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1.3.1: The Processor - Components

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    46352
  • Accumulator

    In a computer's central processing unit (CPU), the accumulator (ACC in the image below) 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 an accumulator because the technology used for the large main memory is slower (but cheaper) than that used for a register. Early electronic computer systems were often split into two groups, those with accumulators and those without.

    Modern computer systems often have multiple general-purpose registers that can operate as accumulators, and the term is no longer as common as it once was. However, to simplify their design, a number of special-purpose processors still use a single accumulator.

    Arithmetic logic unit

    The arithmetic logic unit (ALU) performs the arithmetic and logical functions that are the work of the computer. There are other general purpose registers that hold the input data, and the accumulator receives the result of the operation. The instruction register contains the instruction that the ALU is to perform.

    For example, when adding two numbers, one number is placed in one of the general purpose registers register and the other in another general purpose register. The ALU performs the addition and puts the result in the accumulator. If the operation is a logical one, the data to be compared is placed into the one of the general purpose registers. The result of the comparison, a 1 or 0, is put in the accumulator. Whether this is a logical or arithmetic operation, the accumulator content is then placed into the cache location reserved by the program for the result.

    Instruction register and pointer

    The instruction pointer (CIR in the image below) specifies the location in memory containing the next instruction to be executed by the CPU. When the CPU completes the execution of the current instruction, the next instruction is loaded into the instruction register from the memory location pointed to by the instruction pointer.

    After the instruction is loaded into the instruction register, the instruction register pointer is incremented by one instruction address. Incrementing allows it to be ready to move the next instruction into the instruction register.

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    Memory Address Register

    In a computer, the memory address register (MAR) is the CPU register that either stores the memory address from which data will be fetched to the CPU, or the address to which data will be sent and stored.

    In other words, This register is used to access data and instructions from memory during the execution phase of instruction. MAR holds the memory location of data that needs to be accessed. When reading from memory, data addressed by MAR is fed into the MDR (memory data register) and then used by the CPU. When writing to memory, the CPU writes data from MDR to the memory location whose address is stored in MAR. MAR, which is found inside the CPU, goes either to the RAM (random access memory) or cache.

    Memory Data Register

    The memory data register (MDR) is the register that stores the data being transferred to and from the immediate access storage. It contains the copy of designated memory locations specified by the memory address register. It acts as a buffer allowing the processor and memory units to act independently without being affected by minor differences in operation. A data item will be copied to the MDR ready for use at the next clock cycle, when it can be either used by the processor for reading or writing or stored in main memory after being written.

    This register holds the contents of the memory which are to be transferred from memory to other components or vice versa. A word to be stored must be transferred to the MDR, from where it goes to the specific memory location, and the arithmetic data to be processed in the ALU first goes to MDR and then to accumulated register, and then it is processed in the ALU.

    The MDR is a two-way register. When data is fetched from memory and placed into the MDR, it is written to go in one direction. When there is a write instruction, the data to be written is placed into the MDR from another CPU register, which then puts the data into memory.

    Cache

    The CPU never directly accesses RAM. Modern CPUs have one or more layers of cache. The CPU's ability to perform calculations is much faster than the RAM's ability to feed data to the CPU.

    Cache memory is faster than the system RAM, and it is closer to the CPU because it is physically located on the processor chip. The cache provides data storage and instructions to prevent the CPU from waiting for data to be retrieved from RAM. When the CPU needs data -  program instructions are also considered to be data - the cache determines whether the data is already in residence and provides it to the CPU.

    If the requested data is not in the cache, it's retrieved from RAM and uses predictive algorithms to move more data from RAM into the cache. The cache controller analyzes the requested data and tries to predict what additional data will be needed from RAM. It loads the anticipated data into the cache. By keeping some data closer to the CPU in a cache that is faster than RAM, the CPU can remain busy and not waste cycles waiting for data.

    The  simple example CPU has two levels of cache. Levels 2 is designed to predict what data and program instructions will be needed next, move that data from RAM, and move it ever closer to the CPU to be ready when needed. These cache sizes typically range from 1 MB to 32 MB, depending upon the speed and intended use of the processor.

    CPU clock and control unit (CU in the image)

    All of the CPU components must be synchronized to work together smoothly. The control unit performs this function at a rate determined by the clock speed and is responsible for directing the operations of the other units by using timing signals that extend throughout the CPU.

    Random access memory (RAM)

    Although the RAM, or main storage, is shown in this diagram and the next, it is not truly a part of the CPU. Its function is to store programs and data so that they are ready for use when the CPU needs them.

    Adapted from:
    "Accumulator (computing)" by Multiple ContributorsWikipedia is licensed under CC BY-SA 3.0
    "Memory buffer register" by Multiple ContributorsWikipedia is licensed under CC BY-SA 3.0
    "Memory address register" by Multiple ContributorsWikipedia is licensed under CC BY-SA 3.0
    William Lau, CC BY-SA 4.0, via Wikimedia Commons

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