THE FETCH-EXECUTE CYCLE.

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Presentation transcript:

THE FETCH-EXECUTE CYCLE

The address of the first instruction is copied from the PC to the MAR REGISTERS ACC Check for interrupts Fetch instruction from memory MDR MAR 1 CIR Execute instruction Update Program counter PC 1 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

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

The first instruction is copied into the CIR REGISTERS ACC Check for interrupts Fetch instruction from memory MDR LDA 23 MAR 1 CIR LDA 23 Execute instruction Update Program counter PC 1 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

The program counter is updated to point to the next instruction REGISTERS ACC Check for interrupts Fetch instruction from memory MDR LDA 23 MAR 1 CIR LDA 23 Execute instruction Update Program counter PC 2 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

The address part of the current instruction is placed in the MAR REGISTERS ACC Check for interrupts Fetch instruction from memory MDR LDA 23 MAR 23 CIR LDA 23 Execute instruction Update Program counter PC 2 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

REGISTERS ACC Check for interrupts Fetch instruction from memory MDR 14 MAR 23 CIR LDA 23 Execute instruction Update Program counter PC 2 MEMORY Location Contents The contents of the location whose address is in the MAR are loaded into the MDR THE PROGRAM 1 LDA 23 2 ADD 72 3 STA 53 23 14 53 26 72 35

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

The Status Register is checked for interrupts REGISTERS ACC 14 Check for interrupts Fetch instruction from memory MDR 14 MAR 23 CIR LDA 23 Execute instruction Update Program counter PC 2 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

The address of the second instruction is copied from the PC to the MAR REGISTERS ACC 14 Check for interrupts Fetch instruction from memory MDR 14 MAR 2 CIR LDA 23 Execute instruction Update Program counter PC 2 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

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

The second instruction is copied into the CIR REGISTERS ACC 14 Check for interrupts Fetch instruction from memory MDR ADD 72 MAR 2 CIR ADD 72 Execute instruction Update Program counter PC 2 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

The program counter is updated to point to the next instruction REGISTERS ACC 14 Check for interrupts Fetch instruction from memory MDR ADD 72 MAR 2 CIR ADD 72 Execute instruction Update Program counter PC 3 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

The address part of the current instruction is placed in the MAR REGISTERS ACC 14 Check for interrupts Fetch instruction from memory MDR ADD 72 MAR 72 CIR ADD 72 Execute instruction Update Program counter PC 3 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

REGISTERS ACC 14 Check for interrupts Fetch instruction from memory MDR 35 MAR 72 CIR ADD 72 Execute instruction Update Program counter PC 3 MEMORY Location Contents The contents of the location whose address is in the MAR are loaded into the MDR THE PROGRAM 1 LDA 23 2 ADD 72 3 STA 53 23 14 53 26 72 35

The contents of the MDR are added to the ACC REGISTERS ACC 49 Check for interrupts Fetch instruction from memory MDR 35 MAR 72 CIR ADD 72 Execute instruction Update Program counter PC 3 MEMORY Location Contents THE PROGRAM The contents of the MDR are added to the ACC 1 LDA 23 2 ADD 72 3 STA 53 23 14 53 26 72 35

The Status Register is checked for interrupts REGISTERS ACC 49 Check for interrupts Fetch instruction from memory MDR 35 MAR 72 CIR ADD 72 Execute instruction Update Program counter PC 3 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

The address of the third instruction is copied from the PC to the MAR REGISTERS ACC 49 Check for interrupts Fetch instruction from memory MDR 35 MAR 3 CIR ADD 72 Execute instruction Update Program counter PC 3 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

The third instruction is read into the MDR REGISTERS ACC 49 Check for interrupts Fetch instruction from memory MDR STA 53 MAR 3 CIR ADD 72 Execute instruction Update Program counter PC 3 MEMORY Location Contents THE PROGRAM The third instruction is read into the MDR 1 LDA 23 2 ADD 72 3 STA 53 23 14 53 26 72 35

The third instruction is copied into the CIR REGISTERS ACC 49 Check for interrupts Fetch instruction from memory MDR STA 53 MAR 3 CIR STA 53 Execute instruction Update Program counter PC 3 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

The program counter is updated to point to the next instruction REGISTERS ACC 49 Check for interrupts Fetch instruction from memory MDR STA 53 MAR 3 CIR STA 53 Execute instruction Update Program counter PC 4 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

The address part of the current instruction is placed in the MAR REGISTERS ACC 49 Check for interrupts Fetch instruction from memory MDR STA 53 MAR 53 CIR STA 53 Execute instruction Update Program counter PC 4 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

The contents of the ACC are copied into the MDR REGISTERS ACC 49 Check for interrupts Fetch instruction from memory MDR 49 MAR 53 CIR STA 53 Execute instruction Update Program counter PC 4 MEMORY Location Contents THE PROGRAM The contents of the ACC are copied into the MDR 1 LDA 23 2 ADD 72 3 STA 53 23 14 53 26 72 35

REGISTERS ACC 49 Check for interrupts Fetch instruction from memory MDR 49 MAR 53 CIR STA 53 Execute instruction Update Program counter PC 4 MEMORY Location Contents The contents of the MDR 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

The Status Register is checked for interrupts REGISTERS ACC 49 Check for interrupts Fetch instruction from memory MDR 49 MAR 53 CIR STA 53 Execute instruction Update Program counter PC 4 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