THE

1
THE FETCH-EXECUTE CYCLE
2
REGISTERS

ACC
Fetch instruction from memory
Check for interrupts
MBR
1
MAR

CIR
Execute instruction
Update Program counter
1
PC
MEMORY Location Contents
THE PROGRAM
The address of the first instruction is copied
from the PC to the MAR
1 LDA 23
2 ADD 72
3 STA 53
23 14
53 26
72 35
3
REGISTERS

ACC
Fetch instruction from memory
Check for interrupts
LDA 23
MBR
1
MAR

CIR
Execute instruction
Update Program counter
1
PC
MEMORY Location Contents
THE PROGRAM
The first instruction is read into the MBR
1 LDA 23
2 ADD 72
3 STA 53
23 14
53 26
72 35
4
REGISTERS

ACC
Fetch instruction from memory
Check for interrupts
LDA 23
MBR
1
MAR
LDA 23
CIR
Execute instruction
Update Program counter
1
PC
MEMORY Location Contents
THE PROGRAM
The first instruction is copied into the CIR
1 LDA 23
2 ADD 72
3 STA 53
23 14
53 26
72 35
5
REGISTERS

ACC
Fetch instruction from memory
Check for interrupts
LDA 23
MBR
1
MAR
LDA 23
CIR
Execute instruction
Update Program counter
2
PC
MEMORY Location Contents
THE PROGRAM
The program counter is updated to point to the
next instruction
1 LDA 23
2 ADD 72
3 STA 53
23 14
53 26
72 35
6
REGISTERS

ACC
Fetch instruction from memory
Check for interrupts
LDA 23
MBR
23
MAR
LDA 23
CIR
Execute instruction
Update Program counter
2
PC
MEMORY Location Contents
THE PROGRAM
The address part of the current instruction is
placed in the MAR
1 LDA 23
2 ADD 72
3 STA 53
23 14
53 26
72 35
7
REGISTERS

