Скачать или смотреть What is DATA Bus? Reading a Byte from Memory.

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The data bus -- probably the most important and also complicated communication system in a computer, and it’s bidirectional. In our example the data bus carries exactly 8 bits of data. Because the size of the data bus directly affects the performance of the entire system, it is precisely this that we mean when we say, for example, that the computer is 8-bit, and the game console is 16 bit, and the version of Windows is 32 or 64 bit. In response to the CPU’s request, the ROM chip puts the contents of the memory cell located at address 44206 onto the data bus. Let’s say the value is 112. Naturally, the data will also be presented in binary format. 112 will look like this... While the read operation is in progress, the CPU becomes the receiving side and its data bus lines act as inputs, and the ROM chips data bus pins act as outputs. Our little computer here has an 8-bit data bus, which means the size of a single memory cell is 1 byte, which is exactly 8 bits. So, our CPU has read exactly 1 byte of data and its value equals 112. Like I said before, the ROM chip contains firmware, a computer program in machine code, written by a programmer. So, 112 (which is, again, just a sample value I picked) is the command that tells the CPU to perform a certain task. These commands are also called operation codes, opcodes, or instructions. So, imagine, its the instruction to write a value of 205 to the memory cell located at address 62088... Please allow me to add a RAM chip to write to into our improvised computer. It will be connected in parallel to both the address and the data bus of our system. Like I said, the address bus is unidirectional and in theory we can connect an infinite number of devices to it.

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