Instruction Execution

Types of Instructions

• processor-memory: the instruction transfers data from memory to the processor or from the processor to memory
• processor-I/O: the instruction transfers data from an I/O device to the processor or from the processor to an I/O device
• data processing: the instruction performs an arithmetic or logic operation
• control: the instruction changes the flow of control in the program

Memory Access

The following steps are required to access main memory:

1. The memory address of the data to be read or written is loaded into the MAR.
2. If the memory operation is a write, the data to be written is loaded into the MBR.
3. A signal is sent to the memory to read or write.
4. If the memory operation is a read, the data read is put in the MBR.

Steps in Instruction Execution

• instruction fetch
1. MAR <- PC
2. send read signal to memory
3. MBR <- data from memory
4. IR <- MBR
• decode operation: circuits in control unit analyze instruction to determine the operation and what operands are needed
• calculate address of next instruction: PC <- PC + (size of current instruction)
• calculate address of operand: if operand requires memory access or I/O, determine the address of the operand
• fetch operand
1. MAR <- address of operand --OR-- I/O AR <- address of operand
2. send read signal to memory or input device
3. MBR <- data from memory --OR-- I/O BR <- data from input device
• execute (arithmetic or logic operation)
• store operand
1. MAR <- address to store operand --OR-- I/O AR <- address to store operand
2. MBR <- --OR-- I/O BR <- data from input device
3. send write signal to memory or output device

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