What are Instruction Cycles in computer architecture?

The instruction cycle is the process a microprocessor goes through to process an instruction. First, the microprocessor fetches or reads, the instruction from the memory. Therefore it decodes the instruction, determining which instruction it has fetched. Finally, it implements the operations required to execute the instruction. Each of these functions including fetch, decode and execute contains a sequence of one or more operations.

First, the microprocessor locates the address of the instruction on the address bus. The memory subsystem inputs this address and decodes it to access the desired memory locations. After the microprocessor enables ample time for memory to decode the address and access the requested memory locations, the microprocessor asserts a READ control signal.

The READ signal is ready to read information from the memory or an I/O device. Various processors have a specific name for this signal, but some microprocessors have a signal to implement this function. It relied upon the microprocessor, the READ signal can be active high (asserted=1) or active low (asserted = 0).

When the READ signal is asserted, the memory subsystem installs the instruction code to be fetched onto the computer system’s data bus. The microprocessor then inputs this data from the data bus and saves it in one of its internal registers.

Next, the microprocessor decodes the instruction. Each instruction can need multiple sequences of operations to perform the instruction. When the microprocessor decodes the instruction, it specifies which instruction it is to choose the right sequence of operations to implement. This is done perfectly within the microprocessor and it does not use the system buses.

Finally, the microprocessor executes the instruction. The sequence of operations to execute the instruction change from instruction to instruction. The execute routine can read data from memory, write data to memory, read data from or write data to an I/O device, execute only operations inside the CPU, or perform some sequence of these operations.

CLK − CLK stands for the computer system clock. The microprocessor uses the system clock to synchronize its operations. The microprocessor locates the address onto the bus at the starting of a clock cycle, a 0/1 sequence of the system clock. One clock cycle enables time for memory to decode the address and access its information, the microprocessor asserts the READ signal. This generates memory to locate its data onto the system data bus.

During this clock cycle, the microprocessor reads the data of the system bus and saves it in one of its registers. At the end of the clock cycle, it eliminates the address from the address bus and deasserts the READ signal.

Updated on: 24-Jul-2021

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