ACC
Fetch instruction from memory
Check for interrupts
14
MBR
23
MAR
LDA 23
CIR
Execute instruction
Update Program counter
2
PC
MEMORY Location Contents
The contents of the location whose address is in
the MAR are loaded into the MBR
THE PROGRAM
1 LDA 23
2 ADD 72
3 STA 53
23 14
53 26
72 35
8
REGISTERS
14
ACC
Fetch instruction from memory
Check for interrupts
14
MBR
23
MAR
LDA 23
CIR
Execute instruction
Update Program counter
2
PC
MEMORY Location Contents
THE PROGRAM
The contents of the MBR are copied into the ACC
1 LDA 23
2 ADD 72
3 STA 53
23 14
53 26
72 35
9
REGISTERS
14
ACC
Fetch instruction from memory
Check for interrupts
14
MBR
23
MAR
LDA 23
CIR
Execute instruction
Update Program counter
2
PC
MEMORY Location Contents
THE PROGRAM
The Status Register is checked for interrupts
1 LDA 23
2 ADD 72
3 STA 53
23 14
53 26
72 35
10
REGISTERS
14
ACC
Fetch instruction from memory
Check for interrupts
14
MBR
2
MAR
LDA 23
CIR
Execute instruction
Update Program counter
2
PC
MEMORY Location Contents
THE PROGRAM
The address of the second instruction is copied
from the PC to the MAR
1 LDA 23
2 ADD 72
3 STA 53
23 14
53 26
72 35
11
REGISTERS
14
ACC
Fetch instruction from memory
Check for interrupts
ADD 72
MBR
2
MAR
LDA 23
CIR
Execute instruction
Update Program counter
2
PC
MEMORY Location Contents
THE PROGRAM
The second instruction is read into the MBR
1 LDA 23
2 ADD 72
3 STA 53
23 14
53 26
72 35
12
REGISTERS
14
ACC
Fetch instruction from memory
Check for interrupts
ADD 72
MBR
2
MAR
ADD 72
CIR
Execute instruction
Update Program counter
2
PC
MEMORY Location Contents
THE PROGRAM
The second instruction is copied into the CIR
1 LDA 23
2 ADD 72
3 STA 53
23 14
53 26
72 35
13
REGISTERS
14
ACC
Fetch instruction from memory
Check for interrupts
ADD 72
MBR
2
MAR
ADD 72
CIR
Execute instruction
Update Program counter
3
PC
MEMORY Location Contents
THE PROGRAM
The program counter is updated to point to the
next instruction
1 LDA 23
2 ADD 72
3 STA 53
23 14
53 26
72 35
14
REGISTERS
14
ACC
Fetch instruction from memory
Check for interrupts
ADD 72
MBR
72
MAR
ADD 72
CIR
Execute instruction
Update Program counter
3
PC
MEMORY Location Contents
THE PROGRAM
The address part of the current instruction is
placed in the MAR
1 LDA 23
2 ADD 72
3 STA 53
23 14
53 26
72 35
15
REGISTERS
14
ACC
Fetch instruction from memory
Check for interrupts
35
MBR
72
MAR
ADD 72
CIR
Execute instruction
Update Program counter
3
PC
MEMORY Location Contents
The contents of the location whose address is in
the MAR are loaded into the MBR
THE PROGRAM
1 LDA 23
2 ADD 72
3 STA 53
23 14
53 26
72 35
16
REGISTERS
49
ACC
Fetch instruction from memory
Check for interrupts
35
MBR
72
MAR
ADD 72
CIR
Execute instruction
Update Program counter
3
PC
MEMORY Location Contents
THE PROGRAM
The contents of the MBR are added to the ACC
1 LDA 23
2 ADD 72
3 STA 53
23 14
53 26
72 35
17
REGISTERS
49
ACC
Fetch instruction from memory
Check for interrupts
35
MBR
72
MAR
ADD 72
CIR
Execute instruction
Update Program counter
3
PC
MEMORY Location Contents
THE PROGRAM
The Status Register is checked for interrupts
1 LDA 23
2 ADD 72
3 STA 53
23 14
53 26
72 35
18
REGISTERS
49
ACC
Fetch instruction from memory
Check for interrupts
35
MBR
3
MAR
ADD 72
CIR
Execute instruction
Update Program counter
3
PC
MEMORY Location Contents
THE PROGRAM
The address of the third instruction is copied
from the PC to the MAR
1 LDA 23
2 ADD 72
3 STA 53
23 14
53 26
72 35
19
REGISTERS
49
ACC
Fetch instruction from memory
Check for interrupts
STA 53
MBR
3
MAR
ADD 72
CIR
Execute instruction
Update Program counter
3
PC
MEMORY Location Contents
THE PROGRAM
The third instruction is read into the MBR
1 LDA 23
2 ADD 72
3 STA 53
23 14
53 26
72 35
20
REGISTERS
49
ACC
Fetch instruction from memory
Check for interrupts
STA 53
MBR
3
MAR
STA 53
CIR
Execute instruction
Update Program counter
3
PC
MEMORY Location Contents
THE PROGRAM
The third instruction is copied into the CIR
1 LDA 23
2 ADD 72
3 STA 53
23 14
53 26
72 35
21
REGISTERS
49
ACC
Fetch instruction from memory
Check for interrupts
STA 53
MBR
3
MAR
STA 53
CIR
Execute instruction
Update Program counter
4
PC
MEMORY Location Contents
THE PROGRAM
The program counter is updated to point to the
next instruction
1 LDA 23
2 ADD 72
3 STA 53
23 14
53 26
72 35
22
REGISTERS
49
ACC
Fetch instruction from memory
Check for interrupts
STA 53
MBR
53
MAR
STA 53
CIR
Execute instruction
Update Program counter
4
PC
MEMORY Location Contents
THE PROGRAM
The address part of the current instruction is
placed in the MAR
1 LDA 23
2 ADD 72
3 STA 53
23 14
53 26
72 35
23
REGISTERS
49
ACC
Fetch instruction from memory
Check for interrupts
49
MBR
53
MAR
STA 53
CIR
Execute instruction
Update Program counter
4
PC
MEMORY Location Contents
THE PROGRAM
The contents of the ACC are copied into the MBR
1 LDA 23
2 ADD 72
3 STA 53
23 14
53 26
72 35
24
REGISTERS
49
ACC
Fetch instruction from memory
Check for interrupts
49
MBR
53
MAR
STA 53
CIR
Execute instruction
Update Program counter
4
PC
MEMORY Location Contents
The contents of the MBR are copied into the
memory location whose address is in the MAR
THE PROGRAM
1 LDA 23
2 ADD 72
3 STA 53
23 14
53 49
72 35
25
REGISTERS
49
ACC
Fetch instruction from memory
Check for interrupts
49
MBR
53
MAR
STA 53
CIR
Execute instruction
Update Program counter
4
PC
MEMORY Location Contents
THE PROGRAM
The Status Register is checked for interrupts
1 LDA 23
2 ADD 72
3 STA 53
23 14
53 49
72 35