Essential Registers for Instruction Execution

These are various registers required for execution of instruction : Program Counter (PC), Instruction Register (IR), Memory Buffer (or Data) Register (MBR or MDR), and Memory Address Register (MAR).

These are explained as follows below.


  1. Program Counter (PC) :
    It contains the address of an instruction to be executed next. The PC is updated by the CPU after each instruction executed so that it always points to the next instruction to be executed. A branch or skip instruction will also modify the content of the PC.




  2. Instruction Register (IR) :
    it contains the instruction most recently fetched or executed. The fetched instruction is loaded into an IR, where the opcode and operand specifier are analysed.


  3. Memory Buffer (or Data) Register (MBR or MDR) :
    it contains a word of data to be written to memory are the words most recently read. Contents of MBR are directly connected to the data bus.


  4. Memory Address Register (MAR) :
    It contains the address of a location of main memory from where information has to be fetched for information has to be stored. Contents of MAR is directly connected to the address bus.

Apart from these registers, we may use other registers which may be invisible to the user, e.g., temporary Buffering registers.

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