Discussion of Buses

    In this post, I'm going to introduce about the Bus System which is include in computer architecture. Bus System are separate in two main type, Internal Bus and External Bus. For Internal Bus, it is a bus located a bus located strictly within a CPU chip for communications among the components in a CPU chip. Now talk about External Bus, it is outside a CPU chip for connecting the rest of the system components to the CPU. There are supported by third-party hardware and software, and are more likely to adhere to one of a number of industry standards for buses.


    System Bus consists of a common set of parallel wires, which is Address buses, Data buses, and Control  buses. Address buses frequently transfer addresses from the program counter (PC), or stack register to memory (but not the reverse). Data buses are used to carry data, instructions, and also addresses between main memory system and the ALU. For Control buses, it carry signals (interrupt, timing) from the control unit to other components of the computer and back to the control unit to specify the operations to be performed.


    Now, let’s talk about the bus hierarchy. The most modern PCs have at least four buses. Firstly, the Processor Bus is the highest-level bus that the chipset used to send information to and from the processor. After that, it is Cache Bus which is higher-level architectures, employ a dedicated bus for accessing the system cache. The second-level system bus, Memory Bus is use to connect the memory subsystem to the chipset and the processor. For the Local I/O Bus, it is a high-speed input/output bus used for connecting performance-critical peripherals to the memory, chipset, and processor. And last one, it is Standard I/O Bus used for slower peripherals (mice, modems, regular, sound cards, low-speed networking) and also for compatibility with older devices.


    There are few characteristics of bus. Every bus is composed of two distinct parts: the Data bus and the Address bus. The Data bus is the lines that actually carry the data being transferred. The Address bus is the set of lines that carry information about wherein memory the data is to be transferred to or from. In addition, the Control bus are control lines that control how the bus functions, and allow users of the bus to signal when data is available. Besides that, Bus Width also a characteristics of bus. It is the number of bits that can travel in parallel down the bus. The wider the bus, the more information can flow over the channel, as a wider highway can carry more cars than narrow one. The Bus Speed, the speed of the bus reflects how many bits of information can be sent across each wire each second. Most buses transmit one bit of data per line, per clock cycle, although newer high-performance buses like AGP may actually move two bits of data per clock cycle, doubling performance. For Bus Bandwidth, it refers to the total amount of data that can theoretically be transferred on the bus in a given unit of time.


   So how do we define the performance of the bus? Performance of a bus is defined by its transfer time. Transfer time is the amount of time it takes for data to be delivered in a single transaction. For example, the transfer time defines how long a processor will have to wait when it fetches an instruction from memory. We can also define bus performance from bandwidth too. For example, if the bus has 32 data lines and can deliver 1,000,000 packets per second, it has a bandwidth of 32Mbps.