Essential Registers for Instruction Execution
These are various registers required for the 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.
- Program Counter (PC) :
It contains the address of an instruction to be executed next. The PC is updated by the CPU after each instruction is 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.
- 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 is analyzed.
- Memory Buffer (or Data) Register (MBR or MDR) :
It contains a word of data to be written to memory or the words most recently read. Contents of MBR are directly connected to the data bus.
- 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 are 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.
- Memory Data Register (MDR):
Holds the data being accessed or stored in memory.
Used to transfer data between the processor and memory.
- Accumulator (ACC):
Holds the intermediate results of computations.
Used for arithmetic and logical operations.
- Status Register (SR):
Holds status information about the processor and the current instruction.
Used to indicate whether an operation was successful, to set and clear flags, and to store error information.
- Index Registers (IX):
Holds an offset value to be added to a memory address.
Used to access data structures or arrays.
- Stack Pointer (SP):
Holds the memory address of the last item placed on the stack.
Used for managing the program stack during function calls and returns.
These essential registers work together to perform instruction execution. The program counter fetches the next instruction, which is stored in the instruction register. The instruction is decoded and the memory address to be accessed is stored in the memory address register. The data is then accessed from memory and stored in the memory data register. The accumulator holds the intermediate results of computations, and the status register holds the status information. The index registers and stack pointer are used for accessing data structures and managing the program stack during function calls and returns. Together, these essential registers enable the processor to execute instructions efficiently and accurately.
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