CPU design-Bus classification and device control

Bus structure of the CPU each device to do what things, generally through the control signal control, control signal all determine the state of the bus structure. All control signals are from the controller. The control signal is generally set up separately for the transmission of the line, each signal has its own direction and target. Therefore, the internal control line of the CPU is more dedicated, rather than as the general textbook said, the use of public lines as a control bus. Generally, unless it is a long-distance control signal transmission, taking into account the savings of the line, the use of public line transmission, but both ends are to be restored to a dedicated control line. So in the CPU less control bus, more talk control line more appropriate.

Address lines are often common in the CPU and data buses, unless the number of address lines exceeds the number of lines, then there will be dedicated to run the address signal part of the line, commonly known as address bus is also somewhat reluctant, called the internal bus is more appropriate.

Control line of the entire arrangement of the word, called the bus control word, also known as computer control Word or CPU control word. Each value of the bus control word determines a basic state of the CPU, due to the different bus control word value, you can determine the bus structure of the different state of the CPU, it is to determine the most basic action of the CPU control data, so called micro-instructions.

The process of switching a basic state of a CPU to another basic state is called a basic action, and a micro-instruction can determine the basic action of the CPU. A series of basic actions can accomplish a certain task, set the sequence of basic actions, and encode it, and then use the code to start the series of basic actions, this code is the machine instructions.

The basic action of the CPU is time-order, so the CPU can not leave the time drive. The basic time to drive the CPU is usually done with a quartz oscillator (see figure 3-2). The pulse wave is intercepted by a certain length of section, and the pulse number in the segment is taken to determine the sequence beats of the basic action in the machine instruction.

A beat can contain a clock pulse period, or it can contain multiple clock pulse cycles. After the hardware organization of a CPU is determined, the local or overall state will reflect the basic steps of information processing. A basic action of the CPU takes place within a beat time. The actual cycle length is related to the physical characteristics of the electronic devices that make up the CPU, as well as the manufacturing process. Machine instructions for various computers are always implemented by a finite number of beats, so in many cases people refer to computers as finite state machines.

CPU design-Bus classification and device control