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Point, Open, Take

Any data transfer within the computer occurs always between two storage devices. By "storage" we mean any kind of device capable of holding data, such as a CPU register, a memory cell or an UART buffer.

Usually, the transfer takes three steps in time. First, both devices needs to be addressed. Second, the source devices needs to be opened to the bus. Third, the destination device needs to be clocked to latch the data from the bus. We call these steps: Point, Open and Take respectively. Each step has a duraction calculated to allow data to estabilize before commiting the operation.

The sequence of these steps is called a Machine Cycle (MC). There are different types of MCs as we shall see, but they are all essencially the same: Point (address), Open (three-state buffer) and Take (clock latch device). Some times we can skip one or two steps, making the MC shorter.