Check for interrupts

The Status Register is checked for interrupts

49

35

ADD 72

3

3

REGISTERS

ACC

MDR

MAR

CIR

PC

1 LDA 23

2 ADD 72

3 STA 53

THE PROGRAM

23 14

53 26

72 35

MEMORY

Location Contents

Fetch instruction from memory

Update Program counter

Execute instruction

Check for interrupts

The address of the third

instruction is copied from the PC to the MAR

49

STA 53

ADD 72

3

3

REGISTERS

ACC

MDR

MAR

CIR

PC

1 LDA 23

2 ADD 72

3 STA 53

THE PROGRAM

23 14

53 26

72 35

MEMORY

Location Contents

Fetch instruction from memory

Update Program counter

Execute instruction

Check for interrupts

The third instruction is read into the

MDR

49

STA 53

STA 53

3

3

REGISTERS

ACC

MDR

MAR

CIR

PC

1 LDA 23

2 ADD 72

3 STA 53

THE PROGRAM

23 14

53 26

72 35

MEMORY

Location Contents

Fetch instruction from memory

Update Program counter

Execute instruction

Check for interrupts

The third instruction is

copied into the CIR

49

STA 53

STA 53

3

4

REGISTERS

ACC

MDR

MAR

CIR

PC

1 LDA 23

2 ADD 72

3 STA 53

THE PROGRAM

23 14

53 26

72 35

MEMORY

Location Contents

Fetch instruction from memory

Update Program counter

Execute instruction

Check for interrupts

The program counter is

updated to point to the next instruction

49

STA 53

STA 53

53

4

REGISTERS

ACC

MDR

MAR

CIR

PC

1 LDA 23

2 ADD 72

3 STA 53

THE PROGRAM

23 14

53 26

72 35

MEMORY

Location Contents

Fetch instruction from memory

Update Program counter

Execute instruction

Check for interrupts

The address part of the current instruction is placed in the

MAR

49

49

STA 53

53

4

REGISTERS

ACC

MDR

MAR

CIR

PC

1 LDA 23

2 ADD 72

3 STA 53

THE PROGRAM

23 14

53 26

72 35

MEMORY

Location Contents

Fetch instruction from memory

Update Program counter

Execute instruction

Check for interrupts

The contents of the ACC are

copied into the MDR

49

49

STA 53

53

4

REGISTERS

ACC

MDR

MAR

CIR

PC

1 LDA 23

2 ADD 72

3 STA 53

THE PROGRAM

23 14

53 49

72 35

MEMORY

Location Contents

Fetch instruction from memory

Update Program counter

Execute instruction

Check for interrupts

The contents of the MDR are

copied into the memory location whose address is in the MAR

49

49

STA 53

53

4

REGISTERS

ACC

MDR

MAR

CIR

PC

1 LDA 23

2 ADD 72

3 STA 53

THE PROGRAM

23 14

53 49

72 35

MEMORY

Location Contents

Fetch instruction from memory

Update Program counter

Execute instruction

Check for interrupts

The Status Register is checked for interrupts