Monday, August 17, 2009

The Central Processing Unit (CPU)

Memory Buffer Register (MBR)
This register serves as an interface between the CPU and main memory. Anything needed by the CPU (instruction or data) is first placed here before it goes to its final destination (such as the accumulator, IR, PC or other registers). Also, anything in the CPU that is to be stored in main memory comes here first before being copied into the main memory at the location specified by the address contained in the MAR.

Memory Address Register (MAR)
When another instruction is needed in the IR, or a value is to be loaded into the accumulator, or an operand is needed to perform some arithmetic or logic instruction, this register contains the memory address where the desired information can be found. It also serves as a pointer to the location in memory where the contents of some CPU register is to be stored.

Instruction Register (IR)
A special register in the CPU that holds the bit pattern corresponding to the next instruction to be performed within the CPU. The Control Unit accesses this register to decide which circuits need to be activated.

Instruction Buffer Register (IBR)
It is a multipurpose register in a computer (or a microprocessor) system strictly dedicated to temporarily hold extra instructions and provide other functions within the processor.

Program Counter (PC)
This CPU register always contains the memory address where the next instruction to be performed by the CPU can be found. Its contents is copied into the MAR before an instruction is fetched from the main memory. While the instruction is being fetched, the Control Unit updates the contents of the PC so that it will again point to the next instruction to be performed.

Arithmetic Logic Unit (ALU)
The part of the CPU where arithmetic and logic operations are performed. Sometimes called the arithmetic unit.

In CPU, accumulator is a register in which intermediate arithmetic and logic results are stored. Without a register like an accumulator, it would be necessary to write the result of each calculation (addition, multiplication, shift, etc.) to main memory , perhaps only to be read right back again for use in the next operation. Access to main memory is slower than access to a register like the accumulator because the technology used for the large main memory is slower (but cheaper) than that used for a register.

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