Computer Systems Organization

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

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

: : : 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

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

: : : 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

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

: : : 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

Instruction #4 JMP 510 – Jump to memory location 510

: : : 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