1. Computer Systems Organization

2. Example 1:
Describe the sequence of events carried out during the machine cycle when executing the following instructions.
Address
Contents
500
LDA 1000
501
ADD 1001
502
STO 1002
503
JMP 510
1000 4
1001 6
1003

3. Instruction #1
LDA 1000 – Load to the accumulator the contents of the memory location 1000.

4. : : : MM 4 4 6 1000 500 4 501 500 Decode 4 CPU R W IR MAR MDR PC TMP
502
503 MM :
LDA 1000
LDA 1000
ADD 1001
STO 1002
JMP 510 : :
1000
1001
1002
1003 4 4 6 R W IR
MAR
MDR PC
TMP
ACC
1000
500
LDA 1000 4
Data
bus
Address
bus
501
500
LDA 1000
Decode 4
CPU

5. Instruction #2
ADD 1001 – add the contents of location 1001 and the contents of the accumulator and store the result back in the accumulator.

6. : : : MM 4 6 6 1001 501 6 501 502 Decode 4 6 CPU 10 R W IR MAR MDR PC
500
501
502
503 MM :
LDA 1000
ADD 1001
ADD 1001
STO 1002
JMP 510 : :
1000
1001
1002
1003 4 6 6 R W IR
MAR
MDR PC
TMP
ACC
1001
501 6
ADD 1001
Data
bus
Address
bus
501
502
ADD 1001
Decode 4 6
Add
CPU 10

7. Instruction #3
STO 1002 – store the contents of the accumulator to the memory location 1002.

8. : : : MM 4 6 10 1002 502 10 502 503 Decode 10 CPU R W IR MAR MDR PC
500
501
502
503 MM :
LDA 1000
ADD 1001
STO 1002
STO 1002
JMP 510 : :
1000
1001
1002
1002
1003 4 6 10 R W IR
MAR
MDR PC
TMP
ACC
1002
502
STO 1002 10
Data
bus
Address
bus
502
503
STO 1002
Decode 10
CPU

9. Instruction #4
JMP 510 – Jump to memory location 510

10. : : : MM 4 6 10 503 510 504 503 Decode CPU R W IR MAR MDR PC TMP ACC
500
501
502
503 MM :
LDA 1000
ADD 1001
STO 1002
JMP 510
JMP 510 : :
1000
1001
1002
1003 4 6 10 R W IR
MAR
MDR PC
TMP
ACC
503
JMP 510
Data
bus
Address
bus
510
504
503
JMP 510
Decode
CPU