It is communicated to the different ALUs and execution units of the processor that act.It interprets the instructions and sends them to the different execution units to which it is connected.In general, it is considered that the units of the capture stage or Fetch are part of the hardware that we call the control unit and this hardware is also called the Front-End of a processor.They are in charge of coordinating the movement and the order in which of the data that moves inside and outside the processor, as well as the different subunits that are in charge of it.The control unit is the most complex part that exists in a processor and its tasks are as follows: These 4 sub-stages occur in all processors whatever their utility, architecture and binary compatibility or what we call ISA.
Instruction Register: The final part of the fetch stage is the writing of the instruction in the instruction register, from which the processor control unit will copy its content for the second stage of the instruction cycle.The MDR, unlike the MAR, is connected to the data pins of the RAM and not to the addressing pins and in the case of a write instruction the content of what you want to write in the RAM is also written in the MDR Memory Data Register or memory data register : In the event that the CPU has to perform a memory reading, what the MDR does is copy the content of that memory address to an internal register of the CPU, which is a temporary pass register before its content is copied to the Instruction Register.
Memory Address Register: The MAR copies the content of the PC and sends it to the RAM through the addressing pins of the CPU, which are wired with the addressing pins of the RAM itself.Its value is incremented by 1 each time a complete instruction cycle is completed or when a jump instruction changes the value of the program counter. Program Counter or Program Counter: Which points to the next memory line where the next processor instruction is located.
The first stage of the instruction cycle is responsible for capturing the instructions in the RAM memory assigned to the processor through a series of units and registers that are the following: There is a fourth stage, which is Write-Back, which is when the execution units write the result, but this is usually counted within the execution stage of the instruction cycle.įirst stage of the instruction cycle: Fetch These three stages are fulfilled in every processor